EPA/600/R-93/224
December 1993
EPA TRACEABILITY PROTOCOL FOR ASSAY AND CERTIFICATION
OF GASEOUS CALIBRATION STANDARDS
(Revised September 1993)
U.S. Environmental Protection Agency (MD-77B)
Atmospheric Research and Exposure Assessment Laboratory
Quality Assurance and Technical Support Division
Research Triangle Park, NC 27711
-------
DISCLAIMER
The information in this document has been funded wholly by the
U.S. Environmental Protection Agency under contract number
68-D1-0009 to Research Triangle Institute. It has been
subjected to the Agency's peer and administrative review, and it
has been approved for publication as an EPA document.
Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
-------
Intentionally Blank Page
-------
TABLE OF CONTENTS
Section Page
List of Figures ix
List of Tables x
.Acknowledgments xi
1 INTRODUCTION 1-1
2 EPA TRACEABILITY PROTOCOL FOR ASSAY AND CERTIFICATION
OF COMPRESSED GAS CALIBRATION STANDARDS 2-1
2.1 GENERAL INFORMATION 2-1
2.1.1 Purpose and Scope of the Protocol 2-1
2.1.2 Reference Standards 2-1
2.1.2.1 Gas Manufacturer's Intermediate Standard 2-3
2.1.2.2 Recertification of Reference Standards 2-4
2.1.3 Using the Protocol 2-4
2.1.4 Certification Documentation 2-4
2.1.5 Certification Label 2-6
2.1.6 Assay/Certification of Compressed Gas Calibration
Standards 2-6
2.1.6.1 Incubation of Newly Prepared Compressed
Gas Calibration Standards 2-6
2.1.6.2 Stability Test for Reactive Gas Mixtures 2-6
2.1.6.3 Certification Periods for Compressed Gas
Calibration Standards 2-6
2.1.6.4 Minimum Cylinder Pressure 2-8
2.1.6.5 Assay/Certification of Multicomponent
Compressed Gas Calibration Standards 2-8
2.1.7 Analyzer Calibration 2-9
2.1.7.1 Basic Analyzer Calibration Requirements 2-9
2.1.7.2 Analyzer Multipoint Calibration 2-9
2.1.7.3 Zero and Span Gas Checks 2-13
2.1.7.4 Reference Standards for Multipoint Calibrations
and Zero and Span Gas Checks 2-14
2.1.7.5 Uncertainty of the Calibration Curve 2-15
2.1.8 Uncertainty of the Estimated Concentration of
the Candidate Standard 2-16
2.1.9 Zero Gas 2-18
2.1.10 Accuracy Assessment of Commercially Available
Standards 2-18
22 PROCEDURE G1: ASSAY AND CERTIFICATION OF A
COMPRESSED GAS CALIBRATION STANDARD WITHOUT
DILUTION 2-21
2.1.1 Applicability 2-21
iii
-------
TABLE OF CONTENTS (continued)
Section Page
2.2.2 Limitations 2-21
2.2.3 Assay Apparatus 2-21
2.2.4 Pollutant Gas Analyzer 2-23
2.2.5 Analyzer Calibration 2-23
2.2.5.1 Multipoint Calibration 2-23
2.2.5.2 Analyzer Range 2-23
2.2.5.3 Linearity 2-23
2.2.5.4 Zero and Span Gas Checks 2-24
22.6 Assay Gases 2-25
2.2.6.1 Candidate Standard 2-25
2.2.6.2 Reference Standard 2-25
2.2.6.3 Zero Gas 2-25
2.2.7 Assay Procedure 2-26
2.2.8 Stability Test for Newly Prepared Candidate Standards 2-28
2.2.9 Certification Documentation 2-28
2.2.10 Recertification Requirements 2-28
2.3 PROCEDURE G2: ASSAY AND CERTIFICATION OF A
COMPRESSED GAS CALIBRATION STANDARD USING
DILUTION 2-29
2.3.1 Applicability 2-29
2.3.2 Limitations 2-29
2.3.3 Assay Apparatus 2-29
2.3.4 Pollutant Gas Analyzer 2-32
2.3.5 Analyzer Calibration 2-33
2.3.5.1 Multipoint Calibration 2-33
2.3.5.2 Analyzer Range 2-34
2.3.5.3 Linearity 2-34
2.3.5.4 Zero and Span Gas Checks 2-34
2.3.6 Selection of Gas Dilution Flow Rates or Gas
Concentration Settings 2-35
2.3.7 Flowmeter Type and Flowmeter Calibration 2-36
2.3.8 Assay Gases 2-37
2.3.8.1 Candidate Standard 2-37
2.3.8.2 Reference Standard 2-37
2.3.8.3 Zero Gas 2-37
2.3.9 Assay Procedure 2-37
2.3.10 Stability Test for Newly Prepared Standards 2-40
2.3.11 Certification Documentation 2-41
2.3.12 Recertification Requirements 2-41
iv
-------
TABLE OF CONTENTS (continued)
Section Page
3 EPA TRACEABILITY PROTOCOL FOR ASSAY AND CERTIFICATION
4 OF PERMEATION DEVICE CALIBRATION STANDARDS 3-1
3.1 GENERAL INFORMATION 3-1
3.1.1 Purpose and Scope of the Protocol 3-1
3.1.2 Reference Standards 3-1
3.1.3 Selecting a Procedure 3-3
3.1.4 Using the Protocol 3-3
3.1.5 Certification Documentation 3-3
3.1.6 Certification Label 3-4
3.1.7 Assay/Certification of Candidate Permeation Device
Calibration Standards 3-4
3.1.7.1 Permeation Device Design 3-4
3.1.7.2 Precautions for Use and Storage of
Permeation Devices 3-4
3.1.7.3 Equilibration of Newly Prepared Permeation
Devices 3-5
3.1.7.4 Certification Conditions for Permeation
Device Calibration Standards 3-5
3.1.8 Technical Variances 3-6
32. PROCEDURE P1: ASSAY AND CERTIFICATION OF
PERMEATION DEVICE CALIBRATION STANDARDS
REFERENCED TO A PERMEATION DEVICE REFERENCE
STANDARD 3-6
3.2.1 Applicability 3-6
3.2.2 Limitations 3-6
3.2.3 Assay Apparatus 3-7
3.2.4 Pollutant Gas Analyzer 3-9
3.2.5 Analyzer Calibration 3-9
3.2.5.1 Multipoint Calibration 3-9
3.2.5.2 Analyzer Range 3-10
3.2.5.3 Linearity 3-10
3.2.5.4 Zero and Span Gas Checks 3-10
3.2.6 Selection of Gas Dilution Flow Rates 3-10
3.2.7 Flowmeter Type and Flowmeter Calibration 3-10
3.2.8 Permeation Devices 3-11
3.2.8.1 Candidate Standard 3-11
3.2.8.2 Reference Standard 3-12
3.2.8.3 Zero Gas 3-12
3.2.9 Assay Procedure 3-12
3.2.10 Stability Test for Newly Prepared Permeation
Devices 3-15
v
-------
TABLE OF CONTENTS (continued)
Section Page
3.2.11 Certification Documentation 3-16
3.2.12 Recreatification Requirements 3-16
3.3 PROCEDURE P2: ASSAY AND CERTIFICATION OF
PERMEATION DEVICE CALIBRATION STANDARDS
REFERENCED TO A COMPRESSED GAS REFERENCE
STANDARD 3-16
3.3.1 Applicability 3-16
3.3.2 Limitations 3-16
3.3.3 Assay Apparatus 3-17
3.3.4 Pollutant Gas Analyzer 3-19
3.3.5 Analyzer Calibration 3-19
3.3.5.1 Multipoint Calibration 3-19
3.3.5.2 Analyzer Range 3-20
3.3.5.3 Linearity 3-20
3.3.5.4 Zero and Span Gas Checks 3-20
3.3.6 Selection of Gas Dilution Flow Rates 3-20
3.3.7 Flowmeter Type and Flowmeter Calibration 3-20
3.3.8 Candidate Standard 3-21
3.3.9 Reference Standard 3-22
3.3.10 Zero Gas 3-22
3.3.11 Assay Procedure 3-22
3.3.12 Equilibrium Test for Newly Prepared Permeation
Devices 3-26
3.3.13 Certification Documentation 3-26
3.3.14 Recertification Requirements 3-26
3.4 PROCEDURE P3: ASSAY AND CERTIFICATION OF
PERMEATION DEVICE CALIBRATION STANDARDS
REFERENCED TO A MASS REFERENCE STANDARD 3-26
3.4.1 Applicability 3-26
3.4.2 Limitations 3-26
3.4.3 Assay Apparatus 3-27
3.4.3.1 Analytical Balance 3-27
3.4.3.2 Temperature-Controlled Chamber 3-27
3.4.4 Weighing Interval 3-29
3.4.5 Assay Procedure 3-29
3.4.6 Number of Weighings of the Candidate Standard 3-31
3.4.7 Calculation of Certified Permeation Rate 3-31
3.4.8 Uncertainty of Estimated Permeation Rate for
Candidate Standard 3-32
3.4.9 Certification Documentation 3-32
3.4.10 Recertification Requirements 3-32
vi
-------
TABLE OF CONTENTS (continued)
Section Page
4 REFERENCES 4-1
Appendix
A EXAMPLE UNCERTAINTY CALCULATIONS A-1
vii
-------
Intentionally Blank Page
-------
LIST OF FIGURES
Number Page
2-1 One possible design of the apparatus for the assay of compressed
gas calibration standards without dilution (Procedure G1) 2-22
2-2 One possible design of the apparatus using flow controllers for assay
of compressed gas calibration standards with dilution (Procedure G2) 2-30
2-3 One possible design of the apparatus using a gas dilution system for
assay of compressed gas calibration standards with dilution
(Procedure G2) 2-31
3-1 One possible design of the apparatus for the assay of permeation
device calibration standards referenced to a permeation device
reference standard (Procedure P1) 3-8
3-2 One possible design of the apparatus for the assay of permeation
device calibrationstandards referenced to a compressed gas reference
standard (Procedure P2) ..... 3-18
3-3 Chamber for maintaining permeation tubes at constant
temperature 3-28
ix
Preceding page blank
-------
LIST OF TABLES
Number Page
2-1 SUMMARY OF COMPRESSED GAS SRMs THAT ARE AVAILABLE
FROM NIST 2-2
2-2 CERTIFICATION PERIODS FOR COMPRESSED GAS
CALIBRATION STANDARDS IN ALUMINUM CYLINDERS 2-7
2-3 WORKSHEET FOR ALL TYPES OF LINEAR RELATIONSHIPS (After
NBS Handbook No. 91) 2-11
2-4 CALCULATION OF THE 95-PERCENT UNCERTAINTY FOR THE
REGRESSION-PREDICTED CONCENTRATION OF AN INDIVIDUAL
STANDARD (After NBS Handbook No. 91) 2-12
2-5 SOME LINEARIZING TRANSFORMATIONS FOR MULTIPOINT
CALIBRATION DATA (After NBS Handbook No. 91) 2-17
2-6 WORKSHEET FOR LINEAR RESPONSE CORRECTION BASED ON
ZERO AND SPAN CHECKS 2-19
2-7 CALCULATION OF THE 95-PERCENT UNCERTAINTY FOR THE
REGRESSION-CORRECTED RESPONSE TO AN INDIVIDUAL
STANDARD 2-20
3-1 NIST SRM PERMEATION DEVICE REFERENCE STANDARDS 3-2
x
-------
ACKNOWLEDGMENTS
The information in this document has been funded wholly by the U.S. Environmental
Protection Agency (EPA) under Contract No. 68-D1-0009 to the Research Triangle Institute
(RTI), Research Triangle Park, North Carolina. It has been subjected to the Agency's peer
and administrative review and has been approved for publication as an EPA document.
This trace ability protocol was prepared by Robert S. Wright, Robert W. Murdoch, and
Michael J. Messner of RTI under RTI Project No. 91U-6960-208. It is a revision of an earlier
protocol that was prepared in 1987 by Richard C. Shores of RTI. The authors thank the EPA
work assignment manager, Berne i. Bennett, for his guidance and technical discussion as the
protocol was being written. The authors also thank the following RTI staff members who
provided technical, editorial, word processing, and graphic arts support: Mary T. Denton,
W. Cary Eaton, Linda M. Gaydosh, Mary H. Hoffman, Jan L Shirley, and Philip M. Winstead.
Numerous reviewers from government and industry provided useful insights on draft versions
of the guidebook. The authors thank the following organizations for their assistance:
Air Products and Chemicals, Inc.
Airco Gases
Alphagaz
Calibrated Instruments, Inc.
California Air Resources Board
Kin-Tek Laboratories, Inc.
Liquid Carbonic Specialty Gas Corporation
MG Industries
National Institute of Standards and Technology
Sandia National Laboratories
Scott Specialty Gases, Inc.
U.S. Environmental Protection Agency, Acid Rain Division
U.S. Environmental Protection Agency, Quality Assurance and Technical
Support Division
VICI Metronics.
xi
-------
SECTION 1
INTRODUCTION
in 1987, the U.S. Environmental Protection Agency (EPA) in Research Triangle Park,
North Carolina, revised its traceability protocols for the assay and certification of compressed gas
and permeation-device calibration standards.1,2 These protocols allow producers of gaseous
standards, users of gaseous standards, and other analytical laboratories to establish traceability
between their protocol gases and gaseous Standard Reference Materials (SRMs) produced by
the National Institute of Standards and Technology (NIST). Parte 50, 58, 60, and 75 of Title 40
of the Code of Federal Regulations require using SRMs or gaseous standards traceable to SRMs
for calibrating and auditing ambient air and stationary source pollutant monitoring systems.3"6
EPA is revising the traceability protocols for gaseous standards to:
1. Reflect advances in gas blending and dilution technology that have occurred since
1987;
2. Incorporate changes required by changes in the EPA regulations for protocol gases;
3. Address problems that EPA has identified in the assay and certification of protocol
gases; and
4. Improve the quality assurance requirements for the assay and certification of
standards prepared under the protocols.
The current revisions have several significant changes from the 1987 version of the
protocols. These changes are listed below:
1. The Revised Traceability Protocol Nos. 1 and 2 of 1987 have been consolidated
into a single protocol. As a result of this consolidation, dilution of analytical
reference standards and candidate compressed gas calibration standards may now
be used during the assay of standards for continuous emissions monitoring
systems. Previously, the candidate standards for this application could be assayed
only without dilution.
2. Standards may be prepared for any gas mixture that has a corresponding gaseous
NIST SRM. Previously, standards could be prepared only for a more limited set
of gas mixtures.
3. The certification period for most standards has been extended.
4. The requirements for certification documentation have been changed. Additionally,
laboratories are required to retain analytical records for a specified period of time.
1-1
-------
5. The analyzer must have had a multipoint calibration within 1 month prior to the
assay date, rather than within 3 months as previously specified. This calibration
is not used directly to interpret the analyzer response during the assay of the
standard. The data reduction technique for the assay corrects the analyzer
response on the assay date for any minor calibration drift which may have
occurred during the period between the multipoint calibration and the assay date.
The corrected analyzer response is then used with the multipoint calibration data
to calculate a concentration for the candidate standard. Previously, the multipoint
calibration data were not used to quantitatively interpret the analyzer response
during the assay, but were used only to establish the calibrated range of the
analyzer and its response linearity.
6. The multipoint calibration no longer has a linearity specification. In its place, a
specification has been established for the 95-percent uncertainty of concentrations
that are predicted from the linear regression equation for the calibration data. The
uncertainty specification is more relevant than the linearity specification in
assessing the overall uncertainty of the candidate standard's certified
concentration.
7. The concentration range over which standards may be assayed is now dependent
on this uncertainty, rather than being over a fixed range relative to the
concentration of the analytical reference standard. The concentration of the
candidate standard may be greater than or lesser than the reference standard
concentration it both concentrations fall within the well-characterized region of the
analyzer's calibration curve.
8. Limits are placed on the amount of calibration drift that may occur between the
multipoint calibration date and the assay date. Previously, no drift limits existed
and the analyst could adjust the analyzer's response to approximate that which
was observed during the multipoint calibration.
9. Multiple-component standards (e.g., S02 and NO in N2) may be certified under this
revised protocol if gaseous NIST SRMs that contain the individual components of
the standards exist. If any component in the multiple-component standard
interferes with the assay of any other component, the analyst must conduct an
interference study to determine an interference correction equation.
10. The protocol permits the assay of multiple candidate standards during the same
assay session. Previously, a multiple-standard assay was neither allowed nor
disallowed explicitly.
11. Permeation tube output rates may now be assayed gravimetrically. Previously,
these rates could only be assayed using a pollutant gas analyzer and other
gaseous reference standards.
1-2
-------
12. The protocol now has a method for estimating the analytical uncertainty associated
with the assay of the candidate standard. This uncertainty estimate does not
include the uncertainty of the reference standard. It includes the uncertainty of any
gas manufacturer's intermediate standard (QMIS) or interference correction
equation that may be used.
13. The protocol permits calibration standard producers, calibration standard users,
and other analytical laboratories to assay and certify calibration standards
according to the protocol.
1-3
-------
SECTION 2
EPA TRACEABILITY PROTOCOL FOR ASSAY AND CERTIFICATION OF
COMPRESSED GAS CALIBRATION STANDARDS
2.1 GENERAL INFORMATION
2.1.1 Purpose and Scope of the Protocol
This protocol describes two procedures for assaying the concentration of compressed gas
calibration standards and for certifying that the assayed concentrations are traceable to a National
Institute of Standards and Technology (NIST) Standard Reference Material (SRM). This protocol
is mandatory for certifying the compressed gas calibration standards used for the pollutant
monitoring that is required by the regulations of 40 CFR Parts 50, 58, 60 and 75 for the
calibration and audit of ambient air quality analyzers and continuous emission monitors. This
protocol may be used to assay and certify gas mixtures that have the same components as
compressed gas SRMs. A multiple-component standard may be assayed and certified under this
protocol if compressed gas SRMs that contain the individual components in the standard exist
This protocol may be used by specialty gas producers, standard users, or other analytical
laboratories. The assay procedure may involve the direct comparison of the standards to
reference standards without dilution (i.e., Procedure G1) or the indirect comparison of the
standards to reference standards with dilution (i.e., Procedure G2). A candidate standard having
a concentration that is lower or higher than that of the reference standard may be certified under
this protocol if both concentrations (or diluted concentrations) fall within the well-characterized
region of the pollution gas analyzer's calibration curve.
2.1.2 Reference Standards
Parts 50, 58, 60, and 75 of the monitoring regulations require that gaseous pollutant
concentration standards used for calibration and audit of ambient air quality analyzers and
continuous emission monitors be traceable to either an NIST SRM or an NIST/EPA-approved
Certified Reference Material (CRM).7 The CRM program has been terminated by EPA and NIST.
It has been replaced by a similar, but expanded, program called the NIST Traceable Reference
Material (NTRM) Program. EPA has been part of the decision-making process, concurs with this
new program, and recognizes NTRMs as being equivalent to CRMs. NTRMs are compressed
gas calibration standards that are similar to CRMs, except in two significant ways.8 First, the
concentrations of candidate NTRMs will not have to be identical to that of the SRM reference
standards. Second, the producer of the NTRMs rather than EPA will pay NIST to check the
concentration of the candidate NTRMs. Note that CRMs may be used as reference standards
in this protocol for as long as they remain available. Accordingly, the reference standard used
for assaying and certifying a compressed gas calibration standard under this protocol must be an
SRM, a CRM, an NTRM, or a suitable intermediate reference standard (discussed below). The
reference standard must be within its certification period. The compressed gas SRMs that are
available from NIST are listed in Table 2-1.
2-1
-------
TABLE 2-1. SUMMARY OF COMPRESSED GAS SRMs
THAT ARE AVAILABLE FROM NIST*
Certified component
Balance
gas
Concentration range
for SRMs
Aromatic organic gasesb
Nitrogen
0.25 to 10 ppm
Aliphatic organic gases0
Nitrogen
0.20 to 10 ppm
Benzene
Nitrogen
0.25 to 10 ppm
Carbon dioxide and oxygen (i.e.,
blood gas)
Nitrogen
5 to 10 percent C02
0 to 20 percent 02
Carbon dioxide and nitrous oxide
Air
340 to 380 ppm CO,
300 to 330 ppb N20
Carbon dioxide
Nitrogen
0.5 to 14 percent
Carbon monoxide
Air
10 to 45 ppm
Carbon monoxide
Nitrogen
10 ppm to 8 percent
Carbon monoxide, propane, and
carbon dioxide
Nitrogen
1.6 to 8 percent CO
600 to 3,000 ppm C3H8
0 to 14 percent C02
Hydrogen sulfide
Nitrogen
5 to 20 ppm
Methane
Air
1 to 10 ppm
Methane and propane
Air
4 ppm CH4, 1 ppm C3H8
Nitric oxide
Nitrogen
5 to 3,000 ppm
Oxides of nitrogen
(i.e., nitrogen dioxide plus
nitric acid)
Air
100 to 2,500 ppm
Oxygen
Nitrogen
2 to 21 percent
Propane
Air
3 to 500 ppm
Propane
Nitrogen
100 ppm to 2 percent
Propane and oxygen
Nitrogen
100 ppm C3H8
5 to 10 percent 02
Sulfur dioxide
Nitrogen
50 to 3,500 Ram
T etrachloroethylene
Nitrogen
0.25 to 10 ppm
0 All SRMs may not be available at all times. Other compressed gas may be developed in the
future and could be used as reference standards. Contact NIST for information about SRM
availability.
b Aromatic organic gases are benzene, bromobenzene, chlorobenzene, and toluene.
c Aliphatic organic gases are caibon tetrachloride, chloroform, tetrachloroethylene, and vinyl
chloride.
2-2
-------
The EPA regulations define a "traceable" standard as one that has been compared and
certified, either directly or via not more than one intermediate standard, to a primary standard
such as an NIST SRM, a CRM, or an NTRM.3,4 Comparison of a compressed gas calibration
standard directly to an SRM, a CRM, or an NTRM reference standard is preferred and
recommended. However, the use of an intermediate reference standard in the comparison is
permitted, A Gas Manufacturer's Intermediate Standard (GMIS) (see Subsection 2.1.2.1) that has
been compared directly to an SRM, a CRM, or an NTRM according to Procedure G1 is an
acceptable intermediate reference standard and could be used as the reference standard for the
assay of the compressed gas calibration standard on that basis. However, purchasers of
commercially produced gas standards that have been compared to a GMIS should be aware that,
in conformity with the above definition, such a standard could only be used directly for calibration
or audit. Such a standard could not be used as a second-generation intermediate reference
standard to certify other compressed gas calibration standards.
Volume and temperature reference standards can become traceable to NIST primary
standards by calibration at an NIST-accredited state weights and measures laboratory or at a
calibration laboratory that is accredited by the National Voluntary Laboratory Accreditation
Program (NVLAP).9,
2.1.2.1 Gas Manufacturer's Intermediate Standard-
A GMIS is a compressed gas calibration standard that has been assayed by direct
comparison to an SRM, a CRM, or an NTRM, that has been assayed and certified according to
Procedure G1, and that also meets the following requirements:
1. A candidate GMIS must be assayed on at least three separate dates that are
uniformly spaced over at least a 3-month period. During each of these assays, the
candidate GMIS must be measured at least three times. All these assays must use
the same SRM, CRM, or NTRM as the reference standard to avoid errors associated
with the use of different reference standards for different assays.
2. For each assay, the analyst must calculate the mean and standard deviation for the
three or more measured concentrations of the candidate GMIS. The standard error
of the mean concentration must be less than or equal to 1.0 percent of the mean
concentration. That is,
s < CGM!S
ft " 1°o
where
s = standard deviation of the measured concentrations;
n = the number of measurements of the candidate GMIS; and
cgmis = mean measured concentration of the candidate GMIS.
3. After the three or more assays have been completed, the analyst must calculate the
overall mean and standard deviation for the three or more mean measured
2-3
-------
concentrations (i.e., cGMIS). The standard error of the overall mean concentration
must be less than or equal to 1.0 percent of the overall mean concentration. That is,
smean . coverall
y1
100
'ASSAY
. where
Smean = standard deviation of the mean measured concentrations;
"assay = the number of assays; and
Coverall = the overall mean concentration of the candidate GMIS.
The certified concentration of the GMIS will be the overall mean concentration.
4. The difference between the cGMIS for the first assay and the cGM(S for the last
assay must not exceed 1.5 percent of ^overall • A candidate GMIS may not be
certified for use under this protocol if it cannot be demonstrated to be stable.
5. A GMIS must be recertified every 2 years. The ^gmis tor a single reassay must be
within 1.0 percent of the ^overall from the previous certification. The standard
error of this mean must be less than or equal to 1.0 percent of the mean. If the
reassayed GMIS fails to meet these requirements, it must undergo a full certification
as described in step 1 above before it is used again. There is no requirement that
the same reference standard must be used in the original assays and the
recertification assays, but this practice is desirable if possible.
2.1.2.2 Recertification of Reference Standards-
Recertification requirements for SRMs, CRMs, and NTRMs are specified by NIST. See
Subsection 2.1.2.1 for GMIS recertification requirements.
2.1.3 Using the Protocol
The assay/certification protocol described here is designed to minimize both systematic
and random errors in the assay process. Therefore, the protocol should be carried out exactly
as it is described. The assay procedures in this protocol include one or more possible designs
for the assay apparatus. The analyst is not required to use these designs and may use
alternative components and configurations that produce equivalent-quality measurements. Inert
materials (e.g., Teflon®, stainless steel, or glass) and clean, noncontaminating components should
be used in those portions of the apparatus that are in contact with the gas mixtures being
assayed.
2.1.4 Certification Documentation
Each certified compressed gas calibration standard must be documented in a written
certification report and this report must contain at least the following information:
2-4
-------
1. Cylinder identification number (e.g., stamped cylinder number).
2. Certified concentration of the compressed gas calibration standard, in parts per million
or mole percent. This value should be reported to 3 significant digits. The certified
concentration is the mean of all assayed concentrations for which the candidate
standard is considered to be stable.
3. i Balance gas in the gas mixture.
4. Cylinder pressure at certification and the statement that the standard should not be
used when its gas pressure is below 1.0 megapascals (i.e., 150 psig).
5. Date of the assay/certification.
6. Certification expiration date (i.e., the certification date plus the certification period)
(see Subsection 2.1.6.3).
7. Identification of the reference standard used in the assay: NIST SRM number, NIST
sample number, cylinder identification number and certified concentration for an SRM;
cylinder identification number and certified concentration for a CRM, an NTRM, or a
GMIS. The certification documentation must identify the reference standard as being
an SRM, a CRM, an NTRM, or a GMIS.
8. Statement that the assay/certification was performed according to this protocol and
that lists the assay procedure (e.g., Procedure G1) used.
9. The analytical method that was used in the assay.
10. Identification of the laboratory where the standard was assayed and certified.
11. Chronological record of all certifications for the standard.
12. If applicable, statement that the certified concentration of specified component has
been corrected for analytical interferences from other specified components.
13. An estimate of the total analytical uncertainty associated with the assay of the
candidate standard. This estimate must include the uncertainty associated with the
assay of a GMIS, if one is used as the reference standard for the assay of the
candidate standard. The uncertainty of an SRM, a CRM, or an NTRM is not to be
included in this estimate. The uncertainty of interference corrections must also be
included in the estimate.
This certification documentation must be given to the purchaser of the standard. The specialty
gas producer must maintain laboratory records and certification documentation for 3 years after
the standard's certification date. A specialty gas producer or other vendor may redocument an
assayed and certified standard that it has purchased from another specialty gas producer and that
it wishes to sell to a third party. However, the new certification documentation must clearly list
the specialty gas producer or other laboratory where the standard was assayed.
2-5
-------
2.1.5 Certification Label
A label or tag bearing the information described in items 1-6, 8, and 10 of Subsection
2.1.4 must be attached to the standard.
2.1.6 Assay/Certification of Compressed Gas Calibration Standards
2.1.6.1 Incubation of Newly Prepared Compressed Gas Calibration Standards-
Newly prepared compressed gas calibration standards must be incubated at least 4 days
before being assayed and certified.
2.1.6.2 Stability Test for Reactive Gas Mixtures-
Compressed gas calibration standards that contain reactive gas mixtures, including
hydrogen sulfide (H2S), nitric oxide (NO), nitrogen dioxide (N02), sulfur dioxide (S02), and carbon
monoxide (CO), and that have not been previously certified, must be tested for stability as
discussed herein. Conduct an initial assay of the candidate standard and determine a
concentration for the standard. The candidate standard must be measured at least three times
during the assay. Reassay the standard at least 7 days after the first assay and compare the two
assayed concentrations. If the second assayed concentration differs from the first assayed
concentration by 1.0 percent or less, the standard may be considered to be stable, and the mean
of the two assayed concentrations should be reported as the certified concentration. Otherwise,
wait for an additional 7 days or more and repeat the stability test, using the second and third
assays in the stability calculations as if they were the first and second assays. The mean of the
second and third assayed concentrations should be reported as the certified concentration if the
standard is found to be stable. Candidate standards that fail both the initial and the repeat
stability tests are unstable and are disqualified for further use under this protocol.
2.1.6.3 Certification Periods for Compressed Gas Calibration Standards-
The certification of a compressed gas calibration standard is valid for only a specified
period following its certification date which is the date of its last assay. In general, the certification
period should be no longer than the period for which similar standards have been shown to be
stable.11"13 The certification periods for various standards are specified in Table 2-2. These
certification periods are for standards that are contained in aluminum cylinders, if cylinder
materials other than aluminum are used, the certification period is 6 months.
The certification periods given in Table 2-2 apply to specific concentration ranges over
which the gas mixtures have been found to be stable. These concentration ranges match the
concentration ranges for NIST SRMs. The protocol described here allows the certification of
standards with concentrations that may be lower than those of the corresponding SRMs. If the
concentration of the standard is less than the applicable concentration range given in Table 2-2,
the initial certification period for this standard is 6 months. After this period, the standard must
be recertified before further use. The standard must be measured at least three times. The
reassayed mean concentration must be within 1.0 percent of the original certified concentration.
If the reassayed mean concentration meets this specification, the standard may be recertified for
the period shown in Table 2-2 (e.g., 24 months for a 35-ppm sulfur dioxide in nitrogen standard).
The certified concentration of the recertified standard should be reported as the mean of all
assayed and reassayed concentrations.
2-6
-------
TABLE 2-2. CERTIFICATION PERIODS FOR COMPRESSED GAS CALIBRATION
STANDARDS IN ALUMINUM CYLINDERS
Applicable Certification
concentration period
Certified components Balance gas range (months)
Aromatic organic gases
Nitrogen
20.25 ppm
36
Aliphatic organic gases
Nitrogen
>0.20 ppm
36
Benzene
Nitrogen
>0.25 ppm
36
Carbon dioxide
Nitrogen or air8
2300 ppm
36
Carbon monoxide
Nitrogen or air
>8 ppm
36
Hydrogen sulfide
Nitrogen
>4 ppm
12
Methane
Air
21 ppm
36
Nitric oxide
Oxygen-free
nitrogen13
>4 ppm
24
Nitrous oxide
Air
>300 ppb
36
Oxides of nitrogen
(i.e., sum of nitrogen dioxide
and nitric acid)
Air
>80 ppm
24
Oxygen
Nitrogen
>0.8%
36
Propane
Nitrogen or air
21 ppm
36
Sulfur dioxide
Nitrogen or air
40 to 499 ppm
24
Sulfur dioxide
Nitrogen or air
>500 ppm
36
T etrachloroethylene
Nifrogen
>0.25 ppm
36
Multicomponent mixtures
—
—
See text
Mixtures with lower concentrations
—
—
See text
a When used as a balance gas, "air" is defined as a mixture of oxygen and nitrogen where the
minimum concentration of oxygen is 10 percent and the concentration of nitrogen is greater than
60 percent.
b Oxygen-free nitrogen contains <0.5 ppm of oxygen.
2-7
-------
If a standard is to be used after the certification period has ended, it must be recertified
in accordance with this protocol. The recertification assay must be performed using the same
analytical procedure (e.g., Procedure G1) and analytical method as was used for the original
assay of the standard. The purpose of this assay is to determine whether the standard has
remained stable since its original certification. The standard must be measured at least three
times during the assay. A second assay is not needed for recertification of the standard. There
is no requirement that the same reference standard must be used in the original and
recertification assays, although this practice is desirable if possible. Record the results of the
recertification assay in the laboratory's records. The reassayed concentration for a standard must
be within 1.0 percent of its previous certified concentration. If the recertification assay
demonstrates that the standard has remained stable, the second certification period for the
standard is the same as that given in Table 2-2. The certified concentration of a recertified
standard should be reported as the mean of all assayed and reassayed concentrations. If the
reassayed concentration is not within 1.0% of the previous certified concentration, the analyst
must either disqualify the standard for further use under this protocol or investigate why there is
an apparent difference between the two concentrations. This difference may be due to an actual
instability of the gas mixture, to a reference standard problem, to an analytical instrumentation
problem, or to some other problem. If the analyst can find a reasonable explanation for the
difference and if this cause is not instability, then the standard can be recertified. The analyst
must append a brief report on the investigation to the recertification documentation and to the
laboratory's records.
A multiple-component standard can be certified for a period equal to that of its most briefly
certifiable component. For example, a standard containing sulfur dioxide, carbon monoxide, and
propane in nitrogen can be certified for 24 months because the shortest certification period is 24
months.
A standard may be recertified if the gas pressure remaining in the cylinder is greater than
3.4 megapaseals (i.e., 500 psig).
2.1.6.4 Minimum Cylinder Pressure-
In general, a compressed gas calibration standard should not be used when its gas
pressure is below 1.0 megapaseals (i.e., 150 psig). NIST has found that some gas mixtures (e.g.,
nitric oxide in nitrogen) have exhibited a concentration change when the cylinder pressure fell
below this value.
2.1 .<6.5 Assay/Certification of Muiticomponent Compressed Gas Calibration Standards-
This protocol may be used to assay and certify a multiple-component standard if
compressed gas SRMs exist that contain the individual components of the multiple-component
standard. If any component in the multiple-component standard interferes with the assay of any
other component, the analyst must conduct an interference study to determine an interference
correction equation. This study must be conducted using the same analyzer or analyzers as will
be used to assay the standard. The study must use single-component and multiple-component
reference standards that have been assayed using interference-free analyzers. The study must
cover the same range of concentrations for all components as will exist for the standards being
assayed and certified according to this protocol. Data from the interference study must be
evaluated using multiple-variable least-squares regression analysis. The analyst should consult
with a statistician before beginning the study or evaluating its data. The regression analysis must
produce an interference correction equation and an estimate of the 95-percent uncertainty
2-8
-------
associated with the corrected concentrations for the assayed components. The interference
correction equation will be valid for the range of concentrations covered in the study for which the
uncertainty of the corrected concentration is less than or equal to 1 percent of the corrected
concentration. The analyst must add the interference correction uncertainty to the overall
analytical uncertainty of the standard. The certification documentation must include a statement
that the certified concentration of a specified component has been corrected for interferences from
other specified components. An interference study is not needed if the assay analyzer is
interference free.
2.1.7 Analyzer Calibration
2.1.7.1 Basic Analyzer Calibration Requirements-
The assay procedures described in this protocol employ a data reduction technique to
calculate the concentration of a candidate compressed gas standard that corrects for minor
analyzer calibration variations (i.e., drift). This technique does not require the absolute accuracy
of the analyzer's calibration curve at the time of the assay. Requirements for the analyzer follow:
(1) it must have a well-characterized calibration curve for the pollutant of interest (see Subsection
2.1.7.2); (2) it must have good resolution and low noise; (3) its calibration must be known and
must be reasonably stable during the assay/certification process; and (4) all measurements of
candidate standards must fall within a well-characterized region of its calibration curve.
2.1.7.2 Analyzer Multipoint Callbratlon-
The gas analyzer used for the assay must have had a multipoint calibration within 1
month prior to the assay date. This calibration is not used directly to interpret analyzer response
during the assay of the candidate compressed gas calibration standard. The data reduction
technique corrects the analyzer response on the assay date for any minor calibration drift during
the period between the multipoint calibration and the assay date. The corrected analyzer
response is then used with the multipoint calibration data to calculate a concentration value for
the candidate standard.
The multipoint calibration should consist of measurements of the analyzer responses to
at least five different concentrations, including that of a zero gas. Record these measurements
and the analyzer's zero and span control settings in the laboratory's records. These calibration
concentrations should be approximately evenly spaced over the concentration range. The
multipoint calibration is valid only for the concentration range lying between the largest and
smallest measured concentrations. The concentrations may be produced by undiluted reference
standards or by dilution of reference standards using a gas dilution system. See Subsection
2.1.7.4 for reference standard requirements. If a gas dilution system is used, it must have a
specified accuracy of no worse than 1.0 percent of the undiluted reference standard
concentration. The accuracy of the gas dilution system must be checked by the analyst at
periodic intervals by comparing diluted reference standards to undiluted reference standards
having approximately the same concentration. See the appendix to this report for a discussion
of the evaluation of the accuracy of gas dilution systems.
If the analyzer has multiple concentration ranges, a multipoint calibration should be done
for all ranges that will be used later for the assay of candidate standards. A multipoint calibration
that is conducted on one range is not valid for an assay that is conducted on another range.
2-9
-------
Data from the multipoint calibration must be evaluated using least-squares regression
analysis.14 This statistical analysis will be used to determine the analyzer's calibration curve and
to characterize the uncertainty associated with the calibration. The concentration values are the
independent (i.e., X) values in the analysis and their units may be parts per million, mole percent,
or any other appropriate units. The analyzer response values are the dependent (i.e., Y) values
in the analysis and their units may be volts, millivolts, percent of scale or any other measurable
analyzer response units. The analyzer response values should have a resolution of less than or
equal to 1, percent of the maximum measured analyzer response.
The remainder of this subsection is based on the assumption that a straight-line model
is used for the regression analysis. If a quadratic or higher order model is used, the regression
analysis must use other techniques to determine the analyzer's calibration curve and its
uncertainty. The analyst should consult with a statistician before doing these calculations.
Calculate the slope and the Y-axis intercept of the least-squares regression line using the
worksheet given in Table 2-3 or using equivalent techniques (e.g., a calculator). Plot the values
from the multipoint calibration and then plot the regression line. These plots wilt provide a
graphical representation of the calibration and will permit a qualitative assessment of the
uncertainty associated with the calibration. Record the regression calculations and the plots in
the laboratory's records.
However, a quantitative assessment of the calibration's uncertainty is needed to allow the
analyst to determine whether the multipoint calibration data adequately characterizes the "true"
calibration curve for the analyzer. The criterion to be used to evaluate the uncertainty of the
multipoint calibration is the 95-percent uncertainty (i.e., a = 0.05) for a concentration predicted
from the regression line using measured values of the analyzer response. This 95-percent
uncertainty for the predicted concentration can be calculated using the worksheet given in Table
2-4 or using equivalent techniques. Record the uncertainty calculations in the laboratory's
records. A multipoint calibration will be considered to be well-characterized for all concentrations
which are in the range of the multipoint calibration data and for which the 95-percent uncertainties
are all less than or equal to 1 percent of the largest concentration used in the multipoint
calibration. The calibrated range of the analyzer will be considered to extend across aB
concentrations for which the 95-percent uncertainties are less than or equal to this 1-percent
criterion.
The 95-percent uncertainty value is a measure of how well the multipoint calibration data
fit an equation which the analyst assumes is the "true" calibration equation for the analyzer. This
value can be viewed as an index of "linearity" under the assumption of a straight-line calibration
equation. However, this value also has a more general utility for quadratic or higher order
polynomial calibration equations for which linearity is not an appropriate concept. This concept
is discussed further in Reference 14.
A multipoint calibration may fail to meet this uncertainty criterion for several possible
reasons:
• inadequate analytical precision;
• inaccuracy of the reference standards or the gas dilution system; or
2-10
-------
TABLE 2-3. WORKSHEET FOR ALL TYPES OF LINEAR RELATIONSHIPS
(After NBS Handbook No. 91)
X denotes
IX =
Mean X, X =
Y denotes _
ZY =
Mean Y, Y =
Number of measurements: n =
Step (1) LXY
(2) (IX)(ZY)/n =
(3) S;
xy
= Step (1) - Step (2)
(4)
(5)
(6)
IX2
(ZX)2/n
(7) XY
(8) (ZY)2/n
= Step (4) - Step (5) (9) S,
yy
Step (7) - Step (8)
(10) Slope, b, = Sxy/S
(11) Y_
(12) b,X
(13) Y - Intercept,
b0 = Y-b1X
xx ~ Step (3) Step (6)
Step (11)-Step (12)
(14) (Sxy)2/S,
(15) (n - 2) S
y/x
(16)
(17)
tyx
V*
Step (9) - Step (14)
Step (15) (n — 2)
Equation of the regression line
Estimated variance of the slope:
(18) = Sy/x/Sja = steo (16) -s- Step (6)
Estimated variance of intercept:
m S4 = S*„ (1/n ~ X2/S„)
Note: The following are algebraically identical:
S^ = E(X - X)2; Syy « E(Y - Y)2; S^ = E(X - X) (Y - Y).
Ordinarily, in hand computation, it is preferable to compute as shown in the steps above. Carry all
decimal places obtainable—i.e., if data are recorded to two decimal places, carry four places in steps
(1) through (9) in order to avoid losing significant figures in subtraction.
,2-11
-------
TABLE 2-4. CALCULATION OF THE 95-PERCENT UNCERTAINTY FOR THE
REGRESSION-PREDICTED CONCENTRATION OF AN INDIVIDUAL STANDARD
(After NBS Handbook No. 91)
This calculation is based on the linear regression analysis of multipoint calibration data
and replicate measurements of the candidate standard.
n = The number of measurements in the multipoint calibration.
n' = The anticipated number of measurements of the individual standard.
Y' = The anticipated mean analyzer response from measurement of the individual standard.
X' = The regression-predicted concentration of the individual standard.
(1) Choose a for a desired 100 (1 - a)%
confidence level (e.g., a = 0.05 for
95-percent confidence).
(2) Look up t (1. ^ n_2) from a table of
Student's t-distribution.
(3) Obtain b^S^ , Sy/X, and S^ from
Table 2-3.
(4) Calculate C
(5) Calculate the 100(1 - a)% uncertainty
for the regression-predicted
concentration
^(1 -a/2, n-2) ®y/X
c
or -7f
(i + X\c
n rr
(6) Compare this uncertainty to 1-percent of
the largest concentration used in the
multipoint calibration, (max. cone.)
Confidence level
a =
t,
(1 - a/2, n-2) ~ •
K
'xx
Uncertainty
Too (max.conc.)=.
%
'2-12
-------
excessive uncertainty in the analyzer's calibration equation due to incorrect
assumptions about tie form of the equation.
The effect of inadequate analytical precision can be reduced by increasing the number
of replicate measurements at each calibration concentration or by increasing the number of
different concentrations used in the multipoint calibration. Additionally, precision can be improved
by using an averaged analyzer response, rather than an instantaneous analyzer response, for
each measurement. Reference standard inaccuracy is reduced by using the most accurate
reference standards that are available. An inaccurate gas dilution system can be detected by
comparing measurements of the concentration of a diluted reference standard to the theoretically
equal concentration of another, undiluted reference standard. It can also be detected by
comparing measurements of two theoretically equal concentrations obtained by dilution of two
reference standards having significantly different concentrations. An inaccurate gas dilution
system must not be used for the multipoint calibration. The effect of excessive uncertainty in a
straight-line calibration equation can be eliminated by using quadratic or higher order polynomial
regression analysis or by transforming the calibration data mathematically so that they may be
fitted to a straight line regression equation. See Subsection 2.1.7.5 for a discussion of such
mathematical transformations.
The analyzer's zero and span controls may be adjusted before the start of the multipoint
calibration. If a zero or span adjustment is made, allow the analyzer to stabilize for at least one
hour before beginning the multipoint calibration. The waiting period Is necessary because some
analyzers' calibrations drift for a period of time following a zero or span control adjustment.
Note that possibly a more restrictive uncertainty criterion applies for the assay of the
candidate stand aid. The 95-percent uncertainty for the regression-predicted concentration of the
candidate standard must be less than or equal to 1 percent of the certified concentration of the
reference standard.
2.1.7.3 Zero and Span Gas Checks-
On each day that the analyzer will be used for the assay of a candidate standard, its
calibration drift must be measured. This drift is calculated relative to the analyzer response during
the multipoint calibration. The measurement need not be made with the zero gas and the
reference standard that will be used during the assay. The reference standard may be produced
by an undiluted compressed gas calibration standard or by dilution of a compressed gas
calibration standard using a gas dilution system. The reference standard does not have to be one
of the reference standards that are measured during the multipoint calibration or during the assay
of the candidate standard.
The purpose of the zero and span gas checks is to verify that the calibration drift has
remained within acceptable limits since the multipoint calibration. The criterion that is used to
assess the drift is the relative difference between the analyzer's current response and the
corresponding value from the multipoint calibration. The following equation is used for this
calculation:
2-13
-------
Relative Difference = 100
Current Response - Calibration Response
Calibration Response for Reference Standard
Note that the relative difference is always calculated relative to the calibration response for the
reference standard, rather than for the zero gas.
This calculation is performed for the zero gas measurements and for the reference
standard measurements. If the reference standard was not measured during the multipoint
calibration, use the regression-predicted response for a concentration equal to that of the
reference standard.
If the relative differences for the zero and span gas checks are each less than or equal
to 5.0 percent, the analyzer's current calibration is considered to be approximately the same as
during the multipoint calibration and the assay may be conducted. The zero and span controls
do not have to be adjusted following the zero and span checks because the data reduction
technique used in this protocol does not depend on the absolute accuracy of the analyzer
calibration equation at the time of the assay.
If the relative differences for the zero or span gas checks are greater than 5.0 percent,
the analyzer is considered to be out of calibration. A new multipoint calibration must be
conducted before the candidate standard can be assayed. The zero and span controls may be
adjusted to return the analyzer's response to desired levels. However, this adjustment does not
remove the requirement for a new multipoint calibration.
Between the time of the multipoint calibration and the time of the zero and span gas
checks, the analyst may adjust the analyzer's zero and span controls for assays that will not be
certified according to this protocol. However, these controls must be returned to their calibration
settings before the zero and span gas checks or assays under this protocol. This protocol does
not allow the analyst to conduct zero and span gas checks and then adjust the controls to
reproduce the analyzer's readings that were obtained during the multipoint calibration.
The zero gas and reference standard measurements that are performed for the assay of
the candidate standard may also be used for the zero and span gas checks. However, the assay
will not be valid and the analyzer will not be considered to be in calibration, if the relative
difference for the zero gas measurements or the reference standard measurements is greater
than 5.0 percent.
2.1.7.4 Reference Standards for Multipoint Calibrations and Zero and Span Gas Checks-
The reference standards for the multipoint calibration and for the span gas checks must
be diluted or undiluted SRMs, CRMs, NTRMs, or GMISs as specified in Subsection 2.1.2. The
reference standard for the span gas check need not be the same as one of those used for the
multipoint calibration or for the assay of the candidate standard. Diluted pure gases may be used
as the reference standards for the multipoint calibrations, but they may not be used as the
reference standards for the span gas check or for the assay of the candidate standard. Pure
gases may not be diluted by more than a factor of 100. The zero gas must meet the
requirements in Subsection 2.1.8.
2-14
-------
2.1.7.5 Uncertainty of the Calibration Curve-
The data reduction technique used in this protocol is based on the assumption that the
analyzer has a well-characterized calibration curve. The accuracy of the certified concentration
of a candidate compressed gas calibration standard is dependent upon this assumption. The
analyst cannot assume that the analyzer's calibration curve is a straight line between the
measured values for the zero gas and the reference standard. The analyst must calculate the
calibration equation and the uncertainty for its predicted concentrations by the statistical analysis
of the measurements obtained during the multipoint calibration.
The total uncertainty of the certified concentration for a candidate standard is composed
of several components. The first component is the accuracy associated with the certified
concentration of the reference standard. This uncertainty is minimized by using an SRM, a CRM,
an NTRM, or a GMIS as the reference standard. The second component is the precision of the
measurements of the reference and candidate standards. This uncertainty is minimized by
making replicate measurements of these standards. The third component is the uncertainty
associated with the concentrations that are predicted from the analyzer's calibration curve. This
uncertainty concerns whether an assumed calibration equation accurately represents the "true-
calibration curve.
This third component of uncertainty does not exist if the concentrations of the reference
and candidate standards are equal. The assumed calibration equation and the true calibration
curve will pass through the data for the reference standard regardless of whether they diverge
elsewhere and the equation will be accurate for that single concentration. However, the
uncertainty does exist if the concentrations of the reference and candidate standards differ. The
assumed and true calibration curves may pass through different points for concentrations not
equal to that of the reference standard. Analytical errors will develop because of this difference.
The measure of this uncertainty that is most directly useful to the analyst is the 95-percent
uncertainty for a regression-predicted concentration given one or more measurements of the
candidate standard. The 95-percent uncertainty is given by:
± t(1~a/2- "~2)Sy*
(7 - y)2 + ^2 + J_)c
s« n n'
The terms of tills equation are explained in Subsection 2.1.7.2. The 95-percent
uncertainty for the regression-predicted may be calculated using the worksheets given in Tables
2-3 and 2-4. An example of these uncertainty calculations is given in Appendix A.
Several points should be noted about this uncertainty value. First, its magnitude
decreases as n increases where n is the number of measurements in the multipoint calibration.
Second, its magnitude decreases as n' increases, where n' is the number of measurements of
the candidate standard. Third, its magnitude increases as the mean measured analyzer response
(7) for the candidate standard diverges from the overall mean measured analyzer response (y)
for the multipoint calibration. These points mean that it becomes easier to satisfy the uncertainty
criterion as one increases the number of measurements in the multipoint calibration and in the
assay of the candidate standard. Additionally, the absolute uncertainty of the regression predicted
2-15
-------
concentration is larger at the extremes of the calibrated concentration range than at the middle
of the range.
For analyzers having an inherently nonlinear, but precise response, the calibration
equation can be calculated using quadratic or higher order polynomial regression analysis.
Alternatively, a nonlinear equation may be linearized with a simple mathematical transformation
of the multipoint calibration data. Examples of some linearizing transformations are given in
Table 2-5,, which is reproduced from Reference 14. The multipoint calibration data may need
to undergo several different transformations before the optimum transformation is determined.
Using appropriately transformed calibration data, a least squares straight-line calibration equation
can be calculated with an acceptable 95-percent uncertainty for the regression-predicted
concentration. Subsequently, data obtained from the assay of the candidate standard must be
similarly transformed to calculate a concentration for the candidate standard.
If an analyzer has inadequate precision, it is very unlikely that a polynomial equation or
a mathematical transformation will generate an acceptable 95-percent uncertainty for the
regression-predicted concentration.
2.1.8 Uncertainty of the Estimated Concentration of the Candidate Standard
Uncertainty in the concentration estimated for a candidate standard is due to many
different sources, including uncertainty in the reference standards, uncertainty in the analyzer
multipoint calibration, uncertainty in the zero/span correction factors, and measurement
imprecision. Some of these sources can be assessed using standard statistical techniques
(regression analysis), but others cannot be assessed with the limited data that are produced when
implementing this protocol.
For those cases where the candidate standard Is assayed at the same time as the
multipoint calibration, the candidate standard's concentration is determined directly from the
calibration curve. If a straight-line model is used for the regression calculations, Table 2-4 (or its
equivalent) can be used to determine the 95-percent uncertainty of the concentration. An
example of these uncertainty calculations is given in Appendix A. The 95-percent uncertainty of
the reference standards' concentrations has not been factored into these calculations and is
assumed to be negligible. However, if the reference standards are GMISs, the 95-percent
uncertainty of their concentrations must be included in the assessment of the total analytical
uncertainty of the candidate standard's concentration using the following equation:
UncertaintyTOTAL = ^/(UncertaintyASSAY)2 + (UncertaintyGMIS)2 .
If a quadratic or higher order model is used for the regression calculations. Table 2-4 cannot be
used to determine the uncertainty of the concentration. The analyst should consult with a
statistician before doing the uncertainty calculations.
For those cases where the candidate standard Is assayed on a different date from
the multipoint calibration, the uncertainty estimate produced by Table 2-4 neglects a major
contributor of uncertainty; that which is due to the drift correction. In fact, if the analyzer
calibration curve is linear, then the corrected results will have uncertainty that is due only to the
drift correction and have absolutely zero uncertainty that is due to the initial calibration. If the
analyzer calibration curve is nonlinear, then the drift correction will not correct for any drift in the
shape of the curve (e.g., a change in the quadratic coefficient). Unless additional standards are
2-16
-------
TABLE 2-5. SOME LINEARIZING TRANSFORMATIONS
FOR MULTIPOINT CALIBRATION DATA
(After NBS Handbook No. 91)
If the relationship
is of the form:
STEP ONE:
Plot the transformed
calibration Data
STEP TWO:
Fit the
straight line
STEP THREE:
Convert straight line
constants (b0 and b,)
to original constants:
yt=
XT =
Y-j- = bp b1XT
b, =
Y = a + 1
X
Y
1
X
Use the normal
procedures for
calculating the
regression line using
the transformed
calibration data.
Calculate the 95-
percent uncertainties
for the predicted
transformed
concentrations and
compare them to the
a
b
Y = 1 ,
a - bX'
or
1 « a «¦ bX
Y
1
Y
X
a
b
v = x
a + bX
x\>
X
uncertainty criterion.
a
b
Y = ab*
tog Y
X
toga
log b
ii
m
CD
ff
log Y
X
tog a
b log e
aXb
tog Y
tog X
log a
b
Y = a + bX n,
where n is known
Y
Xn
a
b
2-17
-------
used to correct for these higher order drifts, we can only assess the uncertainty that is due to the
simple linear correction.
Uncertainty In an estimate that is based on a drift-corrected response may be determined
using Tables 2-3 and 2-4 (or equivalent techniques), treating the zero/span correction as a two-
point calibration. Tables 2-3 and 2-4 have been adapted for this use and, with minor changes,
are included as Tables 2-6 and 2-7. An example of these uncertainty calculations is given in
Appendix A. As before, the uncertainty of the reference standard's concentration has not been
factored into these calculations and is assumed to be negligible. However, if the reference
standard is a GMIS, the uncertainty of its concentration must be included in the assessment of
the uncertainty of the candidate standard's concentration.
If an interference-correction equation has been used to obtain a corrected concentration
for the candidate standard, the 95-percent uncertainty for the corrected concentration must be
included in the assessment of the total analytical uncertainty of the candidate standard's
concentration using the following equation:
UncertaintyTOTAL = ^(Uncertainty ASSAY)2 + (UncertaintyCORRECT,ON)2 .
The analyst may report the total analytical uncertainty of the candidate standard's certified
concentration on the certification documentation or may report this estimate as a percentage that
is relative to the certified concentration using the following equation:
UncertaintyRELATlvE = 100 [ UncertaintyTOTAL / Certified Concentration] .
2.1.9 Zero Gas
Zero gas used for zero gas checks or for dilution of any candidate or reference standard
should be clean, dry, zero-grade air or nitrogen containing no detectable concentration of the
pollutant of interest. The zeFo gas also should contain no contaminant that causes a detectable
response on the analyzer or that suppresses or enhances the analyzer's response. The oxygen
content of zero air should be approximately that of ambient air, unless it has been demonstrated
that varying the oxygen content does not suppress or enhance the analyzer's response. The
water vapor concentration in the zero gas should be less than 5 ppm.
2.1.10 Accuracy Assessment of Commercially Available Standards
Periodically, the U.S. EPA will assess the accuracy of compressed gas calibration
standards that have been assayed and certified according to this protocol. The accuracy of
representative standards will be assessed by EPA audits. The audit results, identifying the
specialty gas producers or otfier analytical laboratories that assayed and certified the standards,
will be published as public information.
2-18
-------
TABLE 2-6. WORKSHEET FOR LINEAR RESPONSE CORRECTION
BASED ON ZERO AND SPAN CHECKS
X denotes Response predicted from initial
calibration
XX =
Y denotes Observed response
Mean X, X =
ZY =
Mean Y, Y =
Number of measurements: n =
Step (1) XXY
(2) (ZX)(IY)/n =
(3) S:
*y
= Step (1) - Step (2)
(4)
(5)
(6)
IX"
(ZX)2/n
= Step (4) - Step (5)
(7) XY2
(8) (XY)2/n
(9) S„
yy
Step (7) - Step (8)
(10)
(11)
(12)
(13)
Slope, b, =
Y
Step (3) Step (6)
b,X
Y - Intercept,
b0 = Y-b1X
= Step (11) - Step (12)
(14) (Sxy)2/Sxx
(15) (n - 2) S
y/x
(16)
(17)
Jy/x
V*
Step (9) - Step (14)
Step (15) -s- (n - 2)
Equation of the regression line
Estimated variance of the slope:
(18) = step (16) -fr Step (6)
Estimated variance of intercept:
<19> < - sj, (1*1 * X2/S„)
Note: The following are algebraically identical:
S^ - E(X - X)2; Syy = E(Y - Y)2; S^ = £(X - X) (Y - Y).
Ordinarily, in hand computation, it is preferable to compute as shown in the steps above. Carry ad
decimal places obtainable—i.e., if data are recorded to two decimal places, carry four places in steps
(1) through (9) in order to avoid losing significant figures in subtraction.
2-19
-------
TABLE 2-7. CALCULATION OF THE 95-PERCENT UNCERTAINTY FOR THE
REGRESSION-CORRECTED RESPONSE TO AN INDIVIDUAL STANDARD
This calculation is based on the linear regression analysis of the zero and span check
data. These checks effectively constitute a two point calibration. Replicate measurements
are made of the candidate standard.
n = The number of measurements in the two point (zero and span checks) calibration,
n' = The anticipated number of measurements of the individual standard.
Y' = The mean analyzer response from measurement of the individual standard.
X' = (Y' - b0)/b1 = The regression-corrected response of the individual standard.
(1) Choose a for a desired 100 (1 - a)%
confidence level (e.g., a = 0.05 for
95-percent confidence).
(2) Look up t (1. a/2t n.2) from a table of
Student's t-distribution.
(3) Obtain bi-S£. V and from
Table 2-6.
(4) Calculate C =
bl " ^(1 - a/2, n-2))2 Sb,
(5) Calculate the 100(1 - a)% uncertainty
for the regression-estimated response
%
(1 -a/2, n-2) °y/x
(7' -Y)2
1 1
4* _
(6) The interval X'-uncertainty to
X'+uncertainty is a 100(1 -a)%
confidence interval for the response that
would have been observed at the time
of initial calibration.
Confidence level
a =
%
t,
(1 - a/2, n-2)
Sy/x ~
Sxx -
C =
Uncertainty =.
X'- uncertainty
X' + uncertainty
2-20
-------
2.2
PROCEDURE G1: ASSAY AND CERTIFICATION OF A COMPRESSED GAS
CALIBRATION STANDARD WITHOUT DILUTION
2.2.1 Applicability
This procedure may be used to assay the concentration of a candidate compressed gas
calibration standard, based on the concentration of a compressed gas reference standard of the
same gas mixture. This procedure allows a specialty gas producer, a standard user, or other
analytical laboratory to certify that the assayed concentration for the candidate standard is
traceable to the reference standard. The procedure employs a pollutant gas analyzer to compare
the candidate and reference standards' concentrations by direct measurement without dilution of
either gas.
This procedure may be used for the assay of more than one candidate standard during
the same assay session. Criteria that apply to the assay of one candidate standard apply to the
assay of multiple candidate standards.
2.2.2 Limitations
The concentration of the candidate standard may be greater than or lesser than the
concentration of the reference standard. However, both concentrations must lie within the well-
characterized region of the multipoint calibration (see Subsection 2.1.7.2). Additionally, the 95-
percent uncertainty for the regression-predicted concentration of the candidate standard must be
less than or equal to 1 percent of the certified concentration of the reference standard. This
uncertainty is obtained from the statistical analysis of the multipoint calibration data. This criterion
means that the uncertainty associated with the multipoint calibration determines the concentration
range over which candidate standards may be assayed.
The balance gas must be the same in both the candidate standard and the reference
standard, unless it has been demonstrated that the analyzer's response is insensitive to
differences in the balance gas composition.
A source of clean, dry zero gas is required.
2.2.3 Assay Apparatus
Figure 2-1 illustrates one possible design of apparatus for the assay of compressed gas
calibration standards without dilution. This apparatus is designed to allow the convenient routing
of the gas mixtures to the pollutant gas analyzer. The gas mixture to be measured is selected
by rotation of two three-way valves (i.e., V1 and V2). Pressure regulators and gas flow controllers
(i.e., C1 and C2) control the flow rates from the individual cylinders. The gas flow controllers may
be needle values, capillary tubes, thermal mass flow controllers, or other flow control devices.
The gas mixtures are routed to the analyzer through a union tee tube fitting. Gas in excess of
the analyzer's demand is vented, which helps to ensure that the gas entering the analyzer is at
near-ambient pressure. Normally, the excess gas is vented to the atmosphere without any
obstructions in the tubing. However, the excess gas can be routed through an uncalibrated
rotameter by rotation of a three-way valve (i.e., V3). The rotameter is used to demonstrate that
the total gas flow rate exceeds the sample flow rate of the analyzer and that no room air is being
drawn in through the vent line.
The apparatus may be modified in several ways that will not diminish its performance.
The two three-way valves could be replaced by solenoid valves or by a single four-way valve with
three input ports and one output port. Alternatively, a single length of tubing with a gas flow
2-21
-------
Gas Flow
to Vent
Gas Flow
to Vent
Pressure
Regulator
Gas Flow
Controller
(CI)
Three-way
Valve (V3)
Three-wa
Valve (V1
Pressure
Regulator
Gas Flow
Controller
(C2)
Pressure
Regulator
Three-way
Valve (V2)
Rotameter
Gas Flow to
Analyzer
Candidate
Standard
Reference
Standard
Zero
Gas
Figure 2-1. One possible design of the apparatus for the assay of compressed
gas calibration standards without dilution (Procedure G1).
-------
controller could be connected manually to individual cylinders' pressure regulators in succession.
See also Subsection 2.1.3.
2.2.4 Pollutant Gas Analyzer
The pollutant gas analyzer must have a well-characterized calibration curve and must be
capable of measuring directly the concentration of both the candidate and the reference standards
without dilution. See Subsection 2.1.7.1. It must have good resolution, good precision, a stable
response,'and low output signal noise. In addition, the analyzer must have good specificity for
the pollutant of interest so that it has no detectable response to any contaminant that may be
contained in either the candidate or reference standards. If the candidate and reference
standards contain dissimilar balance gases (e.g., air versus nitrogen or different proportions of
oxygen in the balance air), it must have been demonstrated that the analyzer's response is not
sensitive to differences in the balance gas composition. This demonstration may be
accomplished by showing that no difference exists in the analyzer's response when measuring
a compressed gas calibration standard that has been diluted with identical flow rates of different
balance gases.
The analyzer should be connected to a high-precision strip chart recorder or other data
acquisition system to facilitate graphical observation and documentation of the analyzer's
response during the assay. Additionally, a digital panel meter with four-digit resolution, a digital
voltmeter, a data logger or some other data acquisition system must be used to obtain numerical
values of the analyzer's response. More precise values will be obtained if this system has a data-
averaging capability.
If the analyzer has not been in continuous operation, turn it on and allow it to stabilize for
at least 12 hours before beginning the measurements.
2.2.5 Analyzer Calibration
2.2.5.1 Multipoint Calibration-
See Subsections 2.1.7.2 and 2.1.7.4.
2.2.5.2 Analyzer Range-*
The range of the analyzer must include the concentrations of the zero gas, the candidate
standard and the reference standard. The concentrations of the candidate and reference
standards must fall within the well-characterized region of the analyzer's calibration curve (i.e.,
the 95-percent uncertainty of the regression-predicted concentration must be less than or equal
to 1.0 percent of the largest concentration that was used in the multipoint calibration). In general,
the analyst should use a range that will produce the largest analyzer response.
2.2.5.3 Linearity-
The data reduction technique used in this procedure requires that the analyzer have a
well-characterized, but not necessarily linear, calibration curve (see Subsection 2.1.7.5). High-
concentration-range analyzers of the type that are required for this procedure may not be
inherently linear, but in such cases they usually have a predictable, non-linear calibration curve
that can be described by a polynomial equation or can be mathematically transformed to produce
a straight-line calibration curve that is suitable for use in this procedure. Any such polynomial
equation or mathematical transformation should be verified during the multipoint calibration.
Caution should be exercised in using a transformed calibration curve because zero or span
control adjustments to the analyzer may produce unexpected effects in the transformed calibration
curve.
2-23
-------
2.2.5.4 Zero and Span Gas Checks-
See Subsections 2.1.7.3 and 2.1.7.4. Prior to carrying out the assay of the candidate
standard, use zero and span gases to check for calibration drift in the analyzer since the
multipoint calibration. Zero gas and span gas checks must be performed on each day that
candidate standards are assayed. If multiple assays are being performed on the same analyzer
range, the analyst needs to perform only a single set of zero gas and span gas checks for this
range. However, another set must be performed if the range is changed.
The gas mixtures to be used during the zero and span gas checks need not be the same
as any of the reference standards used for the assay of the candidate standard or for the
multipoint calibration. The reference standard for the span gas check must be traceable to an
NIST SRM, a CRM or an NTRM. Information concerning this standard (e.g., cylinder identification
number, certified concentration) must be recorded in the laboratory's records.
Make three or more discrete measurements of the zero gas and three or more
independent measurements of the reference standard. "Discrete" means that the analyst must
change the gas mixture being sampled by the analyzer between measurements. For example,
the analyst might alternate between measurements of the reference standard and measurements
of the zero gas. Record these measurements in the laboratory's records.
Next, verify that the analyzer's precision is acceptable. Calculate the mean and standard
deviation of the analyzer's responses to the zero gas. Repeat the calculations for the reference
standard measurements. Record these calculations in the laboratory's records. The standard
error of the mean for each set of measurements must be less than or equal to 1.0 percent of the
mean response to the reference standard. That is,
s < Rrs
where
s = standard deviation of the analyzer's response;
n = the number of independent measurements of the gas mixture; and
rrs = the mean analyzer response to the reference standard.
The value of the standard error of the mean can be made smaller by increasing the number of
measurements. This calculation will enable the analyst to determine how many replicate
measurements are needed during the assay of the candidate standard to obtain acceptable
precision. The analyst may wish to use a data logger or data acquisition system with averaging
capability to obtain more precise measurements. If the value of the standard error of the mean
is not acceptable, then the analyzer must be repaired or another analyzer must be used for the
assay.
Next, verify that excessive calibration drift has not occurred since the multipoint
calibration. For the zero gas measurements, calculate the relative difference (in percent) between
the current mean analyzer response during the zero gas check and the corresponding response
that is predicted from the multipoint calibration regression equation. That is,
Relative Difference = 100
Current Response - Calibration Response
Calibration Response for Reference Standard
2-24
-------
Note that the relative difference is always calculated relative to the calibration response for the
reference standard, rather than for the zero gas. Repeat this calculation for the reference
standard measurements. Record these calculations in the laboratory's records. If the reference
standard was not measured during the multipoint calibration, use the regression-predicted
response for a concentration equal to that of the reference standard.
Then, if the relative differences for the zero and span gas checks are less than or equaJ
to 5.0 percent, the analyzer is considered to be sufficiently in calibration. The zero and span
controls need not be adjusted and the assay may be conducted. The data reduction technique
does not require the absolute accuracy of the analyzer calibration. Some minor calibration drift
is acceptable because the effect of any drift will be corrected during the reduction of the assay
data.
However, if the relative difference for either the zero or the span gas check is greater than
5.0 percent, then the analyzer is considered to be out of calibration. A new multipoint calibration
must be conducted before the candidate standard can be assayed. The zero and span controls
may be adjusted prior to the multipoint calibration to return the analyzer's response to desired
levels. However, any adjustments that are made do not remove the requirement for a new
multipoint calibration.
Between the time of the multipoint calibration and the time of the zero and span gas
checks, the analyst may adjust the analyzer's zero and span controls for assays that will not be
certified according to this protocol. However, these controls must be returned to their calibration
settings before the zero and span gas checks or assays under this protocol. This protocol does
not allow the analyst to conduct zero and span gas checks and then adjust the controls to
reproduce the analyzer's readings that were obtained during the multipoint calibration.
The zero gas and reference standard measurements that are performed for the assay of
the candidate standard may also be used for the zero and span gas checks. However, the assay
will not be valid and the analyzer will be considered out of calibration if the relative difference for
either the zero gas measurements or the reference standard measurements is greater than 5.0
percent.
2.2.6 Assay Gases
2J2.6.1 Candidate Standard-
See Subsections 2.1.6 and 2.2.2.
2.2.6.2 Reference Standard-
See Subsections 2.1.2 and 2.2.2. The reference standard used for the assay of the
candidate standard must be traceable to an NIST SRM, a CRM, or an NTRM. This standard
need not be the same as any of the reference standards used for the span gas check or for the
multipoint calibration. Information concerning the reference standard (e.g., cylinder identification
number, certified concentration, etc.) must be recorded in the laboratory's records.
2.2.6.3 Zero Gas-
See Subsection 2.1.8. The zero gas should match the balance gas in the candidate
standard aid the reference standard, unless it has been demonstrated that the analyzer is
insensitive to differences in the balance gas composition. Information concerning the zero gas
should be recorded in the laboratory's records.
2-25
-------
2.2.7 Assay Procedure
1, Verify that the assay apparatus is properly configured, as described in Subsection
2.2.3 and shown in Figure 2-1. Inspect the analyzer to verify that it appears to be
operating normally and that all controls are set to their expected values. Record
these control values in the laboratory's records.
2.1 Verify that a multipoint calibration of the analyzer has been performed within 1 month
prior to the assay date. (See Subsections 2.1.7.2, 2.1.7.5 and 2.2.4). Additionally,
verify that the zero and span gas checks indicate that the analyzer is in calibration
(see Subsection 2.2.5.4). Finally, verify that the concentrations of the candidate and
reference standards fall within the well-characterized region of the analyzer's
calibration curve (see Subsection 2.2.2).
3. Measure and adjust the flow rates of the three gas mixtures (i.e., reference standard,
candidate standard, and zero gas) to approximately the same value that will provide
enough flow for the analyzer and sufficient excess to assure that no ambient air will
be drawn into the vent line.
4. In succession, measure the zero gas, the reference standard, and the candidate
standard(s) using the analyzer. Use valves V1 and V2 to select each of the three gas
mixtures for measurement. For each measurement, allow ample time for the analyzer
to achieve a stable reading. If the reading for each measurement is not stable, the
precision of the measurements will decline and the candidate standard may not be
certifiable under this protocol (as is discussed in step 10). Record the analyzer
response for each measurement, using the same response units (e.g., volts, millivolts,
percent of scale, etc.) as was used for the multipoint calibration. At this point do not
convert these data into concentration values using the calibration equation. Do not
perform any necessary mathematical transformation of these data. These steps will
be done later. Do not make any zero control, span control, or other adjustments to
the analyzer during these measurements. Record the analyzer responses in the
laboratory's records.
The analyst may assay multiple candidate standards during the same assay session.
For example, a single set of measurements may involve a zero gas, a reference
standard and three candidate standards. Criteria that apply to the assay of one
candidate standard apply to the assay of multiple candidate standards. The analyst
should be aware that the effect of any short-term calibration drift will be greater when
multiple candidate standards are assayed. This greater effect is due to the longer
period of time between reference standard measurements. Unacceptable standard
errors of the mean concentration for the candidate standards (see step 10 below) may
occur as a result of the longer assay session.
5. Conduct at least two additional sets of measurements, as described in step 4 above.
However, for these subsequent sets of measurements, change the order of the three
measurements (e.g., measure reference standard, zero gas, and candidate standard
for the second set and measure zero gas, candidate standard, and reference
standard for the third set). Changing the order that the gas mixtures are measured
helps the analyst to discover any effect of that one measurement has on subsequent
measurements. The number of sets of measurements will be the same as the
number of measurements that were made during the zero and span gas checks (see
Subsection 2.2.5.4).
2-26
-------
If any one or more of the measurements of a set of measurements is invalid or
abnormal for any reason, discard all three measurements and repeat the
measurements. Such measurements may be discarded if the analyst can
demonstrate that the experimental conditions were inappropriate during these
measurements. Data cannot be discarded just because they appear to be outliers
but may be discarded if they satisfy statistical criteria for testing outliers.15 The
analyst must record any discarded data and a brief explanation as to why these data
were discarded in the laboratory's records.
The analyzer responses must now be corrected for any minor calibration drift which
may have occurred in the analyzer since the multipoint calibration. For each set of
measurements, calculate a corrected analyzer response for the candidate standard
from its measured analyzer response as follows:
Corrected Response
RS,
-Zi
rs2
-Za
Measured Response - Z2
+ Z,
where
RS1 = the analyzer response for the reference standard during the multipoint
calibration;
Z1 = the analyzer response for the zero gas during the multipoint calibration;
RS2 = the analyzer response for the reference standard during this set of
measurements; and
Zj = the analyzer response for the zero gas during this set of measurements.
Record the corrected response in the laboratory's records.
If the reference standard was not measured during the multipoint calibration, use a
predicted response for the reference standard in place of RSr This response is
predicted from the multipoint calibration's regression equation for a concentration
equal to that of the reference standard.
Note that this step is not necessary if the assay of the candidate standard occurs at
the same time as the multipoint calibration.
If the multipoint calibration data underwent any mathematical transformation before
their statistical analysis, perform the same mathematical transformation on the
corrected analyzer responses for the candidate standard. These transformed
responses should then be used in step 9.
Note that this step is not necessary if the multipoint calibration data were not
transformed before their statistical analysis.
The multipoint calibration's regression equation should be used to convert the
corrected analyzer responses for the candidate standard into the corresponding
concentrations for the candidate standard. Record these concentrations in the
laboratory's records.
2-27
-------
10. Calculate the mean, standard deviation, and standard error for the three or more
candidate standard concentrations that were obtained in step 9. Record these
calculations in the laboratory's records. The candidate standard cannot be certified
unless the standard error of the mean is less than or equal to 1.0 percent of the mean
concentration. That is, _
where
s = the standard deviation for the candidate standard concentrations;
_n = the number of sets of measurements of the candidate standard; and
Ccs = the mean concentration of the candidate standard.
Note that the value of the standard error of the mean can be made smaller by
increasing the number of sets of measurements.
If the standard error of the mean is greater than 1.0 percent of the mean
concentration, the analyst must make additional measurements of the candidate
standard, reference standard, and zero gas as described in step 4 above. These
additional measurements are used to calculate additional concentrations, which are
then pooled with the previously determined concentrations to obtain a new value for
the standard error of the mean. When an acceptable value is obtained, record it, the
number of measurements, the mean and the standard deviation in the laboratory's
records. The mean concentration will be the certified concentration for the candidate
standard. If an acceptable value is not obtained, the candidate standard cannot be
certified under this protocol.
The analyst should investigate any of the measurements that appear to be outliers.
Such data may be discarded if the analyst can demonstrate that the experimental
conditions were inappropriate during these measurements. Data cannot be discarded
just because they appear to be outliers but may be discarded if they satisfy statistical
criteria for testing outliers. The analyst must record any discarded data and a brief
summary of the investigation in the laboratory's records.
2.2.8 Stability Test for Newly Prepared Candidate Standards
Newly prepared candidate standards that contain reactive gas mixtures must be assayed
on at least two dates that are separated by at least 7 days (see Subsections 2.1.6.1 and 2.1.6.2).
The mean of the two assayed concentrations should be reported as the certified concentration
if the standard is found to be stable.
2.2.9 Certification Documentation
See Subsections 2.1.4 and 2.1.5.
2.2.10 Recertlflcation Requirements
See Subsections 2.1.6.3 and 2.1.6.4.
2-28
-------
2.3
PROCEDURE G2: ASSAY AND CERTIFICATION OF A COMPRESSED GAS
CALIBRATION STANDARD USING DILUTION
2.3.1 Applicability
This procedure may be used to assay the concentration of a diluted candidate
compressed gas calibration standard, based on the concentration of a diluted compressed gas
reference standard of the same gas mixture. This procedure allows a specialty gas producer, a
standard liser, or other analytical laboratory to certify that the assayed concentration for the
candidate standard is traceable to the reference standard. The procedure employs a
low-concentration-range (i.e., ambient air quality level) pollutant gas analyzer to compare
quantitatively diluted gas samples of both the candidate and reference standards. Dilution of the
candidate and reference standards with zero gas allows greater flexibility in the range of
concentrations of both the candidate and reference standards that can be assayed. Additionally,
dilution allows the use of a low-concentration-range analyzer, which is more likely to have an
inherently linear response than a high-concentration-range analyzer. However, the dilution
technique introduces additional error into the assay which would not be present if the standards
were assayed without dilution. This additional error is measured by an accuracy check of the
assay apparatus which is performed as part of the multipoint calibration.
This procedure may be used for the assay of multiple candidate standards at the same
time. Criteria that apply to the assay of one candidate standard apply to the assay of multiple
candidate standards.
2.3.2 Limitations
1. The concentration of the diluted candidate standard may be greater than or lesser
than the concentration of the diluted reference standard. However, both
concentrations must lie within the well-characterized region of the analyzer's
multipoint calibration (see Subsection 2.1.7.2). Additionally, the 95-percent
uncertainty for the regression-predicted concentration of the diluted candidate
standard must be less than or equal to 1 percent of the concentration of the diluted
reference standard. This uncertainty is obtained from the statistical analysis of the
multipoint calibration data. This criterion means that the uncertainty associated with
the multipoint calibration determines the concentration range over which a diluted
candidate standard may be assayed.
2. An accurate system for flow measurement and gas dilution is required.
3. A source of clean, dry, zero gas is required.
4. The balance gas in both the candidate and reference standards must be identical,
unless either a high dilution flow rate ratio (i.e., at least 50 parts zero gas to 1 part
standard) is used for the assay or it has been demonstrated that the analyzer is
insensitive to differences in the balance gas.
2.3.3 Assay Apparatus
The components of the assay apparatus can be assembled in several different
configurations without diminishing performance. Two possible designs of the assay apparatus
are illustrated in Figures 2-2 and 2-3. The former figure shows a configuration in which discrete
components (i.e., three-way valves, gas flow controllers, and a mixing chamber) are used to dilute
the reference and candidate standards. The latter figure shows a configuration in which a
2-29
-------
Gas Flow
to Vent
Gas Flow
to Vent
Flow
Measurement
Port
Pressure
Regulator
Three-way
Valve (V1)
Gas Flow
Controller (C1)
Q Three-way
Valve (V2)
Pressure
Regulator
Mixing
Chamber
Pressure
Regulator
Gas Flow
Controller (C2)
Three-way
Valve (V4)
Rotameter
Gas Flow to
Analyzer
Three-way
Valve (V3)
Candidate
Standard
Reference
Standard
Flow
Measurement
Port
Zero
Gas
Figure 2-2. One possible design of the apparatus using flow controllers for assay of
compressed gas calibration standards with dilution (Procedure G2).
-------
A Gas Flow A
Pressure
Regulator
M
i
00
Three-way
Valve (V1)
Three-way
Valve (V2)
Pressure
Regulator
Di ution
Pressure
Regulator
System
Rotameter
Candidate
Standard
Gas Flow to
Analyzer
Reference
Standard
Zero
Gas
Figure 2-3. One possible design of the apparatus using a gas dilution system for assay
of compressed gas calibration standards with dilution (Procedure G2).
-------
commercially available gas dilution system is used to dilute the standards. Both designs share
the important characteristic that the candidate standard is diluted by the same components as the
reference standard is.
In Figure 2-2, either zero gas or a diluted standard can be routed to the analyzer by
rotation of three three-way values (i.e., V1, V2, and V3). One gas flow controller (i.e., C1)
regulates the flow rates of the reference and candidate standards. These flow rates can be
measured by a single flowmeter connected to an outlet port on valve V2 or by a flowmeter built
into C1. Another gas flow controller (i.e., C2) regulates the flow rate of the zero gas. This flow
rate can be measured by a flowmeter connected to an outlet port on valve V3 or by a flowmeter
built into C2. The gas flow controllers may be needle valves, capillary tubes, thermal mass flow
controllers, or other suitable devices (see Subsection 2.3.7). If different flow rates are used for
the reference and candidate standards during the assay (see Subsection 2.3.6), separate gas flow
controllers may be used for the two standards. However, the same flowmeter must be used to
measure the two flow rates to minimize error in the measurement (see Subsection 2.3.7). Flow
rates should be controlled and measured with a relative uncertainty of 1 percent or less. For
large dilutions of the standards, the reference and candidate standard flow rates may be quite
small. Therefore, the internal volume of the tubing and components should be kept small to
minimize the flushing time when valve V1 is rotated.
The mixing chamber combines the two gas streams and should be designed to produce
turbulence to ensure thorough mixing of the gas streams. The diluted gas mixtures are routed
to the analyzer through a union tee tube fitting, which vents excess gas flow. Normally, the
excess gas is vented to the atmosphere without any obstructions in the tubing and the gas
entering the analyzer is at near-atmospheric pressure. However, the excess gas can be routed
through an uncaiibrated rotameter by rotation of a three-way valve (i.e., V4). The rotameter is
used to demonstrate that the total gas flow rate exceeds the sample flow rate of the analyzer and
that no room air is being drawn in through the vent line.
The apparatus in Figure 2-2 may be modified in several ways that will not diminish its
performance. For example, the three-way valves could be replaced by solenoid valves.
Alternatively, valve V1 could be replaced by a single length of tubing that is connected manually
to the two standards' pressure regulators in succession (see also Subsection 2.1.3).
In Figure 2-3, the reference and candidate standards are diluted with a gas dilution
system. This gas dilution system may use capillary tubes, positive-displacement pumps, thermal
mass flow controllers, or other suitable devices to dilute the standards. If a gas dilution system
is used, it must have a specified accuracy of not greater than 1.0 percent of the undiluted
reference standard concentration.
The analyst must check the accuracy of the gas dilution system during the multipoint
calibration (see Subsections 2.1.7.2 and 2.3.5.1).
2.3.4 Pollutant Gas Analyzer
The pollutant gas analyzer must have a well-characterized calibration curve aid must
have a range that is capable of measuring the diluted concentration of both the candidate and
the reference standards (see Subsection 2.1.7.1). It must have good resolution, good precision,
a stable response, and low output signal noise. In addition, the analyzer must have good
specificity for the pollutant of interest so that it has no detectable response to any contaminant
that may be contained in either the candidate or reference standards. A suitable analyzer with
acceptable performance specifications may be selected from the list of EPA-designated reference
and equivalent method analyzers.15 If the candidate and reference standards contain dissimilar
2-32
-------
balance gases (e.g., air versus nitrogen or different proportions of oxygen in the balance air),
either a high dilution flow-rate ratio (i.e., at least 50 parts zero gas to 1 part standard) should be
used or it must have been demonstrated that the analyzer's response is not sensitive to
differences in the balance gas composition. This demonstration may be accomplished by
showing that no difference exists in an analyzer's response when measuring a compressed gas
calibration standard that has been diluted with identical flow rates of different balance gases.
The analyzer should be connected to a high-precision strip chart recorder, or other data
acquisition system to facilitate graphical observation and documentation of the analyzer response
during the assay. Additionally, a digital panel meter with four-digit resolution, a digital voltmeter,
data logger, or other data acquisition system must be used to obtain numerical values of the
analyzer's response. More precise values will be obtained if these instruments have some dsa
averaging capability.
If the analyzer has not been in continuous operation, turn it on and allow it to stabilize for
at least 12 hours before beginning any measurements.
2.3.5. Analyzer Calibration
2.3.5.1 Multipoint Calibration-
See Subsections 2.1.7.2 and 2.1.7.4. Following completion of the multipoint calibration,
the accuracy of the assay apparatus must be checked to verify that the error associated with the
dilution is not excessive. This accuracy check involves the measurement of an undiluted or
diluted check standard. The check standard must be traceable to an NIST SRM, a CRM, or an
NTRM. It must have a certified concentration that is different from that of the reference standard
used in the multipoint calibration. Information concerning this standard (e.g., cylinder identification
number, certified concentration) must be recorded in the laboratory's records.
If an undiluted check standard is used, its concentration must fall in the well-characterized
region of the calibration curve. If a diluted check standard is used, the diluted concentration must
fall in the well-characterized region.
Make three or more discrete measurements of the undiluted or diluted check standard.
"Discrete" means that the analyst must change the gas mixture being sampled by the analyzer
between measurements. For example, the analyst might alternate between measurements of the
check standard and the zero gas. Record these measurements in the laboratory's records.
Next the analyst must verify that the dilution error is not excessive. For the check
standard measurements, calculate the relative difference (in percent) between the mean analyzer
response and the corresponding response that is predicted from the multipoint calibration
regression equation and the undiluted or diluted check standard concentration. That is,
If the relative difference is greater than 1.0 percent, the dilution error is considered to be
excessive. The analyst must investigate why the relative difference is excessive. The problem
may be due to errors in the reference standard and check standard concentrations, errors in
assay apparatus or to some other source. Assays may not be conducted until the relative
difference for a subsequent accuracy check is less than or equal to 1.0 percent.
Relative Difference = 100
Mean Analyzer Response - Predicted Response
Predicted Response
2-33
-------
2.3.5.2 Analyzer Range-
The range of the analyzer must include the concentrations of the zero gas and of the
diluted candidate and reference standards (see Subsection 2.3.6). The concentrations of the
diluted reference and candidate standards must fall within the well-characterized region of the
analyzer's calibration curve (i.e., the 95-percent uncertainty of the regression-predicted concen-
trations must be less than or equal to 1.0 percent of the largest concentration that was used in
the multipoint calibration). Because the selection of the dilution ratio or ratios to be used in the
assay provides great flexibility in the choice of concentrations to be measured by the analyzer,
the analyzer range should be selected based on optimum accuracy, stability, and linearity.
2.3.5.3 Llnearlty-
The data reduction technique used in this procedure requires that the analyzer have a
well-characterized, but not necessarily linear, calibration curve (see Subsection 2.1.7.5). Many
low-range analyzers of the type that may be used for this procedure have straight-line calibration
curves. If not, they usually have a predictable nonlinear calibration curve that can be described
by a polynomial equation or can be mathematically transformed to produce a straight-line
calibration curve suitable for use in this procedure. Any such polynomial equation or
mathematical transformation should be verified during the multipoint calibration. Caution should
be exercised in using a transformed calibration curve because zero or span control adjustments
to the analyzer may produce unexpected effects in the transformed calibration curve.
2.3.5.4 Zero and Span Gas Checks-
See Subsections 2.1.7.3 and 2.1.7.4. Prior to carrying out the assay of the candidate
standard, use zero and span gases to check for calibration drift in the analyzer since the
multipoint calibration. Zero gas and span gas checks must be performed on each day that
candidate standards are assayed. If multiple assays are being performed on the same analyzer
range, the analyst needs to perform only a single set of zero gas and span gas checks.
However, another set must be performed if the range is changed.
The gas mixtures to be used during the zero and span gas checks need not be the same
as any of the reference standards used for the assay of the diluted candidate standard or for the
multipoint calibration. The reference standard for the span gas check must be traceable to an
NIST SRM, a CRM or an NTRM. Information concerning this standard (e.g., cylinder identification
number, certified concentration) must be recorded in the laboratory's records.
Make three or more discrete measurements of the zero gas and three or more
independent measurements of the diluted reference standard. Record these measurements in
the laboratory's records.
Next, the analyst must verify that the analyzer's precision is acceptable. Calculate the
mean and standard deviation of the analyzer's response to the zero gas. Repeat these
calculations for the diluted reference standard measurements. Record these calculations in the
laboratory's records. The standard error of the mean for each set of measurements must be less
than or equal to 1.0 percent of the mean response to the diluted reference standard. That is,
s < Rdrs
where
s = standard deviation of the analyzer's response;
n = the number of independent measurements of the gas mixture; and
2-34
-------
rdrs = the mean analyzer response to the diluted reference standard.
The value of the standard error of the mean can be made smaller by increasing the
number of measurements. This calculation will enable the analyst to determine how many
replicate measurements are needed during the assay of the diluted candidate standard to obtain
acceptable precision. The analyst may wish to use a data logger or data acquisition system with
data averaging capability to obtain more precise measurements. If the value of the standard error
of the mean is not acceptable, then the analyzer must be repaired or another analyzer must be
used for the assay.
Next the analyst must verify that excessive calibration drift has not occurred since the
multipoint calibration. For the zero gas measurements, calculate the relative difference (in
percent) between the mean analyzer response during the zero gas check and the corresponding
response that is predicted from the multipoint calibration regression equation. That is,
Relative Difference = 100
Current Response - Calibration Response
Calibration Response for Diluted Reference Standard
Note that the relative difference is always calculated relative to the calibration response
for the diluted reference standard, rather than for the zero gas. Repeat this calculation for the
diluted reference standard measurements. Record these calculations in the laboratory's records.
Then, if the relative differences for the zero and span checks are less than or equal to 5.0
percent, the analyzer is considered to be in calibration. The zero and span controls need not be
adjusted and the assay may be conducted. The data reduction technique used in this procedure
does not require the absolute accuracy of the analyzer's calibration. Some minor calibration drift
is acceptable because the drift will be corrected for during the reduction of the assay data.
However, if the relative difference for either the zero or the span gas checks is greater
than 5.0 percent, then the analyzer is considered to be out of calibration. A new multipoint
calibration must be conducted before the candidate standard can be assayed. The zero and span
controls may be adjusted prior to the multipoint calibration to return the analyzer's response to
desired levels. However, any adjustments that are made do not remove the requirement for a
new multipoint calibration.
Between the time of the multipoint calibration and the time of the zero and span gas
checks, the analyst may adjust the analyzer's zero and span controls for assays that will not be
certified according to this protocol. However, these controls must be returned to their calibration
settings before the zero and span gas checks or assays under this protocol. This protocol does
not allow the analyst to conduct zero and span gas checks and then adjust the controls to
reproduce the analyzer's readings that were obtained during the multipoint calibration.
The zero gas and diluted reference standard measurements that are performed for the
assay of the diluted candidate standard may also be used for the zero gas and span gas checks.
However, the assay will not be valid and the analyzer will be considered out of calibration, if the
relative difference for either the zero gas measurements or the diluted reference standard
measurements is greater than 5.0 percent.
2.3.6 Selection of Gas Dilution Flow Rates or Gas Concentration Settings
The flow rates or settings used for the zero gas, reference standard, and candidate
standard should be selected carefully to provide diluted concentrations for both the candidate and
2-35
-------
reference standards that fall in the well-characterized region of the analyzer's calibration curve.
The diluted concentration of the candidate standard may be greater than or lesser than the diluted
concentration of the reference standard. Any assay error due to the dilution process will be
reduced if the same dilution flow-rate ratio or concentration setting can be used for both the
candidate and reference standards. Select the diluted concentrations of the reference and
candidate standards, and select flow rates or concentration settings that will produce the highest
analyzer responses within the well-characterized region of the analyzer's calibration curve.
If the same dilution flow-rate ratio or concentration setting cannot be used for both the
candidate and reference standards, select different ratios or settings for the candidate and
reference standards to produce concentrations that are approximately equal and that fall in the
well-characterized region of the analyzer's calibration curve. Select flow rates or settings such
that only one of the apparatus controls must be adjusted when switching from the reference
standard to the candidate standard, or vice versa. Where a choice of analyzer ranges is
available, higher dilution ratios or lower concentration settings will reduce the consumption of the
standards.
2.3.7 Flowmeter Type and Flowmeter Calibration
Figure 2-2 shows flow measurement ports on valves V2 and V3. In this configuration, a
single flowmeter can be used to measure both the standard flow rate and the zero gas flow rate.
Such an approach would reduce measurement errors arising from differences in the calibration
of multiple flowmeters. Alternatively, the flow rates can be measured at the outlet of the dilution
apparatus, with the rotameter vent temporarily plugged. In either case, a volumetric flowmeter
such as a wet test meter, a thermal mass flowmeter, or a soap bubble flowmeter can be used.
Each flow rate must be measured separately while the other flow rates are set to zero. Care
must be exercised to ensure that each measured flow rate remains constant when combined with
the other flow rate(s) and between the time of measurement and the time of the assay.
Additionally, care must be taken to ensure that the flowmeter does not cause any back pressure
in the gas stream and any resulting change in the flow rate through the flow controller.
If in-line flowmeters are mounted directly downstream of the flow controllers, they may not
operate at atmospheric pressure because of back pressure from downstream components. Also,
this back pressure may vary as a function of the total flow rate. Thus, the flowmeters must
compensate for the variable in-line pressure. Thermal mass flowmeters do not need to be
corrected for pressure effects. Measurements from pressure-sensitive flowmeters such as
rotameters or from volumetric flowmeters such as wet test meters must be carefully corrected for
the actual gas pressure during the flow measurement. A nonventing backpressure regulator may
be inserted downstream of the flowmeter so that the flowmeter operates at a fixed pressure
regardless of the total flow rate. An in-line flowmeter must not contaminate or react with the gas
mixture passing through it.
The flowmeters used should be stable, repeatable, and linear and have good resolution.
If possible, select flow rates or a flowmeter range such that the flow rates to be measured fall in
the upper half of the flowmeter's range. The flowmeters should be carefully calibrated at several
flow rates to prove linearity. The calibration should be accurate to plus or minus 1 percent and
must be referenced to an accurate flow rate or volumetric standard traceable to an NIST primary
standard. Flowmeter calibrations should be checked and recertified periodically, as determined
by stability information such as a chronological control chart of calibration data.
All volumetric flow-rate measurements must be corrected or referenced to the same
temperature and pressure conditions, such as EPA-standard conditions (i.e., 760 millimeters of
mercury (mm Hg), 25 °C) or the ambient temperature and pressure conditions prevailing in the
2-36
-------
laboratory during the assay. Measurements using wet test meters and soap bubble flowmeters
also must be corrected for the saturation of the gas stream with water vapor in the moist interiors
of these flowmeters. The equation to correct the flow rate for temperature, pressure, and humidity
effects is given below:
Volume
PM ~ pwv
V
Time
Ps
v y
l>j
where
PM = measured barometric pressure (mm Hg);
Pwv = partial pressure of water vapor (mm Hg);
Ps = standard pressure (mm Hg);
Ts = standard temperature (K = °C + 273.2); and
Tm = measured ambient temperature (K).
Measurement of reference and candidate standard flow rates with the sane flowmeter and
measurement of both dilution zero gas flow rates with the same flowmeter tend to reduce
measurement errors, associated with the use of multiple flowmeters. These errors are more
pronounced at higher dilution flow rate ratios. Note that the impact of any flow measurement
error is reduced if the same dilution ratio can be used for both the reference standard and
candidate standard measurements.
2.3.8 Assay Gases
2.3.8.1 Candidate Standard-
See Subsections 2.1.6, 2.3.2, and 2.3.6.
2.3.8.2 Reference Standard-
See Subsections 2.1.2, 2.1.6.4, 2.3.2, and 2.3.6.
2.3.8.3 Zero Gas-
See Subsection 2.1.8. Use the same zero gas for dilution of both candidate and reference
gases.
2.3.9 Assay Procedure
1. Verify that the assay apparatus is properly configured as shown in Figure 2-2 or
Figure 2-3 and as described in Subsection 2.3.3. Inspect the analyzer to verify that
it appears to be operating normally and that all controls are set to their expected
values. Record these control values in the laboratory's records.
2. Verify that the flowmeters, if used in the assay apparatus, are properly calibrated (see
Subsection 2.3.7).
3. Verify that a multipoint calibration of the analyzer has been performed within 1 month
prior to the assay date and that the dilution error is not excessive (see Subsections
2.1.7.2, 2.1.7.5, 2.3.4, and 2.3.5.1). Additionally, verify that the zero and span gas
checks indicate that the analyzer is in calibration (see Subsection 2.3.5.4). Finally,
verify that the concentrations of the diluted reference and candidate standards fall
2-37
-------
within the well-characterized region of the analyzer's calibration curve (see
Subsection 2.3.2),
Determine and establish the flow rates or concentration settings of the three gas
mixtures (i.e., reference standard, candidate standard, and zero gas) that will be used
for the assay (see Subsections 2.3.6, 2.3.7, and 2.3.5.2). Also check that the total
flow rate coming from the mixing chamber will provide enough flow for the analyzer
and sufficient excess to ensure that no ambient air will be drawn into the vent line.
Changes in the sample pressure may change the calibration curve. When using the
same flow rates for both candidate and reference standards, carefully set the delivery
pressures of the two standards' pressure regulators to the same value so that there
is no change in the flow rate when switching from one standard to the other.
Calculate the diluted reference standards' concentration using the following equation:
Diluted Standard Cone.
(Undiluted Standard Cone.) (Standard Flow Rate)
(Standard Flow Rate + Zero Gas Flow Rate)
Record the measured flow rates and the undiluted and diluted reference standard
concentrations in the laboratory's records.
In succession, measure the zero gas, the diluted reference standard and the diluted
candidate standard using the analyzer. For each measurement, adjust the flow rates,
if necessary, to those determined in step 4, are! allow ample time for the analyzer to
achieve a stable reading. If the reading for each measurement is not stable, the
precision of the measurements will decline and the candidate standard might not be
certifiable under this protocol (as discussed in step 11). Record the analyzer
response for each measurement, using the same response units (e.g., volts, millivolts,
percent of scale, etc.) as was used for the multipoint calibration. At this point, do not
convert the data into concentration values using the calibration equation. Do not
perform any mathematical transformations of the data. These steps will be done
later. Do not make any zero control, span control, or other adjustments to the
analyzer during this set of measurements. Record these analyzer responses in the
laboratory's records.
The analyst may.assay multiple candidate standards during the same assay session.
For example, a single set of measurements may involve a zero gas, a diluted
reference standard, and three diluted candidate standards. Criteria that apply to the
assay of one candidate standard apply to the assay of multiple candidate standards.
The analyst should be aware that the effect of any short-term calibration drift will be
greater when multiple candidate standards are assayed. This greater effect is due
to the longer period of time between reference standard measurements.
Unacceptable standard errors of the mean concentration for the diluted candidate
standards (see step 11 below) may occur as a result of the longer assay session.
Conduct at least two additional sets of measurements, as described in step 5 above.
However, for these subsequent sets of measurements, change the order of the three
measurements (e.g., measure the reference standard, zero gas, and candidate
standard for the second set and measure the zero gas, candidate standard, and
reference standard for the third set, etc.). Changing the order that the gas mixtures
are measured helps the analyst to discover any effect that one measurement has on
subsequent measurements. The number of sets of measurements will be the same
2-38
-------
as the number of measurements that were made during the zero and span gas
checks (see Subsection 2.3.5.4).
7. If any one or more of the measurements of a set of measurements is invalid or
abnormal for any reason, discard all three measurements and repeat the set of
measurements. Such measurements may be discarded if the analyst can
demonstrate that the experimental conditions were inappropriate during these
measurements. Data cannot be discarded just because they appear to be outliers,
but may be discarded if they satisfy statistical criteria for testing outliers.15 The
analyst must record the discarded data and a brief explanation as to why these data
were discarded in the laboratory's records.
8. The analyzer response must now be corrected for any minor calibration drift, which
may have occurred in the analyzer since the multipoint calibration. For each set of
measurements, calculate a corrected analyzer response for the diluted candidate
standard from its measured analyzer response as follows:
Corrected Response =
''DRS! -
Lv
DRS2 - Z2
'Measured Response - Z2A
+ Z,
where
DRSt = the analyzer response for the diluted reference standard during the
multipoint calibration;
Z, = the analyzer response for the zero gas during the multipoint calibration;
DRS2 = the analyzer response for the diluted reference standard during this set
of measurements; and
7-2 = the analyzer response for the zero gas during this set of measurements.
Record the corrected responses in the laboratory's records.
If the diluted reference standard was not measured during the multipoint calibration,
use a predicted response for the diluted reference standard in the place of DRSV
This response is predicted from the multipoint calibration's regression equation for a
concentration equal to that of the diluted reference standard.
Note that this step is not necessary if the assay of the diluted candidate standard
occurs at the same time as the multipoint calibration.
9. If the multipoint calibration data underwent any mathematical transformation before
their statistical analysis, perform the same mathematical transformation on the
corrected analyzer responses for the diluted candidate standard. These transformed
responses should then be used in step 10.
Note that this step is not necessary if the multipoint calibration data were not
transformed before their statistical analysis.
10. The multipoint calibration's regression equation should be used to convert the
corrected analyzer responses for the diluted candidate standard into the
2-39
-------
corresponding concentrations for the diluted candidate standard. Record these
concentrations in the laboratory's records.
11. Calculate the mean, standard deviation and standard error of the mean for the three
or more diluted candidate standard concentrations that were obtained in step 10.
Record these calculations in the laboratory's records. The candidate standard cannot
be certified unless the standard error of the mean is less than or equal to 1.0 percent
of the mean concentration. That is,
s < Cpcs
Vn 100
where
s = the standard deviation for the diluted candidate standard concentrations;
n = the number of sets of measurements of the diluted candidate standard; and
cdcs = the mean concentration of the diluted candidate standard.
Note that the value of the standard error of the mean can be made smaller by
increasing the number of sets of measurements of the three gas mixtures.
If the standard error of the mean is greater than 1.0 percent of the mean
concentration, the analyst must make additional measurements of the three gas
mixtures as described in step 5 above. These additional measurements are used to
calculate additional concentrations, which are then pooled with the previously
determined concentrations to obtain a new value for the standard error of the mean.
When an acceptable value is obtained, record it, the mean, the standard deviation
and the overall number of measurements in the laboratory's records, if an acceptable
value is not obtained, the candidate standard cannot be certified under this protocol.
The analyst should investigate any of the measurements that appear to be outliers.
Such data may be discarded if the analyst can demonstrate that the experimental
conditions were inappropriate during these measurements. Data cannot be discarded
just because they appear to be outliers but may be discarded if they satisfy statistical
criteria for testing outliers. The analyst must record any discarded data and a brief
summary of the investigation in the laboratory's records.
12. Finally, the certified undiluted concentration for the candidate standard can be
calculated from the mean concentration of the diluted candidate standard as follows:
Certified Undiluted Cone. - (Mean Diluted Cone.) (Total Gas Flow Rate)
(Standard Flow Rate)
where Total Gas Flow Rate = Standard Flow Rate + Zero Gas Flow Rate .
2.3.10 Stability Test for Newly Prepared Standards
Newly prepared candidate standards that contain reactive gas mixtures must be assayed
on at least two dates that are separated by at least 7 days. See Subsections 2.1.6.1 and 2.1.6.2.
2-40
-------
The mean of the two assayed concentrations should be reported as the certified concentration
if the standard is found to be stable.
2.3.11 Certification Documentation
See Subsections 2.1.4 and 2.1.5.
2.3.12 Recertlfication Requirements
See Subsections 2.1.6.3 and 2.1.6.4.
2-41
-------
SECTION 3
EPA TRACEABILITY PROTOCOL FOR ASSAY AND CERTIFICATION OF
PERMEATION DEVICE CALIBRATION STANDARDS
3.1 GENERAL INFORMATION
3.1.1 Purpose and Scope of the Protocol
This protocol describes three procedures for assaying the permeation rate of a permeation
device calibration standard and for certifying that the assayed permeation rate is traceable to
National Institute of Standards and Technology (NIST) reference standards. This protocol is
mandatory for certifying the permeation device calibration standards used for the pollutant
monitoring that is required by the regulations of the Code of Federal Regulations, Chapter 40,
Parts 50 and 583,4 for the calibration and audit of ambient air quality analyzers. This protocol
covers the assay and certification of sulfur dioxide (SOz) and nitrogen dioxide (N02) permeation
device calibration standards. This protocol may be used by permeation device producers,
standard users, or other analytical laboratories. The assay procedure may involve the
comparison of these standards to permeation device reference standards (i.e., Procedure P1),
to compressed gas reference standards (i.e., Procedure P2), or to mass reference standards (i.e.,
Procedure P3).
3.1.2 Reference Standards
The permeation device reference standards that may be used under this protocol are
NIST Standard Reference Material (SRM) numbers 1625,1626, 1627, or 1629a. These SRMs
(listed in Table 3.1) are permeation tubes containing S02 or N02. In the future, NIST may
develop additional SRMs, which may be used as reference standards under this protocol.
The compressed gas reference standards that may be used under this protocol are NIST
SRMs, certified reference materials (CRMs), NIST traceable reference materials (NTRMs), or gas
manufacturer's intermediate standards (GMISs). These standards are described in Section 2.1.2
of this report.
The mass reference standards that may be used under this protocol must be traceable
to NIST mass standards.17"19 Additionally, they must have an uncertainty of no more than 0.025
percent of their value. Examples of standards that meet these specifications are NIST classes
S and S-1 and American National Standards Institute/American Society for Testing and Materials
(ANSl/ASTM) classes 1 and 3. The mass reference standards must be recalibrated on a regular
basis (e.g., yearly) at an NIST-accredited State weights and measures laboratory or at a
calibration laboratory that is accredited by the National Voluntary Laboratory Accreditation
Program (NVLAP), which is administered by NIST.9'10 The recalibration frequency is to be
determined from records of previous recalibrations of these standards.
3-1
-------
TABLE 3-1. NIST SRM PERMEATION DEVICE REFERENCE STANDARDS
Nominal concentration (in
Device
Nominal
pmol/mol) at various dilution
NIST
Permeation
length
permeation rate
gas flow rates (L/min)
SRM no.
device type
(cm)
at 30 °C (pg/min)
1
5
10
1625
Sulfur dioxide
10
3.7
1.4
0.28
0.14
1626
Sulfur dioxide
5
2.1
0.8
0.16
0.08
1627
Sulfur dioxide
2
0.8
0.3
0.06
0.03
1629a
Nitrogen dioxide
1
2.1
0.8
0.16
0.08
3-2
-------
The temperature reference standards that may be used under this protocol must be liquid-
in-glass thermometers having scale tolerances and uncertainties that conform to NIST Special
Publication 250-23.20 They must have an uncertainty of no more than 0.05 °C. The
thermometers must have serial numbers etched or permanently marked into the glass and a
manufacturer's calibration certificate of traceability to NIST standards. They must be recalibrated
periodically (e.g., yearly) according to NIST guidelines21 by the user, an NIST-accredited State
weights and measures laboratory, or an NVLAP-accredited calibration laboratory.
3.1.3 Selecting a Procedure
Procedures P1 and P2 are applicable to the assay and certification of candidate
permeation device calibration standards using an ambient air quality analyzer. Procedure P1
provides for the assay referenced to a permeation device reference standard. Procedure P2
provides for the assay referenced to a compressed gas reference standard.
Procedure P3 is applicable to the assay and certification of candidate standards using an
analytical balance. This procedure provides for the assay referenced to a mass reference
standard.
3.1.4 Using the Protocol
The assay/certification protocol described here is designed to minimize both systematic
and random errors in the assay process. Therefore, the protocol should be carried out exactly
as it is described. The assay procedures in this protocol include one possible design for the
assay apparatus. The analyst is not required to use this design and may use alternative
components and configurations that produce equivalent-quality measurements. Inert materials
{e.g., Teflon®, stainless steel, or glass) and clean, noncontaminating components should be used
in those portions of the apparatus that are in contact with the gas mixtures being assayed.
3.1.5 Certification Documentation
Each certified permeation device calibration standard must be documented in a written
certification report and this report must contain at least the following information:
1. Permeation device identification number;
2. The contents of the permeation device;
3. Certified permeation rate (in nanograms (ng) per minute);
4. The certification temperature (in degrees Celsius to the nearest 0.1°);
5. The dilution gas (i.e., air or nitrogen) used during the assay (for procedures P1
and P2);
6. Date of the assay/certification;
7. Identification of the reference standards used in the assay: NIST SRM number, NIST
sample number, and certified concentration or permeation rate for an SRM; cylinder
3-3
-------
identification number and certified concentration for a CRM, an NTRM or GMIS;
manufacturer, model number and serial number for a mass or temperature reference
standard. The certification documentation must identify the type of reference
standard used in the assay;
8. Statement that the assay/certification was performed according to this protocol and
that lists the assay procedure (e.g., Procedure P1) used;
9. The analytical method that was used in the assay;
10. Identification of the laboratory where the standard was assayed and certified;
11. Chronological record of all certifications for the standard by the certifying laboratory;
12. A statement that the standard will retain its certification only as long as 5 percent of
the original liquid weight or a visible amount of liquid remains in it;
13. The environmental exposure conditions (e.g., temperature and moisture) that will
invalidate the certification; and
14. A statement of the overall analytical uncertainty estimate associated with the assay
of the candidate standard. The estimate must include the uncertainty associated with
the assay of a GMIS, if one is used as the reference standard for the assay of the
candidate standard. The uncertainty of a mass reference standard, an SRM, a CRM,
or an NTRM is not to be included in this estimate.
This certification documentation must be given to the purchaser of the standard. The permeation
device producer must maintain laboratory records and certification documentation for 3 years after
the standard's certification date. A permeation device producer or other vendor may redocument
an assayed and certified standard that it has purchased from another permeation device producer
and that it wishes to sell to a third party. However, the new certification documentation must
clearly list the permeation device producer or other laboratory where the standard was assayed.
3.1.6 Certification Label
The permeation device calibration standard must be labeled with its identification number.
3.1.7 Assay/Certification of Candidate Permeation Device Calibration Standards
3.1.7.1 Permeation Device Design-
Permeation devices are designed and constructed in various ways, but all devices consist
of a sealed chamber containing liquified gas and a permeable area through which the gas is
allowed to permeate. The permeated gas is swept and diluted with a measured volumetric flow
rate of dry air or nitrogen to create a quantitative concentration of the pollutant gas.
3.1.7.2 Precautions for Use and Storage of Permeation Devlces-
The permeation rate of all permeation devices is critically dependent on temperature; a
permeation device is useful as a concentration standard only when its temperature is precisely
controlled and accurately measured, and an accurately metered dilution gas flow rate is provided.
3-4
-------
The inaccuracy of gaseous pollutant concentration standards that are produced by
permeation devices may increase due to physical or chemical sorption of the permeated gas in
the permeation system. This sorption will have a larger effect on the inaccuracy as the
concentration decreases. Inert materials (e.g., Teflon®, stainless steel, or glass) and clean,
noncontaminating components should be used in those portions of the permeation system that
are in contact with the permeated gas.
The reproducibility of the certified permeation rate of a permeation device may be
adversely affected by exposure of the device to temperatures greater than the specified operating
or storage temperature range for the device or by exposure to excessive moisture. N02
permeation devices must be stored under dry conditions and preferably at a temperature between
20 and 35 °C, or as otherwise recommended by the manufacturer. S02 permeation devices may
be refrigerated for storage.
It appears that there is a limited temperature range at which N02 permeation devices can
be used as standards. This temperature range is conservatively given as 20 to 35 °C.9 Low or
high temperature storage of NOz permeation devices is not recommended.
Candidate standards being certified under Procedure P3 must be stored under constant
temperature conditions between assays. A storage container for this application is described in
the procedure.
When stored at a temperature other than the assay temperature, some permeation
devices require an equilibration period at the assay temperature to reach thermal equilibrium and
a stable, accurate permeation rate. When transferred from a different storage temperature, thin-
walled permeation devices should be maintained at the assay temperature with a fixed dilution
flow rate for at least 48 hours before use or before certification. Temperature changes of >10 °C
may require equilibration periods of up to 15 days for N02 permeation devices to attain a stable
permeation rate.22,23 Upon return to the original temperature, some devices may not return to
the same permeation rate as before the temperature change. Other types of permeation devices
may require longer equilibration periods. Observe any manufacturer's recommendations for
equilibration and use.
3.1.7.3 Equilibration of Newly Prepared Permeation Devices-
A newly prepared permeation device must be equilibrated for at least 48 hours at the
assay temperature before being assayed for the first time. The equilibration period may be 100
hours or longer for some permeation devices.22 This period will vary as a function of tie
permeating compound, the material and the thickness of the permeating surface and the
temperature.
3.1.7.4 Certification Conditions for Permeation Device Calibration Standards—
A standard will retain its certification only as long as 5 percent of the original liquid weight
or a visible amount of liquid remans in it. A standard loses its certification if it is exposed for
prolonged periods of time to excessive moisture or to temperatures greater than 15 °C above its
certification temperature. A standard that loses its certification must be reassayed before it can
be certified for further use.
3-5
-------
3.1.8 Technical Variances
Permeation device producers, standard users, and other analytical laboratories may
petition the U.S. EPA for technical variances to the assay procedures in this protocol. A technical
variance allows the use of a specific alternative assay procedure for candidate standards, which
can be certified under this protocol. The petitioner must send a written request with a detailed
description of the alternative assay procedure and supporting analytical data to the Director of the
Quality Assurance and Technical Support Division, U.S. EPA, Mailcode MD-78A, Research
Triangle Park, NC 27711. The supporting analytical data must demonstrate the equivalence of
the alternative assay procedure with the procedures given in this protocol. Technical variances
may also be given for alternative temperature ranges of certifying or storing permeation devices
provided that supporting analytical data are provided with the written request.
Permeation device producers, standard users, and other analytical laboratories may
petition the U.S. EPA to allow the assay and certification of permeation devices that contain
gases or liquified gases other than S02 and N02. The petitioner must send a written request with
a detailed description of the permeation device and supporting analytical data to the Director of
the Quality Assurance and Technical Support Division. The supporting analytical data must
demonstrate that the permeation rate for the proposed device can be accurately determined, that
only the specified compound is permeating, that the rate is stable over the lifetime of the device,
and that the rate is not changed by temperature and humidity effects.
3.2 PROCEDURE PI: ASSAY AND CERTIFICATION OF PERMEATION DEVICE
CALIBRATION STANDARDS REFERENCED TO A PERMEATION DEVICE
REFERENCE STANDARD
3.2.1 Applicability
This procedure may be used to assay the permeation rate of a candidate S02 or N02
permeation device calibration standard, based on the permeation rate of a permeation device
reference standard of the same pollutant compound, and to certify that the assayed permeation
rate is traceable to the reference standard. The procedure employs a low-concentration range
(i.e., ambient air quality level) pollutant gas analyzer to compare quantitatively diluted
concentrations from the two permeation devices for the assay of the candidate device. This
procedure may be used for the assay of multiple candidate standards during the same assay
session. Criteria that apply to the assay of one candidate standard apply to the assay of multiple
candidate standards. This procedure may be used by permeation device producers, standard
users, or other analytical laboratories.
3.2.2 Limitations
1. The permeation rate of the candidate standard may be greater than or lesser than the
permeation rate of the reference standard. However, the diluted concentrations from
both standards must lie within the well-characterized region of the analyzer's
multipoint calibration (see Subsection 2.1.7.2). Additionally, the 95-percent
uncertainty for the regression-predicted concentration of the diluted candidate
standard must be £1.0 percent of the concentration of the diluted reference standard.
This uncertainty is obtained from the statistical analysis of the multipoint calibration
data. This criterion means that the uncertainty associated with the multipoint
3-6
-------
calibration determines the concentration range over which a diluted candidate
standard may be assayed.
2. A quantitatively accurate flow measurement and dilution system is required.
3. A source of clean, dry zero gas is required.
4., This procedure is designed to assay the permeation rate of a candidate standard that
is mounted in a specially designed assay dilution system; the procedure does not
accommodate the certification of a candidate standard that is mounted in its own self-
contained dilution/flow measurement system.
3.2.3 Assay Apparatus
Figure 3-1 illustrates the components and configuration of one possible design for the
assay apparatus, including a common dilution system for both the reference and candidate
standards. The configuration is designed to allow convenient routing of zero gas and diluted
concentrations of the reference and candidate standards, in turn, to the analyzer for
measurement, as selected by valves V1, V2, and V3. Three gas flow controllers (i.e., C1, C2,
and C3) regulate the total dilution flow rates for both the reference and candidate permeation
devices and the purge gas flow rate. These gas flow controllers may be needle valves, capillary
tubes, thermal mass flow controllers, or other suitable devices. The flow rates must be controlled
to within 1.0 percent variation during the assay.
The total dilution flow rate is measured by a single, common flowmeter (i.e., M1). Valve
V1 directs a portion (usually 10 to 500 mL/min) to the total dilution flow through one or the other
of the two temperature-controlled permeation device chambers to sweep up the permeated
pollutant gas. This sweep flow rate is monitored by an auxiliary flowmeter for each permeation
device (i.e., M2 and M3). These auxiliary flowmeters need not be accurately calibrated, since
only the total dilution flow measured by flowmeter M1 is used in the dilution calculation. Gas flow
controllers C1 and C3 can be used to adjust and balance the flow rates of the two gas streams
sweeping through the permeation device chambers. The permeation device that is not being
analyzed receives a purge gas stream to avoid the buildup of high pollutant concentrations in the
chamber. This purge gas flow is vented through valve V2 and is not measured by flowmeter M1.
The assay apparatus illustrated in Figure 3-1 may be modified by the addition of multiple
candidate standard chambers. These chambers may be set to different temperatures.
If it is necessary to use different dilution flow rates for the candidate and reference
permeation devices (see Subsection 3.2.6), separate flow controllers for the two permeation
devices may be used for the two different flows. However, the same flowmeter should always
be used to measure these two flow rates to minimize systematic flow measurement errors.
The mixing chamber combines the gas streams and should be designed to provide
turbulence in the flow to ensure thorough mixing of the two gas streams. The diluted gas
mixtures are routed to the analyzer through a union tee tube fitting, which vents excess gas flow.
Normally, the excess gas is vented to the atmosphere without any obstructions in the tubing and
the gas entering the analyzer is at near-atmospheric pressure. However, the excess gas can be
routed through an uncalibrated rotameter by rotation of a three-way valve (i.e., V4). The
3-7
-------
A Excess Gas A
Flow to Vent
Rotameter |
Gas
Flowmeter
(M2)
Gas Flow
Controller
(C1>
3-way
Valve (V4)
m
Dilution Gas
Flowmeter
(M1)
3-way
Valve
-------
rotameter is used to demonstrate that the total gas flow rate exceeds the sample flow rate of the
analyzer and that no room air is being drawn in through the vent line (also see Subsection 3.1.4).
Check the apparatus carefully for leaks and correct all leaks before use.
The mean temperatures of the reference standard chamber and the candidate standard
chamber must be controlled to within 0.05 °C of the setpoint with a temperature stability of
±0.05 °C. These temperatures must be measured with an NIST-traceable thermometer having
a measurement uncertainty of ±0.05 °C or less.
3.2.4 Pollutant Gas Analyzer
See Subsection 2.3.4. The pollutant gas analyzer must have a well-characterized
calibration curve and a range capable of measuring the diluted concentrations of both the
candidate and the reference standards. It must have good resolution, good precision, a stable
response, and low output signal noise. In addition, the analyzer must have good specificity for
the pollutant of interest so that it has no detectable response to any contaminant that may be
contained in the standards. A suitable analyzer with acceptable performance specifications may
be selected from the list of EPA-designated reference and equivalent method analyzers.16
The analyzer should be connected to a high-precision strip-chart recorder or other data
acquisition system to facilitate graphical observation and documentation of the analyzer responses
obtained during the assay. Additionally, a digital panel meter with four-digit resolution, a digital
voltmeter, data logger, or other data acquisition system must be used to obtain numerical values
of the analyzer's response. More precise values will be obtained if these instruments have some
data averaging capability.
If the analyzer has not been in continuous operation, turn it on and allow it to stabilize for
at least 12 hours before beginning any measurements.
3.2.5 Analyzer Calibration
3.2.5.1 Multipoint Calibration-
See Subsections 2.1.7.2 and 2.1.7.4. Following completion of the multipoint calibration,
the accuracy of the assay apparatus must be checked to verify that the error associated with the
dilution is not excessive. This accuracy check involves the measurement of a diluted check
standard. This check standard must be traceable to an NIST SRM. It must have a certified
permeation rate that is different from that of the reference standard used during the multipoint
calibration. Information concerning this standard (e.g., permeation device identification nurriber,
certified permeation rate) must be recorded in the laboratory's records. The diluted concentration
of the check standard must fall in the well-characterized region of the calibration curve.
Make three or more discrete measurements of the diluted check standard. "Discrete"
means that the analyst must change the gas mixture being sampled by the analyzer between
measurements. For example, the analyst might alternate between measurements of the diluted
check standard and the zero gas. Record these measurements in the laboratory's records.
Next the analyst must verify that the dilution error is not excessive. For the diluted check
standard measurements, calculate the relative difference (in percent) between the mean analyzer
3-9
-------
response and the corresponding response that is predicted from the multipoint calibration
regression equation and the diluted check standard concentration. That is,
Relative Difference = 100
Mean Analyzer Response - Predicted Response
Predicted Response
If the relative difference is greater than 1.0 percent, the dilution error is excessive. The analyst
must investigate why the dilution error is excessive. The problem may be due to errors in the
reference standard and check standard permeation rates, errors in the assay apparatus or to
some other source. Assays may not be conducted until the relative difference for a subsequent
accuracy check is less than or equal to 1.0 percent.
3.2.5.2 Analyzer Range-
See Subsection 2.3.5.2.
3.2.5.3 Llnearlty-
See Subsection 2.3.5.3.
3.2.5.4 Zero and Span Gas Checks-
See Subsection 2.3.5.4.
3.2.6 Selection of Gas Dilution Flow Rates
The dilution flow rates used for the reference and candidate standards should be selected
carefully to provide diluted concentrations for both standards that fall in the well-characterized
region of the analyzer's calibration curve. Potential errors in the assayed permeation rate due
to dilution flow rate measurement error will be greatly reduced if the same dilution flow rates can
be used for both standards. If the same dilution flow rates cannot be used for both standards,
select different dilution flow rates for the candidate and reference devices to provide
approximately equal diluted concentrations that fall in the well-characterized region. Additionally,
the 95-percent uncertainty for the regression-predicted concentration of the diluted candidate
standard must be <1.0 percent of the concentration of the diluted reference standard.
3.2.7 Flowmeter Type and Flowmeter Calibration
Flowmeter M1, as shown in Figure 3-1, measures in-line flow rates and does not operate
at atmospheric pressure because of backpressure from downstream components. Also, this
backpressure is variable, depending on the total dilution flow rate. Thus, the type of flowmeter
used must compensate for the variable in-line pressure. Measurements from a pressure-sensitive
flowmeter such as a rotameter or a wet test meter must be carefully corrected for the actual in-
line pressure during the total dilution flow rate measurement. A nonventing backpressure
regulator may be inserted downstream of the flowmeter so that the flowmeter operates at a fixed
pressure regardless of the total flow rate.
Alternatively, the flow rates can be measured at the outlet of the dilution apparatus, with
the excess gas flow vent temporarily plugged. In this case, a volume-type meter such as a wet
test meter or a soap film flowmeter can be used, and flow measurements may be conveniently
referenced to atmospheric pressure. Each flow rate must be measured independently while the
3-10
-------
other flow rate is set to zero. Great care must then be exercised to ensure that each measured
flow rate remains constant between the time of measurement and the time of the assay.
The flowmeter used should be stable, repeatable, linear, and have good resolution. The
flowmeter must not contaminate or react with the gas mixture passing through it. If possible,
select flow rates or a flowmeter range such that the measured flow rates fall in the upper half of
the flowmeter's range. The flowmeter should be carefully calibrated at several flow rates to prove
linearity. The calibration should be accurate to ±1.0 percent, referenced to an accurate flow or
volume standard traceable to an NIST primary standard. The flowmeter calibration should be
checked and recertified periodically. The recertification frequency is to be determined from
stability information such as a chronological control chart of calibration data.
All volumetric flow rate measurements must be corrected or referenced to the same
temperature and pressure conditions, such as EPA standard conditions (25 °C and 760 mm Hg)
or the ambient temperature and pressure conditions prevailing in the laboratory during the assay.
Measurements using wet test meters and soap bubble flowmeters also must be corrected for the
saturation of gas stream with water vapor in the moist interiors of these flowmeters. The equation
to correct the flow rate for temperature, pressure, and humidity effects is given below:
Volume
Pm " Pwv
'V
Time
ps J
lT"J
where
PM = measured barometric pressure (mm Hg);
Pyyy = partial pressure of water vapor (mm Hg);
Ps = standard pressure (mm Hg);
Ts = standard temperature (K = °C + 273.2);
Tm = measured ambient temperature (K).
Measurement of both dilution flow rates with the same flowmeter tends to reduce
systematic flow measurement error. Note particularly that flow measurement error is greatly
reduced if the same dilution flow rates can be used for both the reference and candidate
standards.
3.2.8 Permeation Devices
3.2.8.1 Candidate Standard-
See Subsections 3.1.7 and 3.2.2. Follow the manufacturer's instructions for equilibration
and for use of the candidate standard and for selecting the temperature at which it is to be
assayed and certified. The candidate standard should be assayed at the same temperature at
which it will be subsequently used. The mean operating temperature of the candidate standard
chamber must be controlled to within 0.05 °C of the setpoint with a temperature stability of
±0.05 °C. This temperature must be measured with an NIST-traceable thermometer with a
measurement uncertainty of ±0.05 °C or less.
3-11
-------
3.2.8.2 Reference Standard-
See Subsections 3.1.2, 3.1.7.1, 3.1.7.2, and 3.2.2. Follow NIST's instructions for
equilibration and use of the SRM reference standard and for selecting an operating temperature
within its certified range. The mean operating temperature of the reference standard chamber
must be controlled to within 0.05 °C of the setpoint with a temperature stability of ±0.05 °C. This
temperature must be measured with an NIST-traceabie thermometer with a measurement
uncertainty of ±0.05 °C or less.
3.2.8.3 Zero Gas-
See Subsection 2.1.8. Use the same zero gas for dilution of both the candidate and
reference standards.
3.2.9 Assay Procedure
1. Verify that the assay apparatus is properly configured as shown in Figure 3-1 and
described in Subsection 3.2.3. Inspect the analyzer to verify that it appears to be
operating normally and that all controls are set to their expected values. Record
these control values in tie laboratory's records.
2. Determine and establish the operating temperatures for the reference and candidate
standards in their respective temperature-controlled chambers. Install the standards
and, with zero gas flowing over both standards, allow ample time for the standards
to equilibrate (see Subsection 3.1.7.2). Record the temperatures in the laboratory's
records.
3. Verify that flowmeter M1 is properly calibrated (Subsection 3.2.7).
4. Verify that a multipoint calibration of the analyzer has been performed within 1 month
prior to the assay date (see Subsections 2.1.7.2, 2.1.7.5, and 2.3.4). Additionally,
verify that the zero and span gas checks indicate that the analyzer is in calibration
(see Subsection 2.3.5.4).
5. Determine and establish the dilution flow rates and diluted concentrations for the
reference and candidate standards that wilt be used for the assay (see Subsections
3.2.6, 3.2.7, and 3.2.5.2). Use an estimated permeation rate for the candidate
standard in these calculations. Calculate the diluted standard concentrations (in ppm)
using the following equation:
Diluted Standard Cone.
10"
MV
MW
Permeation Rate
Dilution Flow Rate
where
MV = Molar volume of the dilution gas (liters/mole);
MW = Molecular weight of the dilution gas (grams/mole); permeation rate is given
in nanograms/minute; and dilution flow rate is given in liters/minute.
3-12
-------
Ensure that the diluted candidate and reference standard concentrations are within
the well-characterized region of the analyzer's calibration curve (see Subsection
2.3.2). Also check that both dilution flow rates will provide enough flow for the
analyzer, with sufficient excess to ensure that no ambient air will be drawn into the
vent line, and without increasing the pressure of the sample delivered to the analyzer.
If possible, use the same dilution flow rate for both standards. Also adjust the flow
rate of the portion of the dilution flow that passes over the standards using flow
. controllers C1 and C3. Record the measured flow rates in the laboratory's records.
6. In succession, measure the zero gas, the diluted reference standard and the diluted
candidate standard using the analyzer. Use valves V1, V2, and V3 to select each of
the three gas mixtures for measurement. For each measurement, adjust the flow
rates, if necessary, to those determined in step 5, and allow ample time for the
analyzer to achieve a stable reading. If the reading for each measurement is not
stable, the precision of the measurements will decline and the candidate standard
might not be certifiable under this protocol (as discussed in step 12). Record the
analyzer response for each measurement, using the same response units (e.g., volts,
millivolts, percent of scale, etc.) as was used for the multipoint calibration. At this
point, do not convert the data into concentration values using the calibration equation.
Do not perform any mathematical transformations of the data. These steps will be
done later. Do not make any zero control, span control, or other adjustments to the
analyzer during this set of measurements. Record these analyzer responses in the
laboratory's records.
The analyst may assay multiple candidate standards during the same assay session.
For example, a single set of measurements may involve a zero gas, a diluted
reference standard, and three diluted candidate standards. Criteria that apply to the
assay of one candidate standard apply to the assay of multiple candidate standards.
The analyst should be aware that the effect of any short-term calibration drift will be
greater when multiple candidate standards are assayed. This greater effect is due
to the longer period of time between reference standard measurements.
Unacceptable standard errors of the mean concentration for the diluted candidate
standards (see step 12 below) may occur as a result of the longer assay session.
7. Conduct at least two additional sets of measurements, as described in step 6 above.
However, for these subsequent sets of measurements, change the order of the three
measurements (e.g., measure the reference standard, zero gas, and candidate
standard for the second set and measure the zero gas, candidate standard, and
reference standard for the third set, etc.). Changing the order that the gas mixtures
are measured helps the analyst to discover any effect that one measurement has on
subsequent measurements. The number of sets of measurements will be the same
as the number of measurements that were made during the zero and span gas
checks (see Subsection 2.3.5.4).
8. If any one or more of the measurements of a set of measurements is invalid or
abnormal for any reason, discard all three measurements and repeat the set of
measurements. Such measurements may be discarded if the analyst can
demonstrate that the experimental conditions were inappropriate during these
measurements. Data cannot be discarded just because they appear to be outliers,
3-13
-------
but may be discarded if they satisfy statistical criteria for testing outliers.15 The
analyst must record any discarded data and a brief explanation about why the data
were discarded in the laboratory's records.
9. The analyzer response must now be corrected for any minor calibration drift, which
may have occurred in the analyzer since the multipoint calibration. For each set of
measurements, calculate a corrected analyzer response for the diluted candidate
. standard from its measured analyzer response as follows:
DRSt = the analyzer response for the diluted reference standard during the
multipoint calibration;
Z1 = the analyzer response for the zero gas during the multipoint calibration;
DRS2 = the analyzer response for the diluted reference standard during this set
of measurements; and
2L> = the analyzer response for the zero gas during this set of measurements.
Record the corrected responses in the laboratory's records.
If the diluted reference standard was not measured during the multipoint calibration,
use a predicted response for the diluted reference standard in the place of DRSr
This response is predicted from the multipoint calibration's regression equation for a
concentration equal to that of the diluted reference standard.
Note that this step is not necessary if the assay of the diluted candidate standard
occurs at the same time as the multipoint calibration.
10. If the multipoint calibration data underwent any mathematical transformation before
their statistical analysis, perform the same mathematical transformation on the
corrected analyzer responses for the diluted candidate standard. These transformed
responses should then be used in step 12.
Note that this step Is not necessary if the multipoint calibration data were not
transformed before their statistical analysis.
11. The multipoint calibration's regression equation should be used to convert the
corrected analyzer responses for the diluted candidate standard into the
corresponding concentrations for the diluted candidate standard. Record these
concentrations in the laboratory's records.
Corrected Response
DRS2 - Z2 ^
\ J
DRS, - [Measured Response - Z2
where
3-14
-------
12. Calculate the mean, standard deviation and standard error of the mean for the three
or more diluted candidate standard concentrations that were obtained in step 11.
Record these calculations in the laboratory's records. The candidate standard cannot
be certified unless the standard error of the mean is <1.0 percent of the mean
concentration. That is,
s < CDCS
100
where
s = the standard deviation for the diluted candidate standard concentrations;
n = the number of sets of measurements of the diluted candidate standard; and
gdcs = the mean concentration of the diluted candidate standard.
Note that the value of the standard error of the mean can be made smaller by
increasing the number of sets of measurements of the three gas mixtures.
If the standard error of the mean is >1.0 percent of the mean concentration, me
analyst must make additional measurements of the three gas mixtures as described
in step 6 above. These additional measurements are used to calculate additional
concentrations, which are then pooled with the previously determined concentrations
to obtain a new value for the standard error of the mean. When an acceptable value
is obtained, record it, the mean, the standard deviation and the overall number of
measurements in the laboratory's records. If an acceptable value is not obtained, the
candidate standard cannot be certified under this protocol.
The analyst should investigate any of the measurements that appear to be outliers.
Such data may be discarded if the analyst can demonstrate that the experimental
conditions were inappropriate during these measurements. Data cannot be discarded
just because they appear to be outliers but may be discarded if they satisfy statistical
criteria for testing outliers. The analyst must record any discarded data and a brief
summary of the investigation in the laboratory's records.
13. Finally, calculate the certified permeation rate (in nanograms/minute) for the candidate
standard using the equation below:
Certified Permeation Rate
103
"mw"
.
MV
L
Diluted Standard
Cone.
Dilution Flow
Rate
3.2.10 Equilibration Test for Newly Prepared Permeation Devices
A permeation device that has not been previously assayed must be tested for a stable
permeation rate as follows: Reassay the permeation rate at least 24 hours after the first assay
and compare the two assayed concentrations. If the second assayed concentration differs from
3-15
-------
the first assayed concentration by 1.0 percent or less, the device may be considered to be
equilibrated, and the mean of the two assayed concentrations should be reported as the certified
permeation rate. Otherwise, equilibrate the device longer at the operating temperature and repeat
the test, using the second and third assays as if they were the first and second. This process
may be repeated until the rate stabilizes. Permeation devices that are not stable may not be
used for calibration or audit purposes. Candidate standards that fail the initial and the repeat
stability tests are unstable and are disqualified for further use under this protocol.
3.2.11 Certification Documentation
See Subsections 3.1.5 and 3.1.6.
3.2.12 Recertlficatlon Requirements
See Subsection 3.1.7.3.
3.3 PROCEDURE P2: ASSAY AND CERTIFICATION OF PERMEATION DEVICE
CALIBRATION STANDARDS REFERENCED TO A COMPRESSED GAS
REFERENCE STANDARD
3.3.1 Applicability
This procedure may be used to assay the permeation rate of a candidate S02 and NOz
permeation device calibration standard, based on the concentration of a compressed gas
reference standard of the same pollutant compound, and to certify that the assayed permeation
rate is traceable to the reference standard. The procedure employs a low-concentration range
(i.e., ambient air quality level) pollutant gas analyzer to compare quantitatively diluted
concentrations from the permeation device calibration standard with quantitatively diluted
concentrations from the compressed gas reference standard. This procedure may be used for
the assay of multiple candidate standards during the same assay session. Criteria that apply to
the assay of one candidate standard apply to the assay of multiple candidate standards. This
procedure may be used by permeation device producers, standard users or other analytical
laboratories.
3.3.2 Limitations
1. The concentration of the diluted candidate standard may be greater than or less than
the concentration of the diluted reference standard. However, the diluted
concentrations from both standards must lie within the well-characterized region of
the analyzer's calibration curve (see Subsection 2.1.7.2). Additionally, the 95-percent
uncertainty for the regression-predicted concentration of the diluted candidate
standard must be <1.0 percent of the concentration of the diluted reference standard.
This uncertainty is obtained from the statistical analysis of the multipoint calibration
data. This criterion means that the uncertainty associated with the multipoint
calibration determines the concentration range over which a diluted candidate
standard may be assayed.
2. A quantitatively accurate dilution and flow measurement system is required.
3-16
-------
3. A source of clean, dry zero gas is required.
4. This procedure is designed to assay the permeation rate of a candidate standard that
is mounted in a specially designed assay dilution system. The procedure does not
accommodate the certification of a candidate standard that is mounted in its own self-
contained dilution/flow measurement system.
3.3.3 Assay Apparatus
Figure 3-2 illustrates the components and configuration of one possible design for the
assay apparatus, including a common dilution system for both the reference and candidate
standards. The configuration is designed to allow convenient routing of zero gas and diluted
concentrations of the reference standard and the candidate standard, in turn, to the analyzer for
measurement, as selected by valves V1, V2, and V3. Three gas flow controllers (i.e., C1, C2,
and C3) regulate the total dilution flow rate for the candidate standard, the purge gas flow rate,
and the reference standard flow rate. These gas flow controllers may be needle valves, capillary
tubes, thermal mass flow controllers, or other suitable devices. The flow rates should be
controlled to within 1.0 percent variation during the assay. The dilution flow rates for the
reference and candidate standards is measured by a single, common flowmeter (i.e., M1). The
reference standard and purge gas flow rates may be measured at the vent port of valve V2 or by
flowmeters M2 and M3 that are mounted in the two gas streams.
When the candidate standard is being measured, valve V1 directs a portion (usually 50
to 100 mL/min) of the total dilution flow through the candidate standard chamber. This sweep
flow rate is regulated by gas flow controller C1 and is measured by gas flowmeter M2. This
flowmeter need not be accurately calibrated because only the total dilution flow rate, measured
by flowmeter M1, is used in the dilution calculations. When the reference standard is being
measured, valve V1 directs the purge gas through the candidate standard chamber. The purge
gas prevents the buildup of high pollutant concentrations in the chamber. It is vented through
valve V2 and is not measured by flowmeter M1.
The assay apparatus illustrated in Figure 3-2 may be modified by the addition of multiple
candidate standard chambers. These chambers may be set to different temperatures.
If it is necessary to use different dilution flow rates for the reference standard and the
candidate standard (see Subsection 3.2.6), separate flow controllers for the two dilution flow rates
may be used. However, the same flowmeter should be used to measure both dilution flow rates
to help reduce systematic flow measurement errors.
The mixing chamber combines the gas streams and should be designed to provide
turbulence in the flow to ensure thorough mixing of the two gas streams. The diluted gas
mixtures are routed to the analyzer through a union tee tube fitting, which vents excess gas flow.
Normally, the excess gas is vented to the atmosphere without any obstructions in the tubing and
the gas entering the analyzer is at near-atmospheric pressure. However, the excess gas can be
routed through an uncalibrated rotameter by rotation of a three-way valve (i.e., V4). The
rotameter is used to demonstrate that the total gas flow rate exceeds the sample flow rate of the
analyzer and that no room air is being drawn in through the vent line (also see Subsection 3.1.4).
Check the apparatus carefully for leaks and correct all leaks before use.
3-17
-------
Gas Flowmeter
(M2)
Pressure
Regulator
Dilution Gas
Flowmeter
(M1)
3-way
Valve
(V1)
Gas Flow
Controller
(CI)
Candidate
Standard
Chamber
m
O
Gas Flow
Controller
(C2)
4-way
Valve (V2)
Purge Gas Flow
Purge Gas -
Flow to Vent
3-way
Valve (V3)
Excess Gas
Flow to Vent
Rotameter
3-way
Valve (V4)
O
Mixing
Chamber
Zero
Gas
Gas Flow
to Analyzer
Gas
Flow to
Vent
Pressure
Regulator
Reference
Standard
Flowmeter (M3)
Gas Flow
Controller (C3)
Reference
Standard
Figure 3-2. One possible design of the apparatus for the assay of permeation device calibration
standards referenced to a compressed gas reference standard (Procedure P2).
-------
The mean temperature of the candidate standard chamber must be controlled to within
0.05 °C of the setpoint with a temperature stability of ±0.05 °C. This temperature must be
measured with an NIST-traceable thermometer having a measurement uncertainty of not more
than 0.05 °C.
3.3.4 Pollutant Gas Analyzer
See Subsection 2.3.4. The pollutant gas analyzer must have a well-characterized
calibration curve and a range capable of measuring the diluted concentrations of both the
candidate and reference standards. It must have good resolution, good precision, a stable
response, and low output signal noise. In addition, the analyzer must have good specificity for
the pollutant of interest so that it has no detectable response to any contaminant that may be
contained in the standards. A suitable analyzer with acceptable performance specifications may
be selected from the list of EPA-designated reference and equivalent method analyzers.16 If the
balance gas of the reference standard must be different from the zero gas used for dilution (e.g.,
air versus nitrogen or different proportions of oxygen), either a high dilution ratio (i.e., at least 50
parts zero gas to 1 part standard) should be used, or the analyzer must be proven to be not
sensitive to differences in the balance gas composition. The latter may be demonstrated by
showing no difference in an analyzer's response when measuring a calibration standard that has
been diluted with identical flow rates of the different balance gases.
The analyzer should be connected to a high-precision strip-chart recorder, or other data
acquisition system to facilitate graphical observation and documentation of the analyzer response
during the assay. Additionally, a digital panel meter with four-digit resolution, a digital voltmeter,
data logger, or other data acquisition system must be used to obtain numerical values of the
analyzer's response. More precise values will be obtained if these instruments have some data
averaging capability.
If the analyzer has not been in continuous operation, turn it on and allow it to stabilize for
at least 12 hours before beginning any measurements.
3.3.5 Analyzer Calibration
3.3.5.1 Multipoint Calibration-
See Subsections 2.1.7.2 and 2.1.7.4. Following completion of the multipoint calibration,
the accuracy of the assay apparatus must be checked to verify that the error associated with the
dilution is not excessive. This accuracy check involves the measurement of an undiluted or
diluted check standard. The check standard must be traceable to an NIST SRM, a CRM, or an
NTRM. It must have a certified concentration that is different from that of the reference standard
used in the multipoint calibration. Information concerning this standard (e.g., cylinder identification
number, certified concentration) must be recorded in the laboratory's records.
If an undiluted check standard is used, its concentration must fall in the well-characterized
region of the calibration curve. If a diluted check standard is used, the diluted concentration must
fall in the well-characterized region.
Make three or more discrete measurements of the undiluted or diluted check standard.
"Discrete" means that the analyst must change the gas mixture being sampled by the analyzer
3-19
-------
between measurements. For example, the analyst might alternate between measurements of the
check standard and the zero gas. Record these measurements in the laboratory's records.
Next the analyst must verify that the dilution error is not excessive. For the check
standard measurements, calculate the relative difference (in percent) between the mean analyzer
response and the corresponding response that is predicted from the multipoint calibration
regression equation and the undiluted or diluted check standard concentration. That is,
Relative Difference = 100
Mean Analyzer Response - Predicted Response
Predicted Response
If the relative difference is greater than 1.0 percent, the dilution error is considered to be
excessive. The analyst must investigate why the relative difference is excessive. The program
may be due to errors in the reference standard and check standard concentrations, errors in
assay apparatus or to some ofrier source. Assays may not be conducted until the relative
difference for a subsequent accuracy check is less than or equal to 1.0 percent.
3.3.5.2 Analyzer Range-
See Subsection 2.3.5.2.
3.3.5.3 Linearity-
See Subsection 2.3.5.3.
3.3.5.4 Zero and Span Gas Checks-
See Subsection 2.3.5.4.
3.3.6 Selection of Gas Dilution Flow Rates
The dilution flow rates used for the reference standard and the candidate standard should
be selected carefully to provide diluted concentrations for both standards that fall in the well-
characterized region of the analyzer's calibration curve. Potential errors in the assayed
permeation rate due to dilution flow rate measurement error will be reduced if the same dilution
flow rates can be used for both the reference and candidate standards. This should be feasible
by appropriate selection of the reference standard flow rate. Select a combination of reference
standard flow rate and dilution flow rate that produces approximately equal diluted reference
standard and candidate standard concentrations that fall in the well-characterized region of the
analyzer's calibration curve. Additionally, the 95-percent uncertainty for the regression-predicted
concentration of the diluted candidate standard must be <1.0 percent of the concentration of the
diluted reference standard.
3.3.7 Flowmeter Type and Rowmeter Calibration
Flowmeters M1 and M3, shown in Figure 3-2, measure in-line flow rates and do not
operate at atmospheric pressure because of backpressure from downstream components. Also,
this backpressure is variable, depending on the total dilution and reference standard flow rates.
Thus, the flowmeters must compensate for the variable in-line pressure. Thermal mass
flowmeters do not need to be corrected for pressure effects. Measurements from pressure-
sensitive flowmeters such as rotameters or wet test meters must be carefully corrected for the
actual in-line pressure during the flow rate measurements. A nonventing back pressure regulator
3-20
-------
may be inserted downstream of the flowmeter so that the flowmeter operates at a fixed pressure
regardless of the total flow rate.
Alternatively, the flow rates can be measured at the outlet of the dilution apparatus, with
the excess gas flow vent temporarily plugged. In this case, a volume-type meter such as a wet
test meter or a soap film flowmeter can be used, and flow measurements may be conveniently
referenced to atmospheric pressure. Each flow rate must be measured independently while the
other flow,rate is set to zero. Great care must be exercised to ensure that each measured flow
rate remains constant when combined with the other flow rate and between the time of
measurement aid the time of the assay.
The flowmeters used should be stable, repeatable, linear, and have good resolution. The
flowmeters must not contaminate or react with the gas mixture passing through them. If possible,
select flow rates or flowmeter ranges such that the measured flow rates fall in the upper half of
the flowmeters' ranges. The flowmeters should be carefully calibrated at several flow rates to
prove linearity. The calibration of the zero gas flowmeter should be accurate to ±1.0 percent,
referenced to an accurate flow or volume standard traceable to an NIST primary standard. This
flowmeter calibration should be checked and recertified periodically. The recertification frequency
is to be determined from stability information such as a chronological control chat of calibration
data.
It is desirable to measure both dilution flow rates with the same flowmeter (i.e., M1). This
practice reduces measurement errors associated with the use of multiple flowmeters. Note that
the impact of any flow measurement error is reduced if the same dilution ratio can be used for
both candidate standard and reference standard measurements.
All volumetric flow-rate measurements must be corrected or referenced to the same
temperature and pressure conditions, such as EPA-standard conditions (i.e., 760 millimeters of
mercury (mm Hg) and 25 °C) or tie ambient temperature and pressure conditions prevailing in
the laboratory during the assay. Measurements using wet test meters and soap bubble
flowmeters also must be corrected for the saturation of the gas stream with water vapor in the
moist interiors of these flowmeters. The equation to correct the flow rate for temperature,
pressure, and humidity effects is given below:
Volume
Pm ~ Pwv
V
Time
ps J
W
where
PM « measured barometric pressure (mm Hg);
Pyyy » partial pressure of water vapor (mm Hg);
Ps = standard pressure (mm Hg);
Ts = standard temperature (Kelvin = degrees Celsius + 273.2); and
TM « measured ambient temperature (Kelvin).
3.3.8 Candidate Standard
See Subsections 3.1.7 and 3.2.2. Follow the manufacturer's instructions for equilibration
and for use of the candidate standard and for selecting the temperature at which it is to be
3-21
-------
assayed and certified. The candidate standard should be assayed at the same temperature at
which it will be subsequently used. The mean operating temperature of the candidate standard
chamber must be controlled to within 0.05 °C of the setpoint with a temperature stability of
±0.05 °C. This temperature must be measured with an NIST-traceable thermometer with a
measurement uncertainty ±0.05 °C or less.
3.3.9 Reference Standard
-
See Subsections 2.1.2, 2.1.6.4, 2.3.2, 2.3.6, and 3.1.2.
3.3.10 Zero Gas
See Subsection 2.1.8. If possible, the zero gas should be the same as the balance gas
of the reference standard.
3.3.11 Assay Procedure
1. Verify that the assay apparatus is properly configured as shown in Figure 3-2 and
described in Subsection 3.3.3. Inspect the analyzer to verify that it appears to be
operating normally and that all controls are set to their expected values. Record
these control values in the laboratory's records.
2. Determine and establish the operating temperature for the candidate standard in its
temperature-controlled chamber. Install the candidate standard, start the purge gas
flow, and allow ample time for the device to equilibrate (see Subsection 3.1.7.2).
Record the temperature in the laboratory's records.
3. Verify that the flowmeters are properly calibrated (see Subsection 3.3.7).
4. Verify that a multipoint calibration of the analyzer has been performed within 1 month
prior to the assay date (see Subsections 2.1.7.2, 2.1.7.5, and 2.3.4). Additionally,
verify that the zero and span gas checks indicate that the analyzer is in calibration
(see Subsection 2.3.5.4).
5. Determine and establish the reference standard flow rate and the dilution flow rates
and diluted concentrations for the reference standard and the candidate standard that
will be used for the assay (see Subsections 2.3.5J2 and 3.3.6). Ensure that the
diluted reference standard and diluted candidate standards concentrations are within
the well-characterized region of the analyzer's calibration curve (see Subsection
232). Also check that both dilution flow rates will provide enough flow for the
analyzer, with sufficient excess to ensure that no ambient air will be drawn into the
vent line. If possible, use the same dilution flow rate for both the reference standard
and the candidate standard. Also adjust the flow rate of the portion of the dilution
flow that passes over the candidate standard (i.e., flow controller C3), and adjust the
purge flow rate (i.e., flow controller C2) to approximately the same value.
3-22
-------
Calculate the diluted reference standard concentration using the following equation:
Diluted
Standard =
Cone.
(Undiluted Standard Cone.) (Standard Flow Rate)
(Standard Flow Rate + Zero Gas Flow Rate)
Calculate the diluted candidate standard concentration (in ppm) using the following
equation:
Diluted Standard Cone. =
10"
MV
MW
Permeation Rate
Dilution Flow Rate
where
MV = Molar volume of the dilution gas (liters/mole);
MW = Molecular weight of the dilution gas (grams/mole); permeation rate is given
in nanograms/minute; and dilution flow rate is given in liters/minute.
Use an estimated permeation rate for the candidate standard in this calculation.
Record the measured flow rates and the undiluted and diluted reference standard
concentrations in the laboratory's records.
6. In succession, measure the zero gas, the diluted reference standard and the diluted
candidate standard using the analyzer. Use valves V1, V2, and V3 to select each of
the three gas mixtures for measurement. For each measurement, adjust the flow
rates, if necessary, to those determined in step 5, and allow ample time for the
analyzer to achieve a stable reading. If the reading for each measurement is not
stable, the precision of the measurements will decline and the candidate standard
might not be certifiable under this protocol (as discussed in step 12). Record the
analyzer response for each measurement, using the same response units (e.g., volts,
millivolts, percent of scale, etc.) as was used for the multipoint calibration. At this
point, do not convert the data into concentration values using the calibration equation.
Do not perform any mathematical transformations of the data. These steps will be
done later. Do not make any zero control, span control, or other adjustments to the
analyzer during this set of measurements. Record these analyzer responses in the
laboratory's records.
The analyst may assay multiple candidate standards during the same assay session.
For example, a single set of measurements may involve a zero gas, a diluted
reference standard, and three diluted candidate standards. Criteria that apply to the
assay of one candidate standard apply to the assay of multiple candidate standards.
The analyst should be aware that the effect of any short-term calibration drift will be
greater when multiple candidate standards are assayed. This greater effect is due
to the longer period of time between reference standard measurements.
Unacceptable standard errors of the mean concentration for the diluted candidate
standards (see step 12 below) may occur as a result of the longer assay session.
3-23
-------
7. Conduct at least two additional sets of measurements, as described in step 6 above.
However, for these subsequent sets of measurements, change the order of the three
measurements (e.g., measure the reference standard, zero gas, and candidate
standard for the second set and measure the zero gas, candidate standard, and
reference standard for the third set, etc.). Changing the order that the gas mixtures
are measured helps the analyst to discover any effect that one measurement has on
subsequent measurements. The number of sets of measurements will be the same
. as the number of measurements that were made during the zero and span gas
checks (see Subsection 2.3.5.4).
8. If any one or more of the measurements of a set of measurements is invalid or
abnormal for any reason, discard all three measurements and repeat the set of
measurements. Such measurements may be discarded if the analyst can
demonstrate that the experimental conditions were inappropriate during these
measurements. Data cannot be discarded just because they appear to be outliers,
but may be discarded if they satisfy statistical criteria for testing outliers. The
analyst must record any discarded data and a brief explanation about why the data
were discarded in the laboratory's records.
S. The analyzer response must now be corrected for any minor calibration drift, which
may have occurred in the analyzer since the multipoint calibration. For each set of
measurements, calculate a corrected analyzer response for the diluted candidate
standard from its measured analyzer response as follows:
Corrected Response
'drs, -
DRS2 -
Measured Response - Z2
Zi
where
DRS, = the analyzer response for the diluted reference standard during the
multipoint calibration;
Z, = the analyzer response for the zero gas during the multipoint calibration;
DRS2 ® the analyzer response for the diluted reference standard during this set
of measurements; and
Zj = the analyzer response for the zero gas during this set of measurements.
Record the corrected responses in the laboratory's records.
If the diluted reference standard was not measured during the multipoint calibration,
use a predicted response for the diluted reference standard in the place of DRS,.
This response is predicted from the multipoint calibration's regression equation for a
concentration equal to that of the diluted reference standard.
Note that this step is not necessary if the assay of the diluted candidate standard
occurs at the same time as the multipoint calibration.
3-24
-------
10. If the multipoint calibration data underwent any mathematical transformation before
their statistical analysis, perform the same mathematical transformation on the
corrected analyzer responses for the diluted candidate standard. These transformed
responses should then be used in step 12.
Note that this step is not necessary if the multipoint calibration data were not
transformed before their statistical analysis.
11. The multipoint calibration's regression equation should be used to convert the
corrected analyzer responses for the diluted candidate standard into the
corresponding concentrations for the diluted candidate standard. Record these
concentrations in the laboratory's records.
12. Calculate the mean, standard deviation and standard error of the mean for the three
or more diluted candidate standard concentrations that were obtained in step 11.
Record these calculations in the laboratory's records. The candidate standard cannot
be certified unless the standard error of the mean is <1.0 percent of the mean
concentration. That is,
s < Cpcs
100
where
s = the standard deviation for the diluted candidate standard concentrations;
n = the number of sets of measurements of the diluted candidate standard; and
^dcs = the mean concentration of the diluted candidate standard.
Note that the value of the standard error of the mean can be made smaller by
increasing the number of sets of measurements of the three gas mixtures.
If the standard error of the mean is >1.0 percent of the mean concentration, the
analyst must make additional measurements of the three gas mixtures as descrtoed
in step 6 above. These additional measurements are used to calculate additional
concentrations, which are then pooled with the previously determined concentrations
to obtain a new value for the standard error of the mean. When an acceptable value
is obtained, record it, the mean, the standard deviation and the overall number of
measurements in the laboratory's records. If an acceptable value is not obtained, the
candidate standard cannot be certified under this protocol.
The analyst should investigate any of the measurements that appear to be outliers.
Such data may be discarded if the analyst can demonstrate that the experimental
conditions were inappropriate during these measurements. Data cannot be discarded
just because they appear to be outliers but may be discarded if they satisfy statistical
criteria for testing outliers. The analyst must record any discarded data and a brief
summary of the investigation in the laboratory's records.
3-25
-------
13. Finally, calculate the certified permeation rate (in nanograms/minute) for the candidate
standard using the equation below:
Certified Permeation Rate
103
MW
MV
Diluted Candidate
Standard Cone.
Dilution Flow
Rate
3.3.12 Equilibration Test for Newly Prepared Permeation Devices
A permeation device that has not been previously assayed must be tested for a stable
permeation rate as follows: Reassay the permeation rate at least 24 hours after the first assay
and compare the two assayed concentrations. If the second assayed concentration differs from
the first assayed concentration by 1.0 percent or less, the device may be considered to be
equilibrated, and the mean of the two assayed concentrations should be reported as the certified
permeation rate. Otherwise, equilibrate the device longer at the operating temperature and repeat
the test, using the second and third assays as if they were the first and second. This process
may be repeated until the rate stabilizes. Permeation devices that are not stable may not be
used for calibration or audit purposes. Candidate standards that fail the initial and the repeat
stability tests are unstable and are disqualified for further use under this protocol.
3.3.13 Certification Documentation
See Subsections 3.1.5 and 3.1.6.
3.3.14 Recertification Requirements
See Subsection 3.1.7.3.
3.4 PROCEDURE P3: ASSAY AND CERTIFICATION OF PERMEATION DEVICE
CALIBRATION STANDARDS REFERENCED TO A MASS REFERENCE STANDARD
3.4.1 Applicability
This procedure may be used to assay the permeation rate of a candidate S02 or N02
permeation device calibration standard based on mass reference standards, and to certify that
the assayed permeation rate is traceable to the reference standard. The procedure employs an
analytical balance to measure the weight loss in the candidate standard. It may be used for the
assay of multiple candidate standards during the same assay session. Criteria that apply to the
assay of one candidate standard apply to the assay of multiple candidate standards. This
procedure may be used by permeation device producers, standard users, or other analytical
laboratories.
3.4.2 Limitations
1. This procedure is intended only for the assay of candidate standards containing S02
or N02. These liquid compounds must be anhydrous grade (minimum purity 99.99
percent) or phosphorous pentoxide-dried commercial purity grade (minimum purity
99.5 percent).
3-26
-------
2. An accurate analytical balance with an NIST-traceable calibration is required to weign
the candidate standard.
3. A temperature-controlled chamber for maintaining the candidate standard at s
constant, NIST-traceable temperature between weight measurements is required.
4. A source of clean, dry zero gas is required.
3.4.3 Assay Apparatus
3.4.3.1 Analytical Balance-
Choose a balance with adequate vibration-stabilization control and appropriate
specifications for total weighing capacity, accuracy, precision, and readability. The balance
should be chosen such that the manufacturer's specified uncertainty (i.e., three times the standard
deviation or "reproducibility") of the balance divided by the weight of the candidate standard does
not exceed 0.001. The balance must be calibrated annually using NIST-traceable mass reference
standards by the manufacturer or a manufacturer's representative (see Subsection 3.1.2).
If possible, locate the balance in a climate-controlled, draft-free room, preferably dedicated
to the use of balances. If this is not possible, the general guidelines listed below should be
followed to control environmental factors that may affect balance performance:
Locate the balance away from potential sources of drafts such as doors, windows,
aisles with frequent traffic, ventilation ducts, and equipment with fans or moving parts.
Locate the balance out of direct sunlight and away from local heating or cooling
sources such as open flames, hot plates, water baths, ventilation ducts, windows, and
heat-producing lamps.
• Locate the balance on a sturdy base (ideally, a stone weighing table) and away from
any equipment that produces vibrations. If this is not possible, isolate the balance
from such equipment by placing a stabilizing slab under the balance or composite
damping-pads under the balance legs.
• Ensure that the balance-support is sufficiently level to permit leveling of the balance
according to the manufacturer's instructions.
3.4.3.2 Temperature-controlled Chamber-
A temperature-controlled chamber is required for storing the candidate standard between
weighings. One possible design for the chamber is depicted in Figure 3-3.24 Clean, dry zero gas
enters the chamber at the bottom after passing through the heat exchanger tubing (i.e., several
turns of copper tubing). The zero gas' flow rate must be sufficient to purge the chamber
thoroughly. The chamber and the heat exchanger are immersed in a thermostatted bath to the
level shown in the figure. The bath must control the mean temperature of the chamber to within
0.05 °C of the setpoint with a temperature stability of ±0.05 °C. The temperature of the bath or
the chamber must be measured and recorded in the laboratory's records on at least a daily basis.
An NIST-traceable, liquid-in-glass thermometer or a temperature-sensing device must be used
for this measurement (see Subsection 3.1.2). A temperature-sensing device must be calibrated
annually using NIST-traceable temperature reference standards and must have an uncertainty
3-27
-------
Air Outlet
Removable Cap
Thin Tube
Water Level
Water Level
Permeation Tube
— Holder
Perforated
Spacer
Heat
Exchanger
x Tubing
Permeation Tube in
Position
Perforated Disk
Bottom Plate
im
THTfTT
Air Inlet
Figure 3-3. Chamber for maintaining permeation tubes at constant temperature.
3-28
-------
similar to that of these reference standards. The output of a temperature-sensing device may be
recorded by a strip chart recorder, data logger, or other data acquisition system.
3.4.4 Weighing Interval
The minimum time period between weighings of the candidate standard is a function of
the expected permeation rate, the specified uncertainty for the rate, and the analytical balance's
readability, (i.e., the smallest scale division). The following equation is based on a ±1 percent
uncertainty specification for the permeation rate:
Weighing = 100 (Readability)
Interval (Expected Permeation Rate )
where the weighing interval is in minutes; the readability is in grams; and the expected
permeation rate is in grams per minute.
3.4.5 Assay Procedure
1. Turn on the balance and allow it to warm up for the period specified in the operator's
manual.
2. Check the balance level and, if necessary, adjust the level according to the
manufacturer's instructions.
3. For balances with an optical scale, check the scale image for proper focus,
brightness, and alignment. Perform any necessary adjustments according to the
instructions in the operator's manual.
4. Throughout the weighing session, observe the scale image/digital display for
unacceptable drift or fluctuation. If such variation is observed, identify the cause and
take the necessary corrective action before proceeding.
5. Ensure that the balance room temperature is within 15 to 30 °C or, if given, within the
balance manufacturer's specifications and that the balance and mass reference
standards are equilibrated to the balance room temperature. Record the temperature
in the laboratory's records.
6. Before each calibration, calibration verification, or weighing, adjust the scale
image/digital display to zero in accordance with the instructions in the operator's
manual. If the balance cannot be zeroed, identify the possible cause such as
contamination of the pan, the pan brake shaft, etc., and take the necessary corrective
action. If adjustment of the internal zeroing mechanism is necessary, follow the
instructions in the operator's manual.
After each calibration, calibration verification, or weighing, verify that the scale
image/digital display returns to zero before recording the reading or proceeding to the
3-29
-------
next step. If it does not return to zero, identify the cause, correct the problem, and
recalibrate or reweigh.
7. On each day that the candidate standard is to be assayed, verify the balance's
calibration using at least one NIST-traceable mass reference standard. This standard
must have a mass that is similar to that of the candidate standard. Record the dale,
balance identification, standards identification, certified weight of the standard, ar>d
. the measured weight of the standard in the laboratory's records. Calculate the
relative difference (in percent) between the certified and measured weights as follows:
Relative _ 100 (Measured Weight - Certified Weight)
Difference (Certified Weight)
Record the relative difference in the laboratory's records. If the relative difference is
>0.1 percent, the balance cannot be used under this protocol until it has been
recalibrated or repaired and until a subsequent verification has a relative difference
of <0.1 percent.
The analyst may use any built-in mass reference standard that the balance may have
to check the balance's calibration. But such a check does not remove the
requirement for a calibration verification using external standards.
8. Review the recorded bath or chamber temperature readings since the most recent
weighing of the candidate standard, or since the standard was first put into the
temperature-controlled chamber. Record the minimum and maximum temperatures
in the laboratory's records. The minimum and maximum temperatures must not have
deviated from the setpoint by more than 0.1 °C. If these temperatures deviate by
more than this amount, the current assay and all previous assays are invalidated.
9. Record the current bath or chamber temperature in the laboratory's records.
10. Verify that the candidate standard has been in the temperature-controlled chamber
for a long enough time for its permeation rate to have stabilized.
11. Remove the candidate standard from the temperature-controlled chamber and place
it on the balance's pan using stainless steel forceps or a similar noncontaminating
device. Note that Teflon permeation tubes may have a static electricity charge due
to the passage of the dry gas over them between weighings. Such charges should
be removed from the candidate standard before weighing by antistatic deionizing
radiation devices or similar devices. Note that electronic force balances may require
that candidate standards be thermally equilibrated before they can be weighed.
12. Record the date, time and the candidate standard's identification number and current
weight in the laboratory's records.
3-30
-------
13. Return the candidate standard to the temperature-controlled chamber. The standard
should be outside the chamber only for a long enough time to weigh it using
reasonable laboratory technique.
3.4.6 Number of Weighings of the Candidate Standard
The candidate standard must be weighed at least six times after its permeation rate has
stabilized; at the certification temperature. After each weighing, calculate a provisional
permeation rate from the weights of the candidate standard during the current and previous
weighings and from the time that has elapsed between these weighings:
where the weights are given in grams and the time is given in minutes. Prepare a graph of the
provisional permeation rates as a function of the total elapsed time for all weighings. Use this
graph to determine qualitatively when the permeation rate has stabilized. Record the provisional
permeation rates and the times in the laboratory's records.
3.4.7 Calculation of Certified Permeation Rate
The certified permeation rate for the candidate standard is the slope of the least squares
regression line for data from at least six weighings of the candidate standard after the permeation
rate has stabilized. This statistical analysis technique produces permeation rate estimates that
are more precise than those calculated from weight differences between individual weighings.
Although the minimum number of weighings is six, more precise estimates will be obtained for
more weighings.
Calculate the slope of the least squares regression line (bt) and its variance (S^) using
the statistical worksheet in Table 2-3 of Subsection 2.1.7.2. The standard error of the slope is
estimated by Sb1. Record these calculations in the laboratory's records. The standard error of
the slope must be <1.0 percent of the slope. That is,
If the standard error of the slope is >1.0 percent of the slope, the analyst must make additional
weighings of the candidate standard as described in Subsection 3.4.5. These additional
measurements will be pooled with the previously collected measurements. The pooled data will
be used to obtain new estimates of the slope and its standard error. When an acceptable value
for the standard error is obtained, record it and the slope in the laboratory's records. The slope
is the certified permeation rate of the candidate standard, if an acceptable value is not obtained,
the candidate standard cannot be certified under this protocol.
The analyst should investigate any of the measurements that appear to be outliers. Such
data may be discarded if the analyst can demonstrate that the experimental conditions were
inappropriate during these measurements. Data cannot be discarded just because they appear
Provisional
Permeation =
Rate
(Previous Weight - Current Weight)
Elapsed Time Between Weighings
Sb1 £ (bt) /100 .
3-31
-------
to be outliers but may be discarded if they satisfy statistical criteria for testing outliers. The
analyst must record any discarded data and a brief summary of the investigation in the
laboratory's records.
3.4.8 Uncertainty of Estimated Permeation Rate for Candidate Standard
The total analytical uncertainty of the estimated permeation rate is the standard error of
the slope of the least-squares regression line. The analyst may report this uncertainty on the
certification documentation or may report this estimate as a percentage that is relative to the
certified permeation rate using the following equation:
UncertaintyRELATlvE = 100
UncertaintyT0TAL
Certified Permeation Rate
3.4.9 Certification Documentation
See Subsections 3.1.5 and 3.1.6.
3.4.10 Recertification Requirements
See Subsection 3.1.7.3.
3-32
-------
SECTION 4
REFERENCES
1. U.S. Environmental Protection Agency. Quality Assurance Handbook for Air Pollution
Measurement Systems, Volume II. Ambient Air Specific Methods. EPA-600/4-77-027a,
1977.
2. U.S. Environmental Protection Agency. Quality Assurance Handbook for Air Pollution
Measurement Systems, Volume III. Stationary Source Specific Methods. EPA-600/4-77-
027b, 1977.
3. U.S. Environmental Protection Agency. Code of Federal Regulations. Title 40, Chapter
I, Subchapter C, Part 50. "National Primary and Secondary Ambient Air Quality
Standards." Washington, DC. Office of the Federal Register. July 1, 1993.
4. U.S. Environmental Protection Agency. Code of Federal Regulations. Title 40, Chapter
I, Subchapter C, Part 58. "Ambient Air Quality Surveillance." Washington, DC. Office of
the Federal Register. July 1, 1993.
5. U.S. Environmental Protection Agency. Code of Federal Regulations. Title 40, Chapter
I, Subchapter C, Part 60. "Standards of Performance for New Stationary Sources."
Washington, DC. Office of the Federal Register. July 1,1993.
6. U.S. Environmental Protection Agency. Code of Federal Regulations. Title 40, Chapter
I, Subchapter C, Part 75. "Continuous Emission Monitoring." Washington, DC. Office of
the Federal Register. July 1, 1993.
7. "A Procedure for Establishing Traceability of Gas Mixtures to Certain National Bureau of
Standards SRMs." EPA-600/7-81-010, Joint Publication of the National Bureau of
Standards and the Environmental Protection Agency. May 1989. 56 pp.
8. W.J. Mitchell and W.E. May. "Two New Gas Standards Programs at the National Institute
of Standards and Technology. EPA Publication EPA/600/A-93/107. Presented at the
EPA/Air and Waste Management Association International Symposium on Measurement
of Toxic and Related Air Pollutants, Durham, North Carolina, 1993.
9. G.L Harris. State Weights and Measures Laboratories: State Standards Program
Description. National Institute of Standards and Technology. Special Publication 791.
1993.
10. J.L Cigler, J.M. Crickenberger, and C.D. Faison. Calibration Laboratories. Draft
Handbook. National Voluntary Laboratory Accreditation Program. National Institute of
Standards and Technology. 1993.
4-1
-------
11. R.C. Shores, F. Smith, and D.J. von Lehmden. "Stability Evaluation of Sulfur Dioxide,
Nitric Oxide and Carbon Monoxide Gases in Cylinders." EPA/600/4-86, U.S.
Environmental Protection Agency, Research Triangle Park, NC. 1984. 52 pp.
12. S.G. Wechter. "Preparation of Stable Pollution Gas Standards Using Treated Aluminum
Cylinders." In Calibration in Air Monitoring, ASTM STP 598, American Society for Testing
and Materials. Philadelphia, PA. 1976. pp. 40-54.
13. National Council of the Paper Industry for Air and Stream Improvement, Inc. "An Investi-
gation of the Stability of H2S in Air Cylinder Gases." NCASI Special Report No. 90-09,
New York, NY. 1990. 13 pp.
14. M.G. Natrella. Experimental Statistics, National Bureau of Standards Handbook No. 91,
U.S. Government Printing Office. Washington, DC. 1963. pp. 5-1 to 5-46.
15. American Society for Testing and Materials. Standard Practice for Dealing with Outlying
Observations. ASTM Standard Practice E 178-80, 1980.
16. U.S. Environmental Protection Agency. List of Designated Reference and Equivalent
Methods. Current edition available from U.S. Environmental Protection Agency, Quality
Assurance and Technical Support Division, Mail Code MD-77B, Research Triangle Park,
NC 27711.
17. American National Standards Institute (ANSI) and American Society for Testing and
Materials (ASTM). Standard Specification for Laboratory Weights and Precision Mass
Standards. ANSI/ASTM Standard E 617-91, 1991.
18. W. Kupper. "High Accuracy Mass Measurements, From Micrograms to Tons," Instrument
Society of America Transactions. 29(4). 1990.
19. G. Harris. "Ensuring Accuracy and Traceability of Weighing Instruments," ASTM
Standardization News. 21(4):44-51. 1993.
20. J.A. Wise. NIST Measurement Services: Liquid-in-Glass Thermometer Calibration
Service. National Institute of Standards and Technology Special Publication 250-23.
1988.
21. J.A. Wise. A Procedure for the Effective Recalibration of Liquid-in-Glass Thermometers.
National Institute of Standards and Technology Special Publication 819. 1991.
22. E.E. Hughes et al. "Performance of a Nitrogen Dioxide Permeation Device." Analytical
Chemistry. 49(12):1823-1829. 1977.
23. D.L. Williams. "Permeation Tube Equilibration Times and Long-Term Stability."
Calibration in Air Monitoring. ASTM Publication STP 598. American Society for Testing
and Materials. 1976. pp. 183-197.
24. J.K. Taylor, Ed. Microchemical Analysis Section: Summary of Activities, July 1969 to
June 1970. National Bureau of Standards Technical Note 545. 1970.
4-2
-------
APPENDIX A
EXAMPLE UNCERTAINTY CALCULATIONS
-------
EXAMPLE 1
A candidate standard is tested on the day of calibration. The calibration data are as follows:
Observed Cone, of Reference
Voltage Standard
0.0000 0.0000
0.8143 42.39
1.6421 84.78
2.472 127.17
3.294 169.56
4.085 211.95
4.9443 254.34
5.7258 296.73
6.5348 339.12
7.3698 381.51
8.202 423.9
Table 2-3 was completed and the candidate standard was analyzed, producing responses of
4.8322, 4.8197, and 4.8224. Table 2-4 was then used to determine the uncertainty in the
concentration estimated for the candidate standard.
EXAMPLE 2
A candidate standard was tested several days following calibration. The initial calibration
resulted in a quadratic curve, defined by:
Response = -0.004424 + 0.004896 * Cone. - 8.185E-07 * Cone.2
On the day the candidate standard was tested, three zeros and three span checks produced
the following results:
Predicted Observed
Check Cone. Response Response
zero 0 -0.004424 0.0033
zero 0 -0.004424 0.0012
zero 0 -0.004424 0.0040
span 1192.5 4.6701029 4.6881
span 1192.5 4.6701029 4.6750
span 1192.5 4.6701029 4.6777
Tables 2-6 and 2-7 were completed using these data plus the mean response to the
candidate standard, 2.1081. X', the regression-corrected response to the candidate standard,
was then used with the original quadratic regression to estimate the standard's concentration.
The confidence limits computed in step 6 of Table 2-7 were used in the same manner to
produce confidence limits for the concentration. The point estimate is 429.73, while the lower
and upper 95% confidence limits are 427.74 and 431.72, respectively.
A-1
-------
TABLE 2-3. WORKSHEET FOR ALL TYPES OF LINEAR RELATIONSHIPS
(After NBS Handbook No. 91)
XX = 2331.45
XY =
45.0841
Mean X. X = 211.95
MeanY. Y= 4.0986
Number of measurements: n =
11
Step
(1) SXY
13373.557
(2) (XX)(XY)/n
9555.5749
O) Sxy
= 691811.15
3817.9825
(4) X(X2)
(7)
X(Y2)
= 258.52927
(5) (IX)2/n
= 494150.82
(8)
(XY)2/n
= 184.77964
(6) S^
= 197660.33
(9)
S™
= 73,749634
(10) Slope, b, = Sxy/Sx
(11) Y
(12) b,X
(13) Y - Intercept,
b0 = Y- b,X
0.0193158
= 4.0985545
« 4.094
0.0045545
(14) (Sxy)2/Sx
(15) (n - 2) S,
(16)
{17) V
y/x
'y/x
73.747678
0.0019557
0.0002193
0.0147412
Equation of the regression line:
Y =b0 + b1X
- 0.0045545-t-0.0193158X
Sbl - 0.0000331
SbQ - 0.0083151
Estimated variance of the slope:
(18)
Q2
Sy^/Sjoj
1.10 x 10
-9
Estimated variance of intercept:
<«> sj-s*,
S£ = s„1 (1/n ~ x2/^)
0.0000691
Note: The following are algebraically identical:
•*XX
E(X - X)2; Syy - E(Y - Y)2; Sxy - E(X - X) (Y - Y).
*y
Ordinarily, in hand computation, it is preferable to compute as shown in the steps above. Carry all
decimal places obtainable—i.e., if data are recorded to two decimal places, carry four places in steps
(1) through (9) in order to avoid losing significant figures in subtraction.
A-2
-------
TABLE 2-4. CALCULATION OF THE 95-PERCENT UNCERTAINTY FOR THE
REGRESSION-PREDICTED CONCENTRATION OF AN INDIVIDUAL STANDARD
(After NBS Handbook No. 91)
This calculation is based on the linear regression analysis of multipoint calibration data
and replicate measurements of the candidate standard.
n = The-number of measurements in the multipoint calibration = 11.
n' = The anticipated number of measurements of the individual standard = 3.
Y' = The anticipated mean analyzer response from measurement of the individual standard
= 4.8248.
X' = The regression-predicted concentration of the individual standard = 249.54862
- (4.8248 - bo/b,).
(1) Choose a for a desired 100 (1 - a)%
confidence level (e.g., a = 0.05 for
95-percent confidence).
(2) Look up t (1 _ ^ n_2) from a table of
Student's t-distribution.
(3) Obtain b^, , Sy/X, and S^ from
Table
(4) Calculate C =
b"-(t(t-^.n-2))2 <
(5) Calculate the 100(1 - a)% uncertainty
for the regression-predicted
concentration
n-2) %/x
£ .
(7' - Y)2
"'XX
1+1\C
n n
(6)
Compare this uncertainty to 1-percent of
the largest concentration used in the
multipoint calibration, _L (max. conc.)
Confidence level = 95
a = 0.05
^(1 - a/2, n-2) — —2.262
U1
,2
s'
cy/x
^xx
0.0193158
1.10 x 10'9
0.0147412
197660.33
C = 0.0003730
Uncertainty = 1.1373769
100
(max. conc.) = 4.239
%
A-3
-------
TABLE 2-6. WORKSHEET FOR LINEAR RESPONSE CORRECTION
BASED ON ZERO AND SPAN CHECKS
X denotes Response predicted from initial
calibration
IX - 13.997036
Mean X, X = 2.3328394
Y denotes Observed response
IY « 14.0493
Mean Y, Y = 2.3416
Number of measurements: n
Step
(1) XXY
(2) (LX)(£Y)/n
(3)
= 65.571944
- 32.774761
32.747182
(4)
X(X2)
« 65.429643
(7)
£(Y2)
(5)
(IXfjn
- 32.652840
(8)
(XY)z/n
(6)
®xx
- 32.776803
(9)
Syy
65.714812
32.897138
32.817673
(10)
(11)
(12)
(13)
Slope, bt
Y
^X
Y - intercept,
b0 = Y - b,X
VSX
1.0006217
- 2.34155
2.3342899
0.007260
(14)
(15)
(16)
(17)
(Sxy)2/sx
(n-2) s<
s2
ay/x
2y/x
2
32.817573
0.0000999
0.0000249
0.0049983
Equation of the regression line:
Y = b0 + b1X
- 0.007260 4- 1.0006217X
Sbl » 8.73 x 10"*
Sb0 = 0.0028830
Estimated variance of the slope:
(18) S?
7.62 X 10 7
Estimated variance of intercept:
(19) S,
^ - S^ (1/n + X^J
8.31 x 10-6
Note: The following are algebraically identical:
Sxx - £(X - X)2; Syy - £(Y - Y)2; - £(X - X) (Y - Y).
xy
Ordinarily, in hand computation, K is preferable to compute as shown in the steps above. Carry aS
decimal places obtainable—i.e., If data are recorded to two decimal places, carry four places In steps
(1) through (9) in order to avoid losing significant figures in subtraction.
A-4
-------
TABLE 2-7. CALCULATION OF THE 95-PERCENT UNCERTAINTY FOR THE
REGRESSION-CORRECTED RESPONSE TO AN INDIVIDUAL STANDARD
This calculation is based on the linear regression analysis of the zero and span check
data. These checks effectively constitute a two point calibration. Replicate measurements
are made of the candidate standard.
n = The number of measurements in the two point (zero and span checks) calibration = 6.
n' = The anticipated number of measurements of the individual standard = 3.
Y' = The mean analyzer response from measurement of the individual standard = 2.1081.
X' = (Y' - b^/^ = The regression-corrected response of the individual standard = 2.0995345.
(1) Choose a for a desired 100 (1 -
a)%
Confidence level = 95 %
confidence level (e.g., a = 0.05 for
a = 5%
95-percent confidence).
(2) Look up t (1. ^ n_2) from a table of
Student's t-distribution.
t(1 - a/2, n-2) = 2.776
(3) Obtain b,,^ , S /x
Table 2-6.
, and Sxx from
b< = 1.0006217
Sk = 7.62 x 10'7
SL = 0.0049983
SI = 32.776803
(4) Calculate C »
C= 1.0012380
bl " (^1 - a/2, n-2})2 Sb,
(5) Calculate the 100(1 - a)% uncertainty
for the regression-estimated response
Uncertaintv = 0.0097353
n-2) Sy/X
c
CT -Y)2 A
Sxx
fl 11
+ ¦ ¦
n n'
V
C
(6) The interval X'-uncertainty to
X'+uncertainty is a 100(1-a)%
confidence interval for the response that
would have been observed at the time
of initial calibration.
X'- uncertaintv = 2.089799234
X' + uncertaintv = 2.1092698
A-5
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before comple
1. REPORT NO, 2,
EPA/600/R-93/224
3.
4, TITLE AND SUBTITLE
EPA Traceability Protocol for Assay and Certification
of Gaseous Calibration Standards
5. REPORT DATE
^December- 1993
6. PERt-OHMING ORGANIZATION CODE
" A
7. AUTHOR(S)
Robert S. Wright, Robert W. Murdoch, Michael J. Messner
8. PERFORMING ORGANIZATION REPORT NO.
RTI/6960/208-Q1F
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Research Triangle Institute
P.O. Box 12194
Research Triangle Park, NC 27709
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-D1-0009
12. SPONSORING AGENCY NAME ANO ADDRESS -
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
13. TYPE OF REPORT ANO PERIOD COVERED
User's Guide
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The EPA Traceability Protocol revision (September 1993) permits producers and
users of gaseous standards and other analytical laboratories to establish
traceability between commercial gaseous reference standards and Standard Reference
Materials (SRMs) supplied by the National Institute of Standards and Technology
(NIST). The protocol consolidates the 1987 Protocol 1 for continuous emissions
monitors (CEMs) and Protocol 2 for ambient air monitors into a single protocol.
The certification period for most standards has been extended and there are changes
in some of the required documentation. The protocol permits gravimetric calibration
of permeation tubes. The protocol incorporates many advances in gas blending and
dilution technology and unifies regulatory requirements of standards for acid rain
and CEMis.
17. KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
c. cosati Field/Croup
'
•
18. DISTRIBUTION STATEMENT
19. SECURITY CLASS (This Reporti
21. NO. OF PAGES
RS
20. SECURITY CLASS (This page)
22. PRICE .
CPA. Form 2220-1 (R.V, 4-77) previous coition is obsolete
------- |