EPA-600/7S1-010
Revised November 1989
A PROCEDURE FOR ESTABLISHING TRACEABILJTY OF GAS MIXTURES TO
CERTAIN NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY* SRM'S
by
E.E. Hughes
Center for Analytical Chemistry
and
IMandeJ
National Measurement Laboratory
rtional Institute of Standards and Technology
Garthersburg, Maryland 20899
Contract No, EPA4AGO&684
EPA4A&DW-13932911
Project Officer
D. vonLehmden
U.S. Environmental Protection Agency
Quality Assurance Division
Atmospheric Research and Exposure Assessment Laborator
Research Triangle Park, NC 27711
Revised by
W, L Zelirtski, Jt
Center for Analytical Chemistry
National Institute of Standards and Technology
Garthersburg, Maryland 2OS99
NBSIR 81-2227
*Formerty, National Bureau of Standards
U.S. DEPARTMENT OF COMMERCE
Robert A. Mosbacher, Secretary
Lee Mercer, Deputy Under Secretary
for Technology
MIST
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EPA-600/7-81-010
Revised November 1989
A PROCEDURE FOR ESTABLISHING TRACEAB1UTY OF GAS MIXTURES TO
CERTAIN NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY* SRM'S
by
E.E. Hughes
Center for Analytical Chemistry
and
IMandel
Natkmal Measurement Laboratory
National Institute of Standards and Technology
Gatthersburg, Maryland 20899
Contract No. EPA4AGO&E684
EPA4AGOW-13932911
Project Officer
D. von Lehmden
UJS. Environmental Protection Agency
Quality Assurance Division
Atmospheric Research and Exposure Assessment Laboratory
Research Triangle Park, NC 27711
Revised by
WLZelinskLJt
Center for Analytical Chemistry
National Institute of Standards and Technology
Gafthersburg, Maryland 20899
NBSIR 81-2227
'Formerly, National Bureau of Standards
OS. DEPARTMENT OF COMMERCE
Robert A. Mosbache* Secretary
LeeMeroar; Deputy Under Secretary
for Technology
NATIONAL INSTITUTE OF STANDARDS
AND TECHNOLOGY
John W. Lyons, Director
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ABSTRACT
A procedure is described by which the concentration of commercially
produced gas mixtures may be related to certain gas Standard Reference Mate-
rials (SRM's) currently offered by the National Institute of Standards and
Technology. The concentration of the gas mixture as a Certified Reference
Material (CRM), must, by definition, lie within one percent relative of the
concentration of a particular SRM to reduce the error involved in the compara-
tive analysis. Statistical treatment of the traceability process is given by
an example, as is the process by which a body requiring traceability can
evaluate the quality of the CRM. Procedures also are included for recertifica-
tion of CRM's, and for initiating the certification period from the date of
sale rather than from the date of CRM approval by NIST. Appendices are
included for the preparation of CRM's related to carbon monoxide, nitric oxide,
sulfur dioxide, propane, oxygen, and carbon dioxide SRM's.
DISCLAIMER
This" revised report has been reviewed by the Environmental Monitoring
Systems Laboratory, U.S. Environmental Protection Agency, and approved for
publication. Approval does not signify that the contents necessarily reflects
the views and policies of the U.S. Environmental Protection Agency, nor does
mention of trade names or commercial products constitute endorsement or
recommendation for use.
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FOREWORD
One important role of the National Institute of Standards and Technology
(NIST) is to provide those services necessary to assure data quality in
measurements being made by Federal, state, local, and industrial laboratories
participating in environmental measurement programs. The work at NIST is
conducted in appropriate disciplinary Centers (the Center for Analytical
Chemistry, the Center for Radiation Research, and the Center for Chemical
Technology). NIST activities address data quality assurance needs of air and
water monitoring programs. NIST efforts in support of data quality assurance
include:
Studies of the feasibility of production of Standard Reference Materi-
als (SRM's) which could be used for the verification of performance
audit samples for quality control programs or used for the calibration
of field and laboratory instruments.
The development and demonstration of new or improved measurement
methods, particularly when needed for the certification of SRM's.
The evaluation and dissemination of data on the physical and chemical
properties of effluents, products and raw materials of environmental
significance in energy production.
The provision of reference materials for the evaluation and validation
of monitoring methods.
This document was initially issued in May 1981, as one of the Interagency
Energy/Environmental Research and Development Series Reports prepared to
provide detailed information on the development of a NIST measurement standard
or method. The document provides a procedure by which commercial specialty gas
companies could produce gas mixtures as Certified Reference Materials (CRM's)
that would be directly traceable to certain NIST gas SRM's. These CRM's would
be accepted by the U.S. Environmental Protection Agency (EPA) as reliable
substitutes for SRM's in environmental monitoring programs required by EPA
regulations. EPA published an announcement in the Federal Register that such
CRM's could be used in place of SRM's. The traceability procedure described in
the May 1981 issue of this document was jointly reviewed and approved by NIST,
EPA, and the U.S. Compressed Gas Association.
The original issue of this document in May 1981 contained appendices for
the production of four types of gaseous mixtures, covering 43 different NIST
SRM's: CO in N2 or air (Appendix C); NO in N2 (Appendix D); S02 in N2 (Appen-
dix E); and propane in N2 (Appendix F). This revision to the document contains
appendices for two additional types of gaseous mixtures, covering 18 additional
NIST SRM's: 02 in N2 (Appendix H); and C02 in N2 (Appendix I). In addition,
this revised document includes a procedure (Appendix G) for extending the
certification period for existing CRM's and a mechanism by which a CRM producer
can certify a CRM from the date of sale, rather (as required in the May 1981
document) from the date of NIST approval of sale. The advantage of the
procedure for extending this certification period is that an approved CRM can
be recertified for additional incremental periods (each period usually being
ii
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two years), indefinitely, as long as the certified concentration of original
gas mixture in the cylinder has not changed, supported by reanalysis data
developed for each additional certification period. The advantage of the
mechanism for date of sale certification is that a producer of an approved CRM
can certify the concentration of the mixture for the full certification period
authorized for the CRM, at the time of its sale to a customer. This mechanism
eliminates the problem in selling a two-year certified CRM a year after its
approval with only a one-year certification period, as required in the May 1981
document. The three additional appendices included in this revised document
(Appendices G, H, and I) have been reviewed and approved by NIST, EPA, and
representatives of the U.S. specialty gas industry. Questions relating to this
revised document and its procedures and specifications, including the new
appendices, should be addressed to Walter L. Zielinski, Jr., William D. Dorko,
or George C. Rhoderick at NIST, or to Darryl J. von. Lehmden at EPA.
Ranee A. Velapoldi
Chief
Gas and Particulate Science Division
Center for Analytical Chemistry
National Institute of Standards and Technology
111
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CONTENTS
1. INTRODUCTION
2. CERTIFIED REFERENCE MATERIALS
2.1 Description
2.2 Preparation
2.3 Analysis
3. SUGGESTED ANALYTICAL PROCEDURE
3.1 Description
3.2 First Analysis
3.3 Second Analysis
3.3.1 Stability and Homogeneity
3.3.2 Estimation of the Uncertainty of the CRM
4. AUDIT OF THE CRM
4.1 Introduction
4.2 Environmental Protection Agency Audit Progress
4.2.1 Selection of Samples
4.2.2 Analysis
4.2.3 Report of Results
4.3 Examination of Results
4.4 Certification
ACKNOWLEDGEMENTS
APPENDICES
Appendix A. Statistical Analysis of Data
Appendix B. Report Forms
Appendix C. Carbon Monoxide
Appendix D. Nitric Oxide in Nitrogen
Appendix E. Sulfur Dioxide in Nitrogen
Appendix F. Propane
Appendix G. Extension of Certification (Recertification) and
Date-of-Sale Certification
a. Introduction
b. Procedures to Extend the Certification of a CRM
c. Analytical Procedures for Date-of-Sale Certification
d. Converting from Fixed Period Certification to
Date-of-Sale Certification
Appendix H. Oxygen in Nitrogen
Appendix I. Carbon Dioxide in Nitrogen
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1. INTRODUCTION
This, procedure is intended to increase the availability of accurate gas
standards by the creation of a series of secondary standards, to be referred to
herein as "Certified Reference Materials" (CRM's). The CRM's prepared accord-
ing to this procedure will be related, within known limits of uncertainty, to
specific gaseous Standard Reference Materials (SRM's) offered by the National
Institute of Standards and Technology (NIST). The CRM's are intended to
supplement the supply of existing SRM's, and are not intended to offer trace-
ability for gas mixtures at concentrations other than those of the complemen-
tary SRM's, nor for components of such mixtures that are not certified in the
SRM's.
The procedures described are based on experience at NIST relative to the
production and certification of gaseous SRM's, and are intended to assure the
development of reliable CRM's with reasonable effort, but are not intended to
reduce the effort to a minimum. The acceptance of the CRM's by regulatory
agencies, by industry, and by other users, will depend on the reliability of
the CRM's. The CRM's, in turn, will depend on the integrity of the gas
supplier relative to the methods, conditions, and limitations of the procedure
described here and on the reliability and extent of any complementary audit
program.
2. CERTIFIED REFERENCE MATERTAT.S
2.1 Description
The CRM's will consist of compressed gas samples in cylinders prepared in
lots of ten or more of identical concentration. The average concentration for
any lot must lie within 1.0 percent relative to the concentration of a specific
SRM, but preferably the concentration should be as close as technically pos-
sible to the SRM to reduce to a minimum any errors that might arise in sub-
sequent analyses of the CRM's. The concentration for a specific CRM will be
determined by analysis with a specific SRM and the uncertainty assigned to the
concentration of the CRM will consist of the uncertainty of the SRM and the
added uncertainty resulting from the intercomparison of the CRM to the SRM.
The period of time during which the concentration of the CRM is certified, and
the presence of any impurities of consequence will be reported by the supplier.
2.2 Preparation
Each CRM will be prepared as a homogeneous lot containing at least ten
(10) cylinders, all filled from a bulk mixture prepared in a single container
or in several containers ganged together in such a manner that delivery occurs
simultaneously from the several containers, or by use of a dynamic blending
system. The reagent gases with which the bulk mixture is prepared must be
analyzed for the major component(s) and for minor constituents of interest.
The concentration of the component of interest in the bulk mixture must lie
within 1 percent relative to the particular SRM which is being duplicated. The
cylinders into which the bulk mixture is transferred and which will become the
CRM's must be clean, of known compatibility and history, and must be treated to
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assure stability of the mixtures which will be contained. The cylinders will
be equipped with valves of appropriate material which conform to Compressed Gas
Association (CGA) recommendations for the particular gas mixture.
After filling, an "incubation" period must be allowed before final
analysis. This period will vary depending on the nature of the gas mixture and
is described in the appendices of this document.
2.3 Analysis
The analysis consists of a comparison of each CRM to the appropriate SRM.
The requirement that the CRM have a concentration within 1 percent relative of
the SRM reduces the error arising from the intercomparison and simplifies the
calibration procedure for the analytical instruments. In most cases, the
characteristics of the analyzer are such that the response of the instrument
over a small concentration range (1 percent relative or less) can be described
by the linear equation
y = mx + b
where y is the signal, m is the instrument sensitivity in signal units per unit
of concentration, x is the concentration, and b is a constant in signal units.
Consequently, it is only necessary to determine that the sensitivity does not
change significantly over the interval between the concentration of the CRM and
the SRM to compare the two signals directly. It follows then that the closer
in concentration that the CRM is to the SRM, the less possibility for error
exists in assigning a value to the CRM.
The general characteristics of the analyzer response relative to con-
centration is determined by constructing a calibration curve using the SRM
against which the CRM is to be compared as one calibration point, and at least
two other gas mixtures for a minimum of three calibration points. These
additional calibration gases should be SRM's, one higher in concentration and
one lower in concentration, as compared to the SRM which is being used to
analyze the CRM. Obviously, this is not possible when the CRM is being
compared to either the highest or lowest concentration SRM in a particular
series. In this case, the CRM should preferably have a concentration slightly
lower than the SRM, if it is the highest SRM in a series, or slightly higher if
the SRM is the lowest in a series. If the SRM is the lowest in a series, the
use of a "zero" gas may be feasible. However, either the use of a zero gas or
extrapolation beyond the calibration curve established with SRM's is not
recommended without technical justification.
The ideal case where the instrument response over a wide range is directly
proportional to concentration can be readily recognized by measuring the signal
generated by each of the three SRM calibrating gases and dividing each by its
certified concentration to obtain the sensitivity (m) at the three concentra-
tions. If the response is linear and if the calibration line passes through
the origin, the value for m will be essentially the same at all three con-
centrations, and the equation reduces to y = mx. If m is constant over a wide
range of concentrations, then it should follow that it will be constant over a
smaller range. In this case, the concentration of the CRM is simply
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ICRM JCRM *SRM
CCRM ~ - CSRM or CCRM = - since m -
where I is the signal generated by either the SRM or the CRM and C is con-
centration.
In the somewhat less ideal case where the response is linear but does not
pass through the origin, the value of the signal divided by the concentration
will not be equal at the three concentrations but a straight line of slope m
and intercept b can be fitted to the data. It again follows that if m is
constant over a wide range of concentrations, it should be constant over a
small interval, and the concentration of the CRM is expressed as:
^RM'0 ^RM'b ISRM'b
-CCRM = - CSRM or CCRM = - since m = -
^RM'b m CSRM
A third case is that of an analytical instrument having a slight non- linear
response. In this situation, a calibration curve is constructed using three or
more (preferably, five) SRM's, and the concentrations are determined using the
derived equation for the curve. However, in most cases, the component in a CRM
that is being analyzed will be measured with an instrument which is essentially
linear over small ranges of concentration. In other words, the sensitivity of
the instrument - the response in signal units per unit of concentration - will
not change within the limits of the precision of the instrument over short
intervals of concentration. If the sensitivity is found to be essentially
constant over the internal of 1 percent of the concentration of the SRM that is
to be reproduced by the CRM, then the sensitivity measured for the SRM can be
used to calculate the concentration of the CRM. In this case, the closer the
CRM is in concentration to the SRM, the smaller will be the error introduced by
assuming short range linearity.
It is essential in analyzing the CRM's that the analyses be made with the
highest of degree of precision possible so that the uncertainty of the con-
centration of the CRM not be appreciably larger than the total uncertainty of
the concentration of the SRM with which it is compared.
The principal elements that contribute to the imprecision of this
comparison include instrument sensitivity and instability. The sensitivity of
the instrument should be high enough so that differences of concentration of
0.1 percent relative easily can be measured. Instruments are available that
are capable of measuring all current SRM's with this sensitivity. However,
most instruments exhibit drift to one degree or another. The drift has two
components, short term drift (or noise), and a long term drift characterized by
a slowly changing signal under constant operating conditions. The effect of
short term drift can be minimized by a number of techniques including the
multiple analyses of each sample, and signal averaging. The effect of long
term drift can be minimized by frequent calibration of the instrument. The
characteristics of each analytical system should be considered individually to
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determine the optimum frequency of calibration and the approach to compensation
or control of the effect of noise. However, it should be noted that the effort
expended in minimizing these effects will be reflected in the precision of the
analyses relating the CRM to the SRM. In general, if duplicate analyses of
single samples consistently agree within 0.1 percent relative, then the effect
of noise is at an acceptable level, and if repeated analysis of the same sample
performed at intervals during the analysis of a lot agree within the same
limits, then the long term drift is at an acceptable level.
3. SUGGESTED ANALYTICAL PROCEDURE
3.1 Description
The analytical procedure recommended here is considered to be the minimum
effort necessary to adequately reduce the possibility of accepting an unstable
CRM. All CRM's must be stable, and each batch should be homogeneous. An
unstable batch must be rejected, while an inhomogeneous batch may be accep-
table, providing that the reason for the inhomogeneity is known and further
providing that the cause of the inhomogeneity will not result in a future
change in concentration not revealed by the test for stability.
The object of the analysis is to compare a lot of at least ten samples of
presumably identical concentration to an SRM and to express the concentration
of each sample in the lot with error limits defined by the uncertainty of the
SRM and the uncertainty added by the analytical procedure. Obviously, the
simplest procedure would involve direct analysis of the CRM with the SRM, but
with most analytical methods this approach would be unduly extravagant in terms
of SRM consumption. Consequently, it is recommended that the concentration be
determined on a relative basis by comparing one sample from the lot of all
others in the lot. Thus, only the one sample is compared to the SRM and the
concentration of all others in the lot can then be determined from the measured
ratio of each to the one sample. An additional uncertainty is introduced in
this procedure but the size of this uncertainty can be reasonably small, and
the final uncertainty in the CRM will not be increased significantly.
The sample from the lot with which the rest of the lot is to be compared,
the lot "internal standard", is selected at random from the lot. The signal
generated by the internal standard is compared to the signal generated by each
sample. The internal standard is repeated throughout the sequence of analysis
so that any long term instrument drift can be compensated for, if necessary.
The resulting data can be expressed as the ratio of the signal generated
by individual CRM's to that generated by the internal standard. The ratio in
all cases should be very close to 1.000 if the lot is homogeneous.
3.2 First Analysis
An initial analysis of the batch of CRM's should be performed as soon as
practical after the preparation of the CRM's. A sample is selected at random
from the lot to serve as the lot "internal standard" to which a number of
samples from the lot are compared. At least ten samples selected at random
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from each lot should be analyzed during this first analysis. However, if the
lot is small (<20 samples) it is recommended that all samples be analyzed.
The internal standard should be analyzed against the appropriate SRM
during the first analysis to confirm that its concentration lies within 1
percent relative to the SRM and to establish a reference for later analyses
intended to define the stability of the lot. The internal standard should be
analyzed at least ten times using SRM's to calibrate the instrument. Each of
the remaining nine or more samples selected is then analyzed in duplicate,
using the internal standard as a reference.
Some preliminary conclusions regarding the homogeneity and stability of
the batch may be drawn from the results of this first analysis. For instance,
if the batch was inhomogeneous when transferred and if no subsequent reactions
have occurred randomly in the cylinders, the concentration of all samples
analyzed should be the same within the limits of precision of the analytical
method. The appropriate statistical treatment at this step is given in the
appendices. If the statistical test shows lack of homogeneity among the
samples, it is not possible at this time to determine whether such inhomogene-
ity is due to improper preparation or to reaction in the cylinders. If the
test shows reasonable closeness among the results for all samples, then there
is some assurance that the batch is homogeneous.
If the analyses at this point indicate that no serious problem exists, the
entire batch is set aside for the required "incubation" period. If, however,
the batch appears to be somewhat inhomogeneous the analyses should be continued
to include all samples in the batch. An analysis of each sample at this time
may become critical later in differentiating inhomogeneity resulting from
preparation and inhomogeneity arising from instability.
3.3 Second Analysis
After the requiring incubation period has passed, all samples in the lot
are analyzed, including those analyzed during the first analysis. The first
analysis served to define the concentration of the CRM and to test the
stability and homogeneity of the lot on a qualitative basis. Final decisions
regarding these factors depend on the second and more extensive analysis.
Sufficient time will have elapsed so that reactions in individual cylinders,
which were of too low a rate to be recognized by the distribution of concentra-
tions obtained in the first analysis, will be revealed by this subsequent
analysis. Decisions regarding homogeneity, which may have been based on
analysis of a portion of the batch, will now be based on results obtained on
the entire batch.
3.3.1 Stability and Homogeneity
The stability of the lot is assumed if the concentration in each CRM
cylinder included in the first analysis has not changed appreciably at the time
of its second analysis (see appendix A). An inspection of the data of the
first and second analyses may reveal gross instability, and, if such is found,
the lot should be discarded and remade.
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In a lot consisting of cylinders prepared by transfer of a single bulk
mixture to a number of smaller containers, the composition of the gas in all
the smaller cylinders should be identical. Differences observed among
cylinders should reflect only the random error in the analytical process.
Differences greater than this random error arise from two sources. First,
there are the differences that result from faulty mixing or from dilution by
residual gases left in the cylinder prior to transfer of the bulk mixture. The
second and more critical source arises from reactions, chemical or physical,
that occur in the cylinder after transfer. Because these reactions are largely
due to wall effects and because no two cylinder walls are alike, the reactions
generally proceed at different rates in different cylinders. The two effects
may be differentiated by sets of measurements made over a period of time
sufficiently long so that the extent of reaction is greater than the error in
the analysis. For instance, if a lot is analyzed immediately after preparation
and individual samples are found to have concentrations that vary more than can
be accounted for statistically by the analytical imprecision, then the mixture
is either inhomogeneous or some or all samples in the lot are unstable. If the
lot is reanalyzed after some time and each sample has retained essentially its
previous value, then the samples are probably stable, but the lot is inhomo-
geneous. If the concentration of one or more of the samples has changed
significantly, then it is plausible to conclude that the lot is unstable.
An inhomogeneous lot need not necessarily be discarded, but an unstable
lot must be discarded. An inhomogeneous but stable lot in which the range of
values does not exceed 1 percent of the value of the SRM with which it is to be
compared may be used, providing that the reason for the inhomogeneity is
determined and that the cause does not otherwise affect the integrity of the
CRM. If the bulk container consists of several large cylinders ganged together
to allow simultaneous delivery, then some inhomogeneity may arise if the
concentrations in the several bulk containers differ slightly. In the special
case of large scale dynamic dilution systems where two components are mixed
under constant flow conditions and are then compressed into the cylinders in
the lot, some inhomogeneity may result, depending on the design of the delivery
system. If variations in concentration are observed in a lot prepared by this
method, the variation may be related to the position of individual cylinders on
the manifold, and it is therefore important to record the manifold position of
each cylinder for possible future reference.
It should be reemphasized at this point that all samples in a batch which
is not entirely homogeneous should be analyzed both during the first and second
analysis. It is essential in cases where the samples are stable but not
entirely identical in terms of concentration, that each sample should be
certified separately.
The nature of the distribution of the concentration of samples in a lot
may give significant information concerning both the inhomogeneity and stabil-
ity of the lot. A lot which has concentrations that vary no more than can be
accounted for by the error of the analytical method, is probably homogeneous
and stable. A lot in which the range of values exceeds significantly what
might be expected on the basis of analytical error and in which several samples
are considerably below the average, probably is unstable. A lot in which the
range of values is large but all concentrations are essentially symmetrically
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distributed around the average, was probably not prepared or transferred in
such a way that homogeneity was assured. A lot may be found with concentra-
tions grouped around two or more average values (bimodal or multimodal dis-
tributions) . This usually results from errors in the gas blending or transfer
operations. It should be noted, however, that the detection of such situations
is, except for very large number of cylinders, statistically very uncertain.
Occasionally, a single sample in a lot will be found with a concentration
appreciably below the average for the lot. A single such sample probably
represents a fault in the preparation or filling of the cylinder and may be
omitted from the lot, providing the lot is otherwise stable and homogeneous.
The presence of more than one outlier in a lot or the occurrence of a single
outlier in several lots should be cause for concern, and, in the absence of a
reasonable explanation for the multiple outliers, the lots should be remade.
All of the evidence at this point must support the assumption that the lot
is stable., reasonably homogeneous, and that it meets all other requirements for
a CRM. If there is any suspicion that the lot may be unstable or excessively
inhomogeneous, then further analytical work should be conducted before a final
decision is made.
3.3.2 Estimation of the Uncertainty of the CRM
The estimated uncertainty of the CRM is composed of at least three
components: (1) the uncertainty of the SRM with which the CRM is analyzed; (2)
the imprecision of analysis of the internal standard with the SRM; and (3) the
imprecision of analysis of the CRM's with the internal standard. A discussion
of the statistical procedure for estimating and combining these three sources
of error will be found in the appendices. Here we merely mention that the
second component is evaluated on the basis of ten or more measurements of the
concentration of the internal standard, and that the third component is
evaluated from the variability of the results of all samples in the lot. From
the combined variability of these three sources, an upper limit for the
uncertainty of the concentration can be calculated for each individual CRM
sample, for use in the certificate accompanying the CRM.
4. AUDIT OF THE CRM
4.1 Introduction
At this point the producer should have sufficient evidence to make a
decision as to whether or not the batch will qualify as a CRM. This decision
must be based on the measured stability and homogeneity and on the fact that
the measured concentration of any sample in the batch lies within 1 percent
relative to the complementary SRM. The evidence for compliance with the
requirements for a CRM may be so overwhelming that a visual examination of the
data may convince the producer that all is well. However, the final decision
must await the results of an independent examination of the data and of an
independent analysis of at least some of the samples. This independent
analysis will be referred to as the "audit program". Upon its completion, the
data of the CRM producer and those resulting from the audit will be
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intercompared, using appropriate statistical procedures of data analysis which
are described in the appendices.
4.2 Environmental Protection Agency Audit Program
The purpose of the Audit Program is to determine whether or not the CRM's
in a particular lot are of the concentration claimed by the producer. This
will be determined by an analysis of two or more samples from the lot. The
analysis will be performed by a laboratory chosen by the Environmental Protec-
tion Agency (EPA). Analytical results obtained by the auditor and the CRM
producer will be submitted to NIST for evaluation. If the results of the
analysis by the auditor confirm the analysis claimed by the producer, the EPA
will be so informed by NIST. If the results do not agree, either the auditor
or the producer (or perhaps both), will be asked to repeat the analysis. If
the additional analysis demonstrates the validity of the producer's value, the
EPA will be so informed by NIST. If, however, there is still disagreement, the
samples used in the audit will be submitted to NIST for analysis. The auditor
may be required to analyze the CRM's for substances other than that for which
they are certified, depending on the requirements specified in the appendix
concerning the particular substance.
4.2.1 Selection of Samples
The producer will submit a list of samples identified by sample number and
cylinder number to the auditor. The auditor will select two samples at random
which the producer will then send to the auditor. The producer will inform the
auditor of the particular SRM which the CRM is intended to duplicate. This SRM
will be identified by SRM number, sample number, cylinder number and date of
purchase.
4.2.2 Analysis
The analysis of the CRM will be performed with an instrument whose
response characteristics are known at the concentration of the CRM. The
auditor will calibrate the instrument using SRM's, one of which must be of the
same nominal concentration as the CRM. The number of other SRM's used in the
calibration will depend on the characteristics of the instrument but should
include at least two others. The auditor must be able to demonstrate the
validity of the calibration of the instrument in the concentration range
between the SRM and CRM.
Each sample will be analyzed ten times with the calibrated instrument.
The procedure and sequence of analyses will be planned by the auditor and
should be such that a high degree of precision is obtained.
The average concentration for each sample and the standard deviation are
to be reported. In general, the precision of the methods used in the analysis
of gas mixtures of a type represented by the CRM should yield standard devia-
tions which do not exceed 0.5 percent relative. If this value is exceeded,
then the procedure should be carefully examined for sources of imprecision and
the analyses should be repeated. In calculating the average and the standard
deviation of the ten measurements for each sample, it is not permissible to
8
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omit or change any values (possible "outliers") except where experimental
conditions were known to be inappropriate during the measuring process.
4.2.3 Report of Results
Both the producer and the auditor will submit a comprehensive report of
their respective analyses to NIST. Each individual analysis result for each of
the samples analyzed should be reported. The producer will also report the
SRM's, identified by SRM number, sample number, cylinder number, concentration,
and purchase date, which were used in the calibration of the instrument. A
brief description of the method, the calibration procedure and the measured
instrument sensitivities at the calibration points will also be submitted.
The auditor will submit the individual result of each analysis on each of
the two samples, with information concerning the SRM's used, the instrument
employed, the calibration procedure, and any other analytical results obtained
as may be required for the particular mixture.
The report may be made on the forms shown in the appendices, or by a
letter containing the same information.
4.3 Examination of Results
The analytical data submitted by the producer will be examined by NIST to
assess the homogeneity and stability of the batch. The results reported by the
auditor will be used to confirm or reject the concentration claimed by the
producer. It is assumed that an unstable lot would be readily evident to the
producer and would not have been retained for processing as CRM's. It is
further assumed that an inhomogeneous lot would be recognized and would not be
retained except in the very special case where the reason for the inhomogeneity
was known and where the stability was unequivocally demonstrated. Such batches
will be individually considered and further analyses may be requested of the
producer.
4.4 Certification
If the particular batch of CRM's is reasonably homogeneous and definitely
stable, and if there is no significant difference between the concentration
claimed by the producer and that found by the auditor, both the EPA and the
producer will be notified by NIST. The producer may then certify the samples
in that particular batch as having met the requirements of a Certified
Reference Material that has traceability to the respective SRM.
To assure the purchaser of a CRM that it has been prepared according to
the procedures described herein, and that the results of the EPA audit and the
NIST analysis of the analytical data confirm the producer's concentration
within acceptable limits, the producer will supply the purchaser with the
following:
1. A copy of the letter from NIST to EPA and to the producer stating the
particular lot meets the requirements for CRM's. The NIST letter will
list each cylinder in the lot, identified by cylinder and sample
-------
number, and will show the producer's value for the concentration of
each sample in the lot. In addition, the letter will identify the
samples audited by EPA and will give the concentration determined by
the audit.
2. A producer's "Certificate of Analysis" will include, as a minimum, the
cylinder and the sample number, the concentration for the particular
sample, and the uncertainty assigned to that concentration. In
addition, the period of certification and the date of the beginning of
the period will be given.* Finally, the certificate will identify the
SRM used by the producer to establish traceability of the CRM to that
NIST SRM.
*The date of the beginning of the period of certification normally is the date
of the NIST letter to EPA and to the producer. However, the producer may
elect to follow the procedure given in appendix G for date-of-sale certifica-
tion. If appendix G is strictly followed, the date of the beginning of the
period of certification is the date of sale of the CRM by the producer to a
purchaser of the CRM.
10
-------
ACKNOWLEDGMENTS
Special acknowledgment is made to Ernest E. Hughes, the principal
scientist responsible for the development of the NIST gas SRM program, who led
the development and approval of the May 1981 document, and who recently passed
away. It is to his outstanding technical efforts that this revised document is
dedicated.
Appreciation is given to Robert Wright of Research Triangle Institute and
to Darryl J. von Lehmden of EPA for their thorough review of this revised
document.
Acknowledgment also is made to Robert Wright who has conducted the
analytical audits for EPA for the certification of CRM's since the initiation
of the CRM traceability program in 1981.
11
-------
APPENDIX A
Statistical Analysis of Data
I. Introduction
The data submitted to NIST by the producer and the auditor will be
examined in detail to determine that the candidate lot of CRM's is of the
concentration claimed and that the lot is stable and homogenous. The statisti-
cal treatment is illustrated in this appendix by means of an example based, for
the most part, on real experimental data. However, it was necessary to synthe-
size some'data, particularly in describing the auditor results, to produce an
example illustrative of the whole process. It should be noted that this
example does not define the degree of measurement precision required of a CRM,
and, in many cases, the precision shown may not necessarily be attainable, due
to such factors as instrument sensitivity and the chemical and physical
properties of the gases involved.
II. Illustrative Example and Statistical Analysis
1. Producer's Calibration Data:
The producer provided the following data on the calibration of his
instrument:
Table 1
SRM No. Concentration* Signal or Sensitivity
1677
1678
1679
9
44
97
.67 ±
.9 ±
.1 ±
.09
.5
.9
ppm
ppm
ppm
11.
54.
114.
51
10
86
mV
mV
mV
(signal)
(signal)
(signal)
*The ± values are the total uncertainties given in the SRM certificates.
A linear regression of signal vs. concentration provides the equation:
signal = .464 + 1.181 (concentration)
The fit to the straight line is good, and is only minimally affected by
making the intercept zero. This leads to the modified equation:
signal = 1.187 (concentration)
12
-------
The fit provided by this equation is shown in the following table where
the fitted values in the last column should be compared with the observed
values of signal:
Table 2
Signal
Observed
11.51
54.10
114.86
Fitted
11.48
53.30
115.26
We conclude that the instrument gives essentially a linear response and is in
good state of calibration. However, since the samples to be analyzed have a
concentration close to that of SRM 1678 (about 45 ppm), a sensitivity value can
be adopted that is based on a calibration line going through the origin and
through the point whose abscissa is the certified value of SRM 1678 and whose
ordinate is the measured value for this SRM. This gives the calibration
equation which will be used for all further analyses:
signal = 1.205 (concentration)
2. Analysis of Internal Reference Standard
One of the cylinders was randomly selected as the lot "Internal Reference
Standard." The measurements,* in mV are shown in the second column of Table 3.
The third column shows the concentration values, in ppm, obtained by using the
sensitivity 1.205 mV/ppm for the conversion.
The average concentration for the Internal Reference Standard is:
x = 44.6139 ppm
with a standard deviation among single replicate measurements of
sx = 0.018 ppm
The standard error of the average value for this cylinder is:
0.018
= 0.0057 ppm
13
-------
Table 3
Measure-
ment No.
1
2
3
4
5
Signal
(mV)
53.7623
53.7382
53.7454
53.7948
53.7719
Concentration
(ppm)
44.616
44.596
44.602
44.643
44.627
Measure-
ment No.
6
7
8
9
10
Signal
(mV)
53.7683
53.7454
53.7237
53.7707
53.7767
Concentration
(ppm)
44.621
44.602
44.584
44.623
44.628
*Note: It is not required that either producer or auditor provide the actual
measured value of the signal. However, the value of the concentration
calculated from the signal should be expressed with sufficient digits to
reflect the magnitude of the signal. In other words, don't round off the
calculated concentration when submitting the data for evaluation.
3. Analysis of All Samples
The Internal Reference Standard is now used by the producer to analyze all
the cylinders in the lot. This is generally accomplished by measuring the
ratio of the signal for the sample to that for the internal standard. Table 4
shows the signal ratios, denoted as R, and the corresponding calculated con-
centrations, denoted as C, using the value 44.6139 ppm for the internal
standard. Thus, each concentration is obtained by the equation:
C = R (44.6139)
where
R =
signal for sample
signal for internal standard
The last column of Table 4 is used for two purposes: a) to obtain an addition-
al estimate of the standard deviation among replicates, and b) to test whether .
a significant systematic shift has occurred between the two sets ("first
analysis" and "second analysis"). The estimate of the standard deviation,
converted to a single measurement basis is 0.021 and is consistent with that
obtained previously (s = 0.018) [see section 2, preceding]. As to a possible
shift, there is no evidence for such an occurrence. A test of significance can
be carried out as follows:
t =
-0.0048
0.030/yiO
14
= -.50
-------
This value is not significant, when compared with the critical value of
Student's t, for 10-1 = 9 degrees of freedom.
The standard deviations, 0.021 and 0.022, for the two sets of values in
Table 4 are mutually consistent. Moreover, since they are of the same order of
magnitude as the measurement error (as derived from replicate measurements on
the same sample), it may be concluded that no measurable heterogeneity exists
between the cylinders of this lot.
Table 4
Sample
5
6
11
13
15
21
23
29
30
31
33
34
35
39
45 -
46
48
49
50
51
52
Average
Std. dev.
First Analysis
R C
1.000177
1.000102
1.000904
0.999906
0.999350
0.999520
0.999563
0.999514
0.999881
0.999452
44.622
44.618
44.654
44.610
44.585
44.592
44.594
44.592
44.609
44.589
44.6065
0.021
Second Analysis
R C
0.999601
1.000592
1.000592
1.000406
1.000392
0.999482
0.999807
1.000438
1.000368
1.000353
1.000307
1.000050
1.000480
1.000210
1.000050
0.999415
0.999168
0.999800
0.999304
0.999060
0.999321
44.596
44.592
44 . 640
44.632
44.631
44.591
44.605
44.633
44.630
44.630
44.628
44.616
44.635
44.623
44.616
44.588
44.577
44 . 605
44.583
44.572
44.584
44.6099
0.022
Calculated
Concentration
Difference
Between
Duplicates
0.026
-0.014
0.049
-0.020
-0.043
-0.024
-0.029
-0.024
0.026
0.005
-0.0048
0.030*
*Since the numbers in this column are differences of two measurements, the
standard deviation found for this column is J2 times that for single mea-
surements. Consequently, the standard deviation for single measurements
derived from the differences is (0.030)/,/2 = 0.021. This values does not
include possible variability between samples.
15
-------
The best average value for the concentration of the lot is:
(44.6065 x 10) + (44.6099 x 21)
= 44.6088
10 + 21
The standard error of this overall average is:
0.022
731
= 0.0040
This standard error does not include calibration error, or errors in the
value of the SRM and in the value of internal reference standard.
4. Auditor's Calibration Data
Table 5 shows the auditor's calibration results.
Table 5
SRM No.
1677
1678
1679
1680
*The ± values are the
Concentration* Signal (counts)
9.79 ±
45.3 ±
97.1 ±
476. ±
.09
.5
.8
.4
total uncertainties given in the
19,688
91,642
197,990
979,130
SRM certificates.
The sensitivities (signal/concentration) are successively:
2011, 2023, 2039, and 2057 counts/ppm.
These values indicate a trend due, either to the presence of a blank or to
curvature, or to both.
A regression analysis shows a slight amount of curvature, but otherwise
the calibration data appear satisfactory. Since the samples are of the order
of magnitude of SRM 1678, the latter will be used for conversion of signal to
concentration, through the equation:
signal = 2023 (concentration).
16
-------
5. Auditor's Sample Measurements
The auditor made 10 replicate analysis on each of two cylinders, no. 48
and no. 29. The results are shown in Table 6.
Table 6
Signal
89866
90044
89495
89570
90024
89997
89568
89708'
89742
89852
Average
Std. dev.
6.
Cylinder 48
Concentration
44.422
44.510
44.239
44.276
44 . 500
44.487
44.275
44 . 344
44.361
44.415
44.3829
0.099
Evaluation of Uncertainties and
a. Inter_nal Conmarison of
Cylinder 29
Signal Concentration
89781
90121
89874
90050
89558
89874
90226
89888
89987
89655
Intercomparison
Auditor's Resul
44.380
44.548
44.426
44.270
44.270
44.426
44.600
44.433
44.482
44.318
44.4396
0.101
of Results
ts
The standard deviation of a single measurement made by the auditor is
0.10. The results for cylinders 48 and 29 may be compared by Student's £-test:
44.3829 - 44.4396
t = = -1.27
0.100
10 10
17
-------
The t-value is not significant. There is, therefore, no evidence of
heterogeneity between the two cylinders.
b. Total Uncertainty of Producer's Values
The value of each sample obtained by the producer is obtained by a
procedure represented by the following diagram:
SRM
1
Calibration
Factor
If
Internal Reference
Standard
tf
Sample
I
Calibration A B
The total error in the sample value is composed of four parts:
1) the uncertainty in the SRM value
2) the uncertainty in the calibration experiment
3) the uncertainty due to comparison A
4) the uncertainty due to comparison B
We use the rule (derived from the law of propagation of errors) that the
square of the relative error of the final value is equal to the sum of the
squares of the relative errors of the components.
More specifically, if ct is the final concentration value obtained for a
particular sample (denoted by the subscript i) in the lot, we have (from step
B):
a, = R, CB., (1)
where Rt is the ratio of signals for sample i to the reference sample, and CRe£
is the concentration attached to the reference sample. But Cj^f is obtained in
step A by averaging ten values obtained each as
Signal ,,
k < '
where k is the calibration value derived from the calibration experiment. In
our case, k = 1.205. The average of the ten measurements may be described by
S
CR.f=- (3)
k
where S is the average of ten replicate signal values. Combining (1) and (3)
gives
18
-------
s
k
(4)
The value of k is obtained experimentally from a single signal value divided by
the concentration of the SRM. Thus we may write:
(5)
where C0 is the concentration attached to the SRM by the SRM certificate and S0
is the signal corresponding to it.
Combining (4) and (5), we obtain finally:
(6)
The law of propagation of errors gives:
(7)
The first term of the right side represents step B; the second term, step
A; and the third term the uncertainty of the calibration experiment itself.
The last term represents the uncertainty of the SRM used for calibration.
We now estimate these four components.
1)
(step B) is obtained from the last column of Table 4:
0.021
= 4.71 x 10'*
44.61
2) (step A) is obtained from the calculations derived from Table 3:
S
0.0057
= 1.28 x 10'*
44.62
(8)
(9)
19
-------
3) r (calibration experiment) has not been measured, but we can assume the
same precision as in step A for a single measurement:
°S0 0.018
= 4.03 x 10'*
(10)
S0 44.62
4) =- (uncertainty of SRM) is derived from the uncertainty stated in the SRM
Lo
certificate. This stated uncertainty is equal to two standard deviations.
Thus:
"C0 1 0.5
= 55.68 x 1C'
(11)
C0 2 44.9
Adding the squares we have:
r£i I2 r i
= (4.71)2 + (1.28)2 + (4.03)2 + (55.68)2 x 10-e
Ci J * -I
= 3140 x 10"8
Hence:
= ±56 x 10-
Since all CL values are approximately the same and equal to
ci = 44.62 ppm, we obtain
"ci = 56 x 10'* x 44.62 = ±0.25 ppm
(12)
We see that the predominant component of uncertainty, in this case, is that of
the SRM.
c. Comparison of Producer's and Auditor's Values
By a calculation similar to that above, we obtain, for the auditor's value
for a particular cylinder, say cj:
ct
ct
'ct
Ct
(13)
20
-------
Step A is not present for the auditor's data. The symbol ct represents a
count, and ct an average of ten counts; C* represents the value given by the
certificate for the SRM used in the calculation of the calibration factor. We
have:
o.io/yio '
44.4
0.10
44.4
0.25
45.3
= (7.12xlQ-4)2 + (22.52xlQ-*)2 + (55.19x10-*)2
= 3604x10'8 = (60x10-*)2 .
Since c* =44.4 ppm for both samples analyzed by the auditor, we have:
17 c* = 60x10'*x44.4 = ±0.27 ppm
We now obtain the following summary results (Table 7):
Table 7*
Sample
48
Sample
29
Producer
Auditor
44.58 ± .25
44.38 ± .27
44.63 ± .25
44.44 ± .27
*The ± values represent standard errors in this table.
It is apparent that the result obtained by the auditor for each sample is not
significantly different from that of the producer for the same sample. Thus,
the auditor's values in this case, substantiate those provided by the producer.
In general, it may be assumed that there is no significant difference
between the concentration claimed by the producer and that found by the auditor
if the following expression is satisfied:
< 2
21
-------
APPENDIX B
Report Forms
The following forms, A and B, are suggested report forms which will be
made available to both the producer and the auditor.
22
-------
-FORM A. Report of Analysis of Certified Reference Materials .
(Producer)
Prepared by: (Company name & address)
(Individual preparing report
& date)
Description:
Composition:
Corresponding SRM No.:
Date Blended:
Date Transferred:
Analysis
SRM's Used as Calibrants:
No. Sample No. Cylinder No. Concentration Purchase Date
1.
2.
3.
4.
5.
23
-------
FORM A (Continued)
Brief Description of Methods:
Brief Description of Calibration Procedure:
Calibration Data:
SRM No. Signal or Sensitivity (Identify)
1.
2.
3.
4.
5.
Results
Analysis of Internal Standard (if used)
Sample No. Cylinder No.
Concentration Date of Analysis
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Average s.d.
24
-------
FORM A (Continued)
Analysis of CRM's
Concentration on Date Shown
Sample No.
Cylinder No.
Average
Std. Dev.
25
-------
FORM B. Report of Analysis of Certified Reference Materials
(Auditor)
Prepared by:
Description:
Analysis:
SRM's Used as Calibrants:
No. Sample No. Cylinder No. Concentration Purchase Date
1.
2.
3.
4.
5.
Brief Description of Method:
Brief Description of Calibration Procedure:
Calibration Data:
SRM No. Signal or Sensitivity (Identify)
1.
2.
3.
4.
5.
26
-------
FORM B (Continued)
Analysis of CRM
Sample No. Sample No.
Cylinder No. Concentration Cylinder No. Concentration
Average Average
Std. Dev. Std. Dev.
27
-------
APPENDIX C
Carbon Monoxide
a. The Available SRM's are as Follows:
CO in N2
SRM No. Nominal Concentration
1677 10 ppm
2635 25 ppm
1678 50 ppm
1679 100 ppm
2636 250 ppm
1680 500 ppm
1681 1000 ppm
2637 2500 ppm
2638 5000 ppm
2639 1 %
2640 2 %
2641 4 %
2642 8 %
CO in Air
SRM No. Nominal Concentration
2612 10 ppm
2613 18 ppm
2616 42 ppm
The concentrations shown are in parts per million (ppm) by mole and in
mole percent. The actual concentration of a particular SRM may differ by more
than several percent relative from the nominal value and it is therefore
essential that the exact concentration of the SRM to be duplicated be known
before attempting the preparation of the CRM.
28
-------
The maximum permissible levels of impurities in the CRM are as follows:
Water Vapor 10 ppm
Methane 1 ppm (CO in N2)
Methane 5 ppm (CO in air)
Carbon Dioxide 5 ppm (CO in N2)
Carbon Dioxide 400 ppm (CO in air)
b. Cylinders
CRM's must be packaged in new aluminum cylinders or in used aluminum
cylinders that have been used exclusively for mixtures containing carbon
monoxide in nitrogen or air. The history of any cylinder to be reused must be
known and confirmation must be obtained by analysis that the contents have not
been diluted or contaminated. Cylinders to be reused must contain sufficient
gas from the last filling to allow an analysis. The concentration must lie
within ±1 percent relative of the original analysis. The cylinder must be
evacuated before refilling and the concentration of the CRM to be added must
lie within 2 orders of magnitude (±100% relative) of the concentration of the
CRM previously contained.
c. Preparation of Analysis
Prepare a lot of at least 10 cylinders all at identical concentration.
The concentration of carbon monoxide must lie within ±1 percent relative of the
certified concentration of the specific SRM cylinder with which the lot is to
be compared. Analyze a representative number of cylinders by direct comparison
to the SRM or by comparing cylinders in the lot to a single sample selected at
random from the lot (internal standard from the lot). Analyze each sample at
least two times. Calculate the average of all results for all samples and
calculate the standard deviation of the average.
The lot should be put aside to "incubate" for at least one month after
which time all samples in the lot are analyzed. The results for the first and
second analyses should be statistically identical. If the two analyses are the
same, then it may be assumed that the lot is homogeneous and stable. If the
analyses were performed by direct comparison to the SRM, then the analytical
value for the sample will be the certified value. If the analyses were per-
formed by comparison to an internal standard for the lot, then it will be
necessary to determine the concentration of the internal standard. This is
done by repeated comparison of the internal standard to the SRM. The con-
centration measured for the internal standard is then multiplied by the
measured ratio of each sample.
The uncertainty of the CRM is composed of the uncertainty of the SRM and
any random errors introduced by the analysis of the CRM. The error of the SRM,
ESRM, is given on the certificate for the particular SRM cylinder and is
defined as the estimated upper limit of the total uncertainty. The estimated
upper limit of the uncertainty for the CRM should be calculated as follows:
29
-------
E2 (Total Error) = 2 | ^LEH | + a2 + b2 + c2
where E2 is the uncertainty of the CRM; a is the imprecision of intercoraparison
of the internal standard with the lot; b is the imprecision of intercomparison
of the internal standard and the SRM; and c is the error of the calibration
experiment if applicable (see appendix A).
The following table lists the methods of analysis that are applicable to
the analysis of CRM's of carbon monoxide in nitrogen or air. The method chosen
is not necessarily limited to those shown providing that the method chosen is
of adequate sensitivity, precision and linearity. The precision and
sensitivity are evaluated during the analysis described earlier and are
reflected directly in the imprecision of intercomparison. The linearity must
be defined in the concentration range between the CRM and the SRM.
Table 1. Methods for the Intercomparison of CRM's and SRM's of Carbon
Monoxide in Air or Nitrogen
Method Concentration of CO
Gas Chromatography - TC Detector 8% - 0.5%
Gas Chromatography - Ultrasonic Detector 1% - 0.05%
Gas Chromatography Followed by Catalytic
Reduction to Methane with Flame
lonization Detection 1% - 10 ppm
Non-dispersive Infrared 8% - 10 ppm
d. Period of Certification
Experience at NIST indicates that carbon monoxide mixture contained in
aluminum cylinders are stable for periods of three years or more. If no change
in concentration of the CRM's is indicated by the results of the producer's
first and second analysis, then stability for a period of at least two years
from the date of the second analysis is assured with a high degree of
probability.
30
-------
e. Recertification (see appendix G for further details)
CRM's of carbon monoxide in nitrogen or air that have exceeded the period
of certification may be recertified by the original supplier according to the
following provisions.
1. The pressure remaining in the cylinder must be greater than 500 psi
(3.4 MPa).
2. The ratio of the signal generated by the CRM when compared to the
original internal standard must be the same as when originally measured
within the limits of uncertainty generated by both sets of
measurements.
3. The concentration of the internal standard, determined by comparison to
the SRM with which it was originally compared, must be the same within
the limits of precision of the comparison.
4. The concentration of the SRM must be known either by comparison with a
new SRM or the original SRM must have been recently recertified by
NIST.
5. If either the original SRM or the internal standard no longer exists,
then no recertification can be made.
6
. The uncertainty to be assigned to the recertified value must include
the added uncertainty, if any, of the recertification process.
31
-------
APPENDIX D
Nitric Oxide in Nitrogen
a. The Available SRM's are as Follows:
Nominal Concentration of NO
SRM Number ppm (molar)
2627 5
2678 10
2629 20
1683 50
1684 100
1685 250
1686 500
1687 1000
2630 1500
2631 3000
The concentrations shown are in parts per million (ppm) by mole. The
actual concentration of a particular SRM may differ by more than several
percent relative from the nominal value and it is therefore essential that the
exact concentration of the SRM to be duplicated be known before attempting the
preparation of the CRM.
The maximum permissible levels of impurities in the CRM are as follows:
Water vapor 5 ppm
Nitrogen dioxide <1% of the NO concentration
b. Cylinders
Nitric oxide mixtures will remain stable in aluminum cylinders which have
been treated to assure stability. The treatment must be such that the sample
remains stable with regard to adsorption, absorption, and desorption. Because
many methods are available to achieve stability, the choice of method is left
to the producer of the CRM, and confirmation that stability has been achieved
32
-------
will be based on the observed homogeneity of the batch and on the stability
analysis.
Aluminum cylinders that previously contained mixtures of nitric oxide in
nitrogen," either CRM's or other mixtures, may be reused. It is recommended
that if such cylinders are reused, the concentration of the new mixtures be
within one order of magnitude of the previously contained mixture (within 0.1
to 10 times). The history of any reused cylinder must be known, and cylinders
in which mixtures of nitric oxide were observed not to be stable must not be
used for CRM's. Cylinders to be reused must have a residual pressure greater
than one atmosphere of the mixture of nitric oxide in nitrogen before
preparation for use with a CRM's.
c. Preparation and Analysis*
Prepare a lot of at least 10 cylinders of identical concentration. The
concentration of nitric oxide must lie within ±1 percent relative of the
concentration of the specific SRM with which the lot is to be compared.
Analyze at least 10 samples from the batch if the batch contains more than 20
samples. All samples should be analyzed in batches of 20 or less. Analyze the
internal standard from the lot by comparison to the appropriate SRM's. Analyze
each sample at least two times. Calculate the average for the samples analyzed
and calculate the standard deviation of the average using,
Sd2
where d is the difference between a sample and the mean, and Nx is the number
of measurements, or the number of samples times the number of measurements per
sample.
The lot should be put aside to "incubate" for at least two months (60
days) after which time all samples in the lot are analyzed. The internal
standard should be reanalyzed at this time using the same SRM's for comparison
as were used in the first analysis. The results for the first and second
analyses should be statistically identical (see appendix A) . If the two
analyses are the same, then it may be assumed that the lot is homogeneous and
stable. If the analyses were performed by direct comparison to the SRM, then
the analytical value for the sample will be the certified value. If the
analyses were performed by comparison to an internal standard from the lot then
it will be necessary to determine the concentration of the internal standard.
This is done by repeated comparison of the internal standard to the SRM. The
concentration measured for the internal standard is then multiplied by the
measured ratio of each sample to the internal standard to obtain the
concentration of each sample .
*The reader should be familiar with the procedures of analysis given in the
body of this document, particularly Section 3, "Suggested Analytical
Procedure" and appendix A, "Statistical Analysis of Data".
33
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The uncertainty of an individual CRM must include the error of the SRM and
the added uncertainty due to the imprecision of the intercomparison.
The error of the SRM, ESRM, is given on the certificate for the particular
SRM cylinder and is defined as the estimated upper limit of the total uncer-
tainty. The estimated upper limit of the uncertainty for the CRM should be
calculated as follows:
E2(Total Error) = 2
+ a2 + b2 +
c
2
where E2 is the total uncertainty of the CRM; a is the imprecision of the
intercomparison of the internal standard with the lot; b is the imprecision of
the intercomparison of the internal standard and the SRM; and c is the
calibration error.
The recommended method of analysis for nitric oxide in nitrogen is
chemiluminescence. The instrument used must have adequate sensitivity and
precision in the concentration range represented by the SRM which the CRM is
duplicating and as many other SRM's at other concentrations which are necessary
to define the response curve of the instrument. A zero gas may be used as one
calibration point when the lower concentration SRM's (2627 and 2628) are being
prepared.
Some general considerations concerning calibration are given in the main
body of this document. These considerations apply to the calibration of most
gas analysis instruments and may be directly applicable to the calibration of
instruments for the analysis of nitric oxide. In any case, it will be the
responsibility of the producers of CRM's to assess and document the linearity,
stability, and precision of their instruments.
d. Special Considerations
Certain peculiar effects may be observed when analyzing batches of CRM's
consisting of nitric oxide in nitrogen by the procedure outlined in the main
body of this document. If a number of samples are analyzed by comparison to an
internal standard from the batch, the ratio of each sample to the internal
standard may be consistently less than 1.00. This effect has been observed for
nitric oxide mixtures and for certain other reactive gases. It is probably
related to the following: greater amounts of sample are removed from the
cylinder chosen as the internal standard than from each other sample in the lot
because the internal standard is used at frequent intervals throughout the
analysis of the samples. The phenomenon probably results from slight adsorp-
tion on the surface of the valve exposed to the sample inside the cylinder. A
very slight depletion of the nitric oxide probably occurs in the gas phase in
the vicinity of the valve and, because of the small quantity of gas removed
during analysis, the observed concentration will be slightly lower than that of
the bulk of the mixture remaining in the cylinder.
34
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If such a difference exists it may be obscured by the imprecision of the
results, in which case it is compensated for in the final assigned uncertainty.
However, if the degree of precision of the intercomparison of the samples with
the internal standard is high, the difference may be readily observable. In
this case, and if the difference is small (0.1 percent relative or less)
compared to an estimated total uncertainty of greater than 1 percent relative,
a slight correction should be applied. This can be done simply by multiplying
the final concentration of each sample by the reciprocal of the average of the
measured ratios for all samples. For instance, if the average of the measured
ratios is 0.9995, multiply the concentration of each sample by 1.0005.
If the precision of measurement of the ratios in high and the average of
all ratios is significantly less than 1.000, some further work will be neces-
sary to fully evaluate the problem. A simple approach is to reanalyze a large
number of samples choosing a different sample from the batch as the internal
standard. If the large difference is no longer observed then the problem may
have been with the first internal standard. If, however, the difference is
still evident then it is likely that the valves or perhaps the cylinders are
not compatible with the particular gas mixture and the mixture should be remade
using more appropriate containers.
e. Impurities
The impurity of greatest concern in nitric oxide mixtures is nitrogen
dioxide. Careful preparation of the mixture from analyzed reagents and the
exclusion of all oxygen from the system is necessary to maintain the nitrogen
dioxide at the low level required (<1 percent relative to the NO content of the
sample). This quantity of nitrogen dioxide can be measured with most chemi-
luminescent analyzers designed to measure, NO, N02, or NOX. The calibration of
such instruments to measure 1 percent relative or less of N02 in a mixture of
NO and nitrogen may be somewhat difficult. In general, however, if the three
components, NO, N02, and NOZ are measured and if the sum of NO and N02 is equal
to the NOX then the instrument is probably functioning adequately. If this is
not the case, then the instrument should be serviced and reevaluated.
It is possible to measure nitrogen dioxide in the presence of nitric oxide
by chemical means, but the details of such a procedure are complicated and
lengthy and will not be included here.
The nitrogen dioxide should be measured during both the first and second
analyses to be certain that no positive changes are occurring. Decreases in
nitrogen dioxide with time are expected and do not adversely affect the
samples. Increase in nitrogen dioxide, however, can only occur if there is a
substantial amount of oxygen in the sample, and such a case would be cause for
discarding the batch.
It is not necessary to measure the nitrogen dioxide in all samples. If
the concentrations of all samples in the batch lie within the required limits
and the nitrogen dioxide is found to be less than 1 percent relative to the
nitric oxide in ten percent or more of the samples, then it can be assumed that
the batch concentration of nitrogen dioxide is within limits.
35
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If a careful measurement is made of the nitrogen dioxide and if it is
initially found to be at a low relative concentration, then the probability
that any significant amount of oxygen is in the samples is quite low. If the
nitrogen dioxide then remains low or disappears the assumption that no oxygen
is present is probably valid.
The water content of the diluent nitrogen should be measured before
preparing the mixture, and several samples should be analyzed for water content
during the initial analysis. If the concentration is the same in the diluent
and the samples, it may be assumed that the sample cylinders and the transfer
systems were adequately dried before use.
The methods used and the results of analysis for impurities should be
reported along with the other required information for each lot of CRM's.
f. Period of Certification
Experience at NIST indicates that nitric oxide mixtures contained in
aluminum cylinders generally remain stable for periods of time greater than two
years. If no significant changes in the concentration of nitric oxide occurs
during the incubation period and if the batch is reasonably homogeneous to
begin with, there is a high probability that samples will remain stable for at
least two years.
g. Recertification (see appendix G for further details)
CRM's of nitric oxide in nitrogen that have exceeded the period of
certification may be recertified by the original supplier according to the
following provisions.
1. The pressure remaining in the cylinder must be greater than 500 psi
(3.4 MPa).
2. The ratio of the signal generated by the CRM when compared to the
original internal standard must be the same as when originally measured
within the limits of uncertainty generated by both sets of measure-
ments .
3. The concentration of the internal standard, determined by comparison to
the SRM with which it was originally compared, must be the same within
the limits of precision of the comparison.
4. The concentration of the original SRM must be known either by
comparison with a new SRM, or the original SRM must have been recently
recertified by NIST.
5. If either the original SRM or the internal standard no longer exists,
then no recertification can be made.
6. The uncertainty to be assigned to the recertified value must include
the added uncertainty, if any, of the recertification process.
36
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APPENDIX E
Sulfur Dioxide in Nitrogen
a. The Available SRM's are as Follows:
Nominal Concentration of S02
SRM Number ppm (molar)
1693 50
1694 100
1661 500
1662 1000
- 1663 1500
1664 2500
1696 3500
The concentrations shown are in parts per million (ppm) by mole. The
actual concentration of a particular SRM may differ by more than several
percent relative from the nominal value and it is therefore essential that the
exact concentration of the SRM to be duplicated be known before attempting the
preparation of the CRM.
The maximum permissible levels of impurities in the CRM are as follows:
Water vapor 5 ppm
Sulfur compounds other than S02 0.1 ppm
b. Cylinders
Sulfur dioxide mixtures will remain stable in aluminum cylinders that are
dry and which have been treated to assure stability. The treatment must be
such that the mixture remain stable with regard to adsorption, absorption, and
desorption. Because many methods are available to achieve stability, the
choice of method is left to the producer of the CRM and confirmation that
stability has been achieved will be based on the observed homogeneity of the
batch and on the stability.
Aluminum cylinders which previously contained mixtures of sulfur dioxide
in nitrogen, either CRM's or other mixtures, may be reused. It is recommended
that if reused, the concentration of the new mixtures be within one order of
magnitude of the previously contained mixture (within 0.1 to 10 times.) The
37
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history of any reused cylinder must be known and cylinders in which mixtures of
sulfur dioxide were previously observed not to be stable must not be used for
CRM's. Cylinders to be reused must have a residual pressure greater than one
atmosphere of the mixture containing sulfur dioxide before preparation for use
with a CRM.
c. Analysis-General
Several methods of analysis are applicable to the analysis of mixtures of
sulfur dioxide in nitrogen. These include gas chromatography non-dispersive
infrared, electrochemical titration, pulse, fluorescence, and flame photometry.
Most of these are relative methods requiring calibrating gases. Another
method, the "peroxide" method, is an absolute technique requiring no
calibrating gases.
Some general considerations concerning calibration are given in the main
body of this document. These considerations apply to the calibration of most
gas analysis instruments and are directly applicable to the calibration of
instruments for the analysis of sulfur dioxide.
The alternative to the relative instrumental methods, the peroxide method,
is a wet chemical method. It involves collection of the sulfur dioxide from a
mea-sured volume of the sample, in a solution of hydrogen peroxide. The sulfur
dioxide is oxidized to sulfuric acid which is subsequently titrated with stan-
dard sodium hydroxide solution. The method as described in the literature* is
not directly applicable to the analysis of SRM's or CRM's in the concentration
range from 50 to 3500 ppm. Consequently, we will describe the method in some
detail as adapted for the analysis of SRM's. (See section G, below).
d. Preparation and Analysis of the CRM's
Prepare a lot of at least 10 cylinders of identical concentrations of
sulfur dioxide in nitrogen. The concentration must lie within ±1 percent
relative of the concentration of the SRM with which the lot is to be compared.
Analyze at least 10 samples from the lot if the lot contains more than 20
samples. If the lot contains 20 or fewer samples, it is advisable to analyze
all samples. The lot should then be set aside to "incubate" for at least two
months (60 days), after which all samples in the lot are analyzed.
If an instrumental method of analysis is used, it will be necessary to
analyze the internal standard for the batch using the appropriate SRM's. This
analysis should be performed at the time of the first analysis and again at the
time of the second analysis. The analysis must confirm that no change in
concentration of the internal standard has occurred during this time and the
difference between the two values should be no greater than would be predicted
from the precision of the method. In such cases, the concentration assigned to
the internal standard is the average of the two separate sets of measurements.
*See for instance the description in "The Analytical Chemistry of Poisons,
Hazards, and Solvents", by Morris B. Jacobs, Interscience, 1941, p. 250.
38
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If the peroxide method is used for the analysis, the "true" value will
have been directly obtained for each CRM sample. However, it will be necessary
to confirm the analyses by also analyzing the appropriate SRM using the per-
oxide method. The value obtained by analysis using the peroxide method should
agree with the value given on the certificate for the SRM. If the difference
between these two values is small and is consistent with the expected errors of
the analysis, then a correction to the CRM values may be made to align the CRM
value with the certified value for the SRM. This assumes that the difference
between the two values results from an accumulation of small random errors. If
the difference is large, then the source of error must be identified and
corrected.
At this point, the producer of the CRM will have completed two analyses
and will have assigned a concentration to the CRM's. These data will allow
decisions to be made regarding stability and homogeneity, and will indicate
whether there is compliance with the concentration requirements for a CRM.
Details of the procedures of analysis are to be found in the main body of this
document, particularly in section 3, "Suggested Analytical Procedure" and in
appendix A, "Statistical Analysis of Data". In addition, the total error of
the CRM may be calculated from the various errors generated during the analysis
of the CRM. Details of the calculation of the error where instrumental methods
are used will be found in appendix A. In the special case of the peroxide
method, the error will be calculated as described in section G, paragraph 5 of
this Appendix.
e. Period of Certification
Experience at NIST indicates that sulfur dioxide mixtures contained in
aluminum cylinders generally remain stable for periods of greater than two
years. If no significant changes in the concentration of sulfur dioxide occurs
during the incubation period and if the batch is reasonably homogeneous to
begin with, there is a high probability that samples will remain stable for at
least two-years.
f. Recertification (see appendix G for further details)
CRM's of sulfur dioxide in nitrogen which have exceeded the period of
certification may be recertified by the original supplier according to the
following provisions.
1. The pressure remaining in the cylinder must be greater than 500 psi
(3.4 MPa).
2. The ratio of the signal generated by the CRM when compared to the
original internal standard must be the same as when originally measured
within the limits of uncertainty generated by both sets of
measurements.
3. The concentration of the internal standard, determined by comparison to
the SRM with which it was originally compared, must be the same within
the limits of precision of the comparison.
39
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4. The concentration of the SRM must be known either by comparison with a
new SRM or the original SRM must have been recently recertified by
NIST.
5. If either the original SRM or the internal standard no longer exists,
then no recertification can be made.
6. The uncertainty to be assigned to the recertified value must include
the added uncertainty, if any, of the recertification process.
7. The concentration determined by the peroxide method must agree with the
original value to within the combined limits of uncertainty of the two
measurements. The validity of the peroxide method must be confirmed by
analysis of the SRM.
g. Analysis - Hydrogen Peroxide Method
The sample is passed through an absorption train as shown in the
accompanying figure.
The sample cylinder is equipped with a metering valve, and a three-way
valve allows the flow to be set at the appropriate rate using the rotameter
before beginning the analysis. The midget impingers contain a hydrogen
peroxide solution. The wet-test meter measures the total volume of gas passed
through the train.
1. Reagents
a. Hydrogen Peroxide Solution
Prepare a 1.5 percent peroxide solution by diluting 50 mL of 30
percent H202 to 1 liter with distilled water. Using a pH meter,
adjust to a pH of 5.5 by adding dropwise 0.1N nitric acid. This
reagent is stable at room temperature for one month.
b. Standard Sulfuric Acid (0.06N)
Carefully add 5.0 mL of concentrated reagent grade sulfuric acid to 3
liters of distilled water. Standardize against approximately 0.310 g
portions of THAM (tris[hydroxymethyl]aminomethane). This solution may
be stored indefinitely.
c. Standard Sodium Hydroxide (0.05N)
Dissolve approximately 4.1 g of NaOH pellets in 2 liters of
freshly boiled distilled water. Pipette 50.0 mL portions of the
standardized sulfuric acid into a beaker. Add a stirring bar and,
with moderate stirring, titrate with the sodium hydroxide solution
to a pH of 7.0. Store this solution in a polyethylene bottle and
restandardize it bimonthly.
40
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2. Apparatus
a. Absorbers
Two standard midget impingers are connected in series with short
lengths of Teflon tubing.
b. Burettes
Several calibrated burettes will be needed depending on the
concentration of the sulfuric dioxide and the volume of the sample.
Minimum graduation should be 0.2 percent of the volume titrated.
c. Wet-Test Meter
A 1 liter per revolution wet-test meter is used to measure the volume
-of sample. The meter must be calibrated with a maximum inaccuracy of
0.25 percent relative. Details of a procedure by which this accuracy
may be accomplished will be found at the end of this appendix.
3. Procedure
a. Fill each impinger base with 10 mL of hydrogen peroxide solution.
b. Adjust flow to desired rate using rotameter.
c. Rotate 3-way valve to flush tubing and impinger head.
d. Attach second impinger followed by first impinger.
e. Stop flow when desired volume has passed through train.
f. Quantitatively transfer contents of both impingers to a 100 mL beaker
rinsing impinger tube and body with distilled water. Fill beaker to
50 mL level.
g. Titrate solution with standard base to pH 5.5.
h. Determine a reagent blank using 20 mL of peroxide solution.
4. Calculation
At the temperature, pressure, and concentration of the components of the
mixture the following may be assumed to be correct.
V M
S02 S02
S02 S02
VS02 + M
where V is volume and M is moles .
41
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The volumes of both species, S02 and N2, must be expressed at the same
conditions of temperature and pressure. The volume of nitrogen measured with
the wet test meter, for instance, is corrected to 25 "C and one atmosphere (760
mm or 101.3 KPa). The volume of sulfur dioxide is expressed at the same
conditions using the relationship:
1 milliequivalent NaOH = 12.235 pL S02
Volume NaOH (mL) x NHaOH x 12,235 = fiL S02
and
S02
= cone, in ppm
S02
106
The following table is included to give some guidance for the selection of
sample size, burette size and reagent concentration needed to analyze samples
of various concentration. Factors such as the concentration of the alkali and
the sample size may be adjusted to accommodate the skill of the operator or the
size of burettes available.
Table 1
Concentration Volume mL of Burette Volume
of Sulfur Dioxide of Sample 0.05N NaOH (mL)
50 ppm 15 L 1 5
100 ppm 12 L 2 5
500 ppm 15 L 11 50
1000 ppm 6 L 10 50
1500 ppm 3 L 7 50
2500 ppm 3 L 12 50
3500 ppm 3 L 17 50
5. Errors
The peroxide method is essentially an absolute method and the total error
should be definable by the random errors of the various elements of the
procedure. These sources of random error are as follows:
42
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1. Calibration of wet-test meter.
2. Calibration of burettes.
3. Imprecision of preparation of reagent solutions.
4. Imprecision of titration.
5. Analysis of reagent purity.
In addition to these sources of random error there may be biases, but
these biases most likely will be small relative to the random error.
In the absence of blunders, the major sources of error will be the
uncertainty of measurement of the sample volume with the wet-test meter and the
imprecision of the titration of the absorbed sulfur dioxide. Both of these
errors will be combined into the uncertainty of the measured value of the
concentration and will be represented by the uncertainty of the average of
replicate measurements of single samples. Reliable reagents carefully used to
prepare standard solutions and careful standardization will result in very
small errors in the chemistry preceding the titration of the absorbed sulfur
dioxide.
The magnitude of the random error and the possible existence of biases in
the system are evaluated by analysis of an SRM of appropriate sulfur dioxide
concentration. The SRM must be analyzed in triplicate, and the average of the
results should agree within experimental limits with the value appearing on the
certificate for the SRM. The following relationship should be satisfied if the
analysis of the SRM by the procedure has been done correctly:
IA - Bl < 2
[fl
+ b2
where A is the certified value of the SRM with the associated uncertainty, a,
(as given on the SRM certificate) and B is the average of the producer's
replicate analysis of the SRM with the associated standard deviation, b, of the
average of its determination.
If this relationship is not satisfied and the value of b is reasonably
small (<0.5 percent relative), then further work will be necessary to discover
the source of error. Consultation with the appropriate group at NIST is the
logical first step in resolving any conflict at this point.
If the NIST value and the producer's values agree, then it can be assumed
that no significant bias exists between the two values and the error of the
producer's value can be calculated from the observed random errors of the
measurement.
Stepwise, these include the following:
43
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a. The imprecision of standardization of the sulfuric acid based on
titration of at least three weighed portions of THAM. This error,
represented by the relative standard deviation of the average
measured normality of the acid includes both the weighing error
and the titration error.
b. The imprecision of standardization of the hydroxide solution
represented by the standard deviation of the average of replicate
titrations of the base with the acid.
c. The imprecision of measurement of the sulfur dioxide represented
by the standard deviation of the replicate measurements of the
sample. This uncertainty includes both the titration error and
the error associated with the repeatability of the volume
measurement.
d. The error in the measured volume represented by the uncertainty in
calibration of the wet-test meter.
These errors may be summed as follows:
E = 7az + b2 + c2 + d2
In general, the first two factors, a and b, associated with standardization of
solutions" should not exceed 0.1 percent relative. The third factor, c, which
is the imprecision of measurement of the sulfur dioxide, will depend somewhat
on the concentration but should not exceed 0.25 percent relative. The errors
in calibration of the wet test meter should be less than 0.25 percent relative.
Thus, the total error should be in the vicinity of ±0.5 percent relative, or 1
percent at the 2a level.
h. Calibration of Wet Test Meter
Minimum uncertainty in a volume measurement using a wet-test meter is
obtained when the conditions of the calibration duplicate the conditions of use
as nearly as possible. The procedure described here is intended to satisfy
that requirement. The method is basically very simple - a weighed quantity of
gas is passed through the wet-test meter and the volume indicated by the meter
is related to the measured weight of gas. The gas is contained in a cylinder
which is weighed. Gas from the cylinder is then passed through the meter and
the cylinder is once again weighed. The weight loss of the cylinder represents
the mass of gas passed through the meter. This mass can be expressed as a
volume if temperature and pressure are known. The only equipment needed other
than the wet-test meter is a high-capacity balance capable of weighing to
±0.1 g and a small cylinder equipped with a regulator. The weight of the
cylinder filled with gas and equipped with the regulator should not exceed the
capacity of the balance.
The cylinder is filled with air and nitrogen to a pressure that will allow
the release of at least 200 liters of gas at N.T.P. (normal temperature [25 °C]
and pressure [1 atm.]). The regulator is attached and the cylinder is
44
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connected to the wet- test meter. The outlet control valve on the regulator is
adjusted for the flow rate to be used in the analysis, and the cylinder valve
is then shut off without changing the adjustment of either the regulator or the
outlet control valve. The cylinder is then weighed and reattached to the
system. The zero reading of the wet-test meter is recorded, as is the
temperature and barometric pressure. The cylinder valve is opened and about 50
L of gas is allowed to flow through the meter. The cylinder valve is closed
and the cylinder is reweighed. This sequence should be repeated at least three
times .
Fifty liters of air or nitrogen will weigh somewhat over 50 g, and the
weighing error should not exceed about ±0.2 percent relative. The observed
weight loss of the cylinder represents the mass of gas passed through the
meter. This mass is expressed as a volume of gas, V , at the measured
temperature and pressure using the ideal gas relationship,
RT
V =
If the gas from the cylinder is dry and no saturation occurs except in the
wet test meter it will be necessary to reduce the volume indicated by the
meter, Vm , by a fraction representing the volume increase in the sample due to
uptake of water vapor, or more simply:
V = V
H20 "
H20
The water vapor correction (V,, ) is V. , where Pu is the
n2" " "2
saturation pressure of water at the temperature of the wet test meter and P is
the barometric pressure and, thus,
r pH2o i
vn = VB [ i + J
A simpler approach is to saturate the flowing gas between the cylinder and
the wet-test meter. In fact, the apparatus used for collection of sulfur
dioxide may be used in the calibration of the wet-test meter. The system as
shown contains a rotameter to facilitate setting the flow, and the impingers
can be used to saturate the gas, eliminating the need for water vapor
corrections. In this case, the relationship becomes:
V = V
8 »
If, however, the meter does not read correctly, a factor, a, should be
applied such that
a Vm = Vg or
PH20
1 + -
45
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Using this method, and using sufficiently large quantities of gas, it is
possible to calibrate a wet-test meter in place under identical conditions to
those which will be used in the analysis with an absolute uncertainty not
exceeding 0.25 percent relative.
46
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METERING
VALVE
ROTAMETER
BALL JOINTS
"-.
3-WAY VALVE
ii\x
IMPINGERS
SAMPLE
WET TEST METER
SAMPLING TRAIN FOR PEROXIDE METHOD
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APPENDIX F
Propane
a. Introduction
This, appendix describes the preparation of Certified Reference Materials
(CRM's) consisting of propane in nitrogen or propane in air. The reader should
be familiar with the general description of the preparation and analysis of
CRM's as described in the main body of this document.
b. Current SRM's
Following is a list of SRM's of propane in either nitrogen or air for
which CRM's may be made. The concentrations shown are in parts per million
(ppm) by mole and in mole percent. The actual concentration of a particular
SRM may differ by more than several percent relative from the nominal value,
and it is therefore essential that the exact concentration of the SRM to be
duplicated be known before attempting the preparation of a CRM.
C3H8 in Air
SRM No. Nominal Concentration
1665 3 ppm
1666 10 ppm
1667 " 50 ppm
1668 100 ppm
1669 500 ppm
C3H8 in N2
SRM No.
2643
2651
2652
2644
2645
2646
2647
2648
2649
2650
Nominal Concentration
100 ppm
100 ppm C3H8; 5% 02
100 ppm C3H8; 10% 02
250 ppm
500 ppm
1000 ppm
2500 ppm
5000 ppm
1%
2%
c.
Cylinders
CRM's must be packaged in new aluminum cylinders equipped with valves of
appropriate material which conform to CGA recommendations for these particular
mixtures. Aluminum cylinders previously used for propane CRM's or which have
48
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been used only for propane in air or in nitrogen mixtures may be used for
CRM's. The history of a reused cylinder must be known, and, in the case of a
CRM cylinder, an analysis must be performed of the gas mixture remaining in the
cylinder to assure that the contents have not been diluted or contaminated.
The concentration of the remaining gas mixture must lie within ±1 percent
relative of the concentration of the original CRM, as previously certified.
The cylinder must be evacuated before refilling and the concentration of the
new CRM gas mixture must lie within one order of magnitude (within 0.1 to 10
times) of the concentration of the previous CRM or other propane mixtures
previously contained.
d. Preparation and Analysis
Prepare a lot of at least 10 cylinders all at identical concentration.
The concentration of propane must lie within ±1 percent relative of the
concentration of the specific SRM with which the lot is to be compared.
Analyze at least 10 samples from the lot, if the lot contains more than 20
samples. If the lot contains fewer than 20 samples, it is recommended that all
samples be analyzed. Analyze the samples by direct comparison to the SRM or by
comparing samples in the lot to a single sample selected at random from the lot
(designated as the "internal standard" of the lot). Analyze each sample at
least two times. Analyze for impurities as required in Section E, below.
After this first analysis, the lot should be put aside to "incubate" for
at least two months after which time all samples in the lot are analyzed and at
which time the impurities are redetermined. If the two sets of analyses are
the same, then it may be assumed that the lot is homogeneous and stable. If
the analyses were performed by direct comparison to the SRM, then the analyti-
cal value for the sample will be the certified value. If the analyses were
performed by comparison to an internal standard for the lot, then it will be
necessary to determine the concentration of the internal standard. This is
done by repeated comparison of the internal standard to the SRM. The con-
centration of the internal standard must be determined at the time of the first
and second analysis by comparing it to the appropriate SRM's. The measured
concentration of the internal standard should be the same for both analyses
within the limits of error of the analyses. The concentration measured for the
internal standard is multiplied by the measured ratio of each sample to the
internal standard to obtain the concentration of each sample.
The uncertainty of the CRM includes the error of the SRM and the
uncertainty due to the imprecision of the intercomparison. The error of the
SRM, ESRM, is given on the certificate for the particular cylinder, and is
defined as the estimated upper limit of the total uncertainty. The estimated
upper limit of the uncertainty for the CRM should be calculated as follows:
E2 (Total Error) = 2
+ a2 + b2 + c
2
49
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where E2 is the estimated upper limit of the uncertainty of the CRM; a is the
imprecision of the intercomparison of the internal standard with the lot; b is
the imprecision of intercomparison of the internal standard and the SRM; and c
is the error of the calibration.
e. Analytical Method
Analysis of these CRM propane mixtures may be done by gas chromatography
or any other method specific for propane determination. The method chosen
should have adequate sensitivity and precision, and the nature of the instru-
ment response must be known for the concentration region bracketing the SRM and
the CRM. The precision and sensitivity are evaluated during analysis, and are
reflected directly in the "imprecision of intercomparison." The instrument
response characteristic is evaluated over the small concentration range between
the SRM and CRM by observing the response over a wider range using SRM's as
calibration standards.
f. Impurities
The major impurities in SRM's of propane in air or propane in nitrogen are
other organic compounds. The propane content of an SRM is certified, while the
concentrations of other hydrocarbons are given for information only. If a
method of analysis is used that is specific for propane (gas chromatography,
for example), then the presence of other hydrocarbons will not affect the
measurement of propane. However, if a non-specific method (such as using a
total hydrocarbon analyzer) is used, then it is essential that the type and
concentration of each other organic compound be known. Furthermore, the source
of the other organics must be known to assure future assessments of the
stability of the samples.
There are three major sources of organic contamination of SRM's or CRM's.
These include trace hydrocarbons, usually methane, present in the diluent air
or nitrogen, other aliphatic hydrocarbons that are present in the reagent
propane, and solvents, usually chlorinated compounds, which are used to clean
cylinders and valves and which are not always completely removed before
assembly of the cylinder. The first two sources generally do not create
analytical problems, provided that the total concentration of the hydrocarbon
is low and further provided that the specific hydrocarbons are identified. The
last source may cause serious problems if the concentration is high and if the
concentration increases with time as the material desorbs inside the container.
The aliphatic hydrocarbons in the CRM's can be measured by gas
chromatography, and their concentration and identity should be consistent with
the impurities in the reagent propane and in the diluent nitrogen or air (which
should have been analyzed before the preparation of the CRM's). The presence
of other organic material may be determined by exhaustive analyses utilizing
any number of trace analytical techniques, but if reasonable precautions were
used in excluding organic impurities from the preparatory process, a simple
test will suffice to assure that the only impurities are those originating in
the reagents. This involves comparing the hydrocarbons measured with a total
50
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hydrocarbon analyzer, with the sum of the aliphatic hydrocarbons individually
measured by gas chromatography. If the two values are the same or similar,
there is a high degree of assurance that the other organics in the sample are
aliphatic hydrocarbons originating from the reagent gases. If the value
obtained by total hydrocarbon analysis is significantly higher than the sum of
the compounds measured chromatographically, then a serious problem exists, and
either the batch of CRM's must be discarded or the source of the other organics
must be identified. Further, it must be determined that these organic
impurities will not create future analytical problems.
In general, the following requirements must be met in regard to
impurities:
1. The hydrocarbon impurities* must be less than one percent of the
propane.
2. Organic impurities other than aliphatic hydrocarbons must be less than
0.25 percent of the propane.
3. Neither the aliphatic hydrocarbons nor the other organics should
increase between the first and second analysis.
Compliance with these requirements can be confirmed by use of the
following procedure. This procedure is the minimum effort that should be
expended to characterize the impurities, and it is not intended to discourage
more strenuous efforts to identify the exact nature of the impurities:
1. Select at least five samples from the batch for analysis of total
other hydrocarbon [TOH] and total organics [TO]. This analysis must
be performed within 10 days after preparation of the batch, and again
sixty days after the first analysis.
2. Analyze the selected samples for total organics using a flame
ionization total hydrocarbon analyzer. Calibrate the analyzer with
SRM's of propane in air or propane in nitrogen, whichever are
appropriate. (The total concentration of hydrocarbon in the SRM,
[TH]SRM, is the sum of the propane and of the reported other hydrocar-
bon, expressed as propane, whereas the total organics [TO] is the sum
of the propane, the total other hydrocarbon [TOH], and any other
organic compounds, such as chlorinated hydrocarbons.)
3. Calculate the total organics [TO] in the CRM using the following
relationship:
*The concentration of each impurity is expressed as the equivalent
concentration of propane. When measured with a total hydrocarbon analyzer
calibrated with propane, the measured concentration of the impurities is
equivalent to propane. If the individual compounds are measured by
chromatography, the concentration of each is multiplied by -r where n is
the number of carbon atoms in the particular molecule.
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ICRM
= [TH]SRM j-
where i is the signal generated by the CRM or the SRM. The total
other hydrocarbon (TOH) in the CRM is simply the total hydrocarbon
expressed as propane minus the propane. ([TH] = [TOH] + Propane).
4. Analyze the CRM's for aliphatic hydrocarbons using gas chromatography.
The analysis must be such that hydrocarbons from Ci through C7 can be
quantitatively determined. Several different analyses may be required
to accomplish this.
5. Calculate the total hydrocarbon [TH] in the CRM by summing the
individual hydrocarbons after expressing each as the equivalent
propane concentration. This is done as follows:
[TH]CRM = 1/3[CHJ + 2/3[C2H6] + .... | [CnH2n + 2]
6. Compare the value for total organics measured with the hydrocarbon
analyzer to the total hydrocarbon measured by chromatography after
first subtracting the propane concentration from each. The concentra-
tion of other organics is at an acceptable level if the other organics
([TO] - [C3H8]) do not exceed the total other hydrocarbons ([TH] -
[C3H8]) by more than 25 percent or
([TO] - [C3H8]) < 1.25 ([TH] - [C3H,]).
Furthermore, the total organics must not exceed 1 percent of the
propane, or
[TO] < 1.01 [C3H8]
Finally, the value for total organics must not differ between the
first and second analysis in excess of what would be calculated from
the uncertainty of the two analyses.
In addition to the limit for the concentration of organic impurities, the
water vapor content must not exceed 5 ppm by mole.
g. Reporting Results
A complete report concerning the concentration of impurities will be
required in addition to the information concerning the concentration of propane
and the stability of the propane concentration. The report on impurities
should summarize the results obtained in steps 2 through 6 of the previous
section on impurities.
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h. Period of Certification
Experience at NIST indicates that propane mixtures contained in aluminum
cylinders are stable for periods of more than two years. If no change in
concentration of the CRM's is indicated by the results of the producer's first
and second analysis, then stability for a period of two years from the date of
the second analysis is assured with a high degree of probability.
i. Recertification (see appendix G for further details)
CRM's of propane in air or in nitrogen that have exceeded the period of
certification may be recertified by the original supplier according to the
following provisions.
1. The pressure remaining in the cylinder must be greater than 500 psi
(3.4 MPa).
2. The ratio of the signal generated by the CRM when compared to the
original internal standard must be the same as when originally
measured within the limits of uncertainty generated by both sets of
measurements.
3. The concentration of the internal standard, determined by comparison
the SRM with which it was originally compared, must be the same within
the limits of precision of the comparison.
4. The concentration of the SRM must be known either by comparison with a
new SRM or the original SRM must have been recently recertified by
NIST.
5. If either the original SRM or the internal standard no longer exists,
then no recertification can be made.
6. The uncertainty to be assigned to the recertified value must include
the added uncertainty, if any, of the recertification process.
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APPENDIX G
Extension of Certification (Recertification)
and Date-of-Sale Certification
a. Introduction
In Appendices C through F and Appendices H and I of this document, the
period of certification for a Certified Reference Material (CRM) has been
limited to two years from the date of the second analysis of its CRM lot by its
producer. This limitation is necessary in the absence of confirmatory
measurements proving long-term stability (>2 years). Short-term stability (up
to 2 years) of CRM's must be established before they can be certified, but
long-term stability can only be proven by periodic reanalyses after
certification. Guidelines for establishing short-term stability are given in
the main body of this document (see section 3.3.1). Changes which might not
affect the certified concentration over a two-year period may become evident
after more than two years.
The purpose of this appendix is to provide CRM producers with procedures
for extending the certification of CRM samples that either are unsold or are
still in use at the end of the original period of certification.
Two approaches may be taken to achieve this: (1) the first approach is to
extend the certification of all unsold (or sold samples, if returned to the CRM
producer for recertification) in a batch for a specified period following
reanalysis of these samples (see section b, below); and (2) the second approach
is to certify individual samples in a lot for a specified period, beginning at
the time of sale of each sample to a purchaser (see "date-of-sale" certifica-
tion; section c, below). The procedures to be followed to demonstrate long-
term stability are slightly different in each of these two approaches. Simply
stated, date-of-sale certification (section c) requires that the CRM producer
retain the samples for a longer period of time and perform a third analysis,
six months after preparation, before submitting data for approval of the lot as
CRM's, and, subsequently, perform additional reanalyses at 18 and 30 months on
samples of the lot remaining unsold.
This appendix also contains a procedure for converting existing lots of
CRM's (that had originally been limited to two years from the date of the
second analysis of the lot by the CRM producer) to date-of-sale certification
(see section d, below).
b. Procedures to Extend the Certification of a CRM
The certification period for a CRM may be extended if it can be
demonstrated that the concentration, on reanalysis, has not changed from that
of its initial certification. The period of recertification will begin at the
time of completion of the reanalysis and will continue for an additional period
of time not to exceed two years. A CRM may be recertified more than one time,
as long as the concentration, on reanalysis, has not changed from that of its
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initial certification. This section is written to describe the first
recertification of a CRM, which represents its third analysis (the first two
analyses being performed during the original certification of the CRM). Hence,
in the discussion given below, for subsequent recertifications of the same CRM
sample, the new recertification analysis is compared to the most recent
previous recertification analysis. The recertified CRM may be one or more of
the samples remaining unsold in a particular lot, or may be a sample that has
been returned by a purchaser for recertification.
The analyses performed during recertification must be sufficient to show
that the concentration of the sample has not changed within the limits of the
combined error of the original two analyses and the recertification analysis.
For CRM samples that are recertified more than once, the analyses performed
during any subsequent recertification must be sufficient to show that the
concentration of the sample has not changed within the limits of the combined
error of the original two analyses and all subsequent recertification analyses.
Any recertification analysis is performed in an identical manner to the
second analysis of the original certification (see section 3.3 in the main body
of this document), except that a smaller number of samples may be involved.
The statistical test to demonstrate that the concentration has not changed
between the second and the third analyses is the same as that which was used to
show that no change had occurred between the first and the second analyses.
The third analysis must be performed with reference standards that are
traceable to the same source as was used for the first and second analyses.
These standards must include one of the following combinations:
1. The original SRM and the original internal standard of the lot;
2. A new SRM and the original internal standard of the lot; or
3. The original SRM and a new internal standard of the lot.
Any SRM used in the recertification of a CRM must either be within the original
period of its certification by N1ST or must have been recertified by NIST and
still be within the period of its recertification.
The linearity and precision attained with the instrument used for the
third analysis must be similar to that attained with the instrument used in the
first and second analyses. The linearity in the concentration region between
the CRM and the SRM must be confirmed by the method outlined in section 2.3 of
the main body of this document and as illustrated in section II of appendix A.
The imprecision of the measurement method is evaluated by ten (10) replicate
measurements of the internal standard of the lot (e.g., see section II in
appendix A). The value of the imprecision thus obtained should be the same as
or smaller than that obtained during the first and second analyses.
The analyst should be aware of the effect of precision on the decision
concerning the stability of the samples being analyzed. Ultimately, a decision
must be made as to whether an observed difference between the second and third
analyses is real, or, if no difference is observed, whether the third analysis
55
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was sufficiently precise to reveal a difference. Thus, if the imprecision of
the third- analysis is the same as or smaller than that of the first or second
analyses, it will be possible to recognize small real differences in
concentration that have occurred during the interval between the second and
third analyses. However, if the imprecision of the third analysis is large
(i.e., two or more times that of the second analysis), then larger differences
in sample concentration values would need to be observed to conclude that a
group of samples was unstable (i.e., it would not be possible to state with
confidence that samples in a lot are stable and, hence, acceptable for
recertification, if the imprecision of the third analysis is large).
The analyst should not proceed with the analysis of the samples in the lot
if the imprecision of the analysis (evaluated by replicate measurements of the
internal standard, as stated above) or if the concentration of the internal
standard is questionable. The concentration of the internal standard should be
the same as measured during the second analysis. This can be determined by
observation if agreement is close and the imprecision is small, or, more
rigorously, by application of a simple "t" test of the form:
t =
-------
1. An obvious and unquestionable change in the concentration of some or
all samples in the batch has occurred.
2. No significant difference is found between the second and third
analyses.
3. A small but real change in concentration is revealed on application of
statistical tests for differences.
In the first situation, an obvious problem exists which requires further
action, probably of a drastic nature. In the second case, all is well and the
certification period may be safely extended. In the third case, we are faced
with deciding whether or not a difference is significant. That is, will a
statistically significant difference translate into an analytically significant
difference, considering the total uncertainty of the certified concentration
and the length of the period of certification. For example, suppose that the
second and third analyses have been executed properly and that the imprecision
of the measurement of the difference between the two sets is less than 0.1
percent relative. In principle, it should be possible to detect real differ-
ences of 0.1 percent relative between the second and third analyses. Because
of the nature of the changes which take place in compressed gases contained in
cylinders, a change of 0.1 percent relative over one year can be extrapolated
to a change of no more than 0.3 percent relative over three years. Therefore,
a 0.1 percent relative change will not compromise the analytical utility of the
sample, and may be safely ignored. Changes greater than 0.1 percent relative
per year, however, may or may not be acceptable with respect to extending the
certification period. A final decision may have to be based on factors in
addition to the results of the third analysis. Factors which might be con-
sidered are: (i) the results of the first and second analyses as compared to
the third^ (ii) the relative contributions of random errors and systematic
errors to the total error of the CRM; and/or (iii) the sign and nature of the
observed change.
Evaluation of the Data
The producer must submit to the NIST the results of the analysis of the
internal standard, the imprecision and linearity data, and the individual
results obtained for each sample for which extension of the period of cer-
tification is sought. NIST will examine these data and will decide whether or
not the results justify an extension of the period of certification. The
producer will be notified of NIST's decision in writing. If the decision is
negative, the producer will be supplied with the reason and a summary of all
statistical treatments of the data. In either regard, EPA will be notified of
the decision in writing and will be informed of any further action to be taken
either by the producer or by NIST.
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Consequence of the Third Analysis
If the evaluation of the data of the third analysis concludes that the
batch is stable or that observed changes will not become analytically sig-
nificant during the period of the recertification, then all samples analyzed as
part of the third analysis may be certified for two years or more. If changes
have occurred that can be extrapolated to become significant changes by the end
of the requested period of recertification, the samples cannot be recertified
and will remain in certification only through the original period of certifica-
tion. If the analysis reveals changes that are substantial and are considered
significant even during the original period of certification, then all samples
still within the certification must be recalled from the purchasers. However,
before a "recall" decision is made, NIST and the CRM producer will make every
effort to determine whether or not the change is real or if it is the result of
an analytical error committed at the time of the second or third analyses.
Since a recall could occur, a CRM producer should maintain records of the names
and addresses of all purchasers of CRM's.
c. Analytical Procedures for Date-of-Sale Certification
This section describes procedures that allow a CRM producer to certify an
individual CRM for a two-year period starting from the date of its sale to a
customer. These procedures differ slightly from those for regular CRM cer-
tification. The principal difference is that all samples in the lot must be
analyzed three times rather than twice before approval as a date-of-sale
certified CRM. An initial analysis of the lot should be performed as soon as
practical after its preparation, followed by reanalyses at 3 and 6 months after
its preparation. After the third analysis, the CRM producer should submit a
comprehensive report of the three analyses to NIST for review and approval.
The CRM producer should at this time also send two samples, randomly selected
by EPA from the lot, for an EPA Performance Audit. If the review of the
producer's and auditor's analytical results by NIST indicates that the batch
has satisfied the requirements of a CRM, NIST will notify EPA and the producer
by letter that the lot may be certified for sale. If reservations concerning
the stability or homogeneity of the lot arise as a result of the review of the
analytical results by NIST, an additional analysis may be required or,
alternately, the lot may be rejected.
The period of certification for each CRM sample in an approved lot will be
two years from the date of sale to a customer for those sales that occur within
two years after preparation of the lot. Individual CRM's may continue to be
sold after this two-year period with a two-year certification from date of sale
if additional analyses are performed at 18 and at 30 months. For these
analyses, only 20 percent of the lot (or at least 4 cylinders) need to be
reanalyzed. The results of the reanalysis should be submitted to NIST for
review.
During each analysis of the lot, ten (10) replicate measurements must be
performed on an internal standard from the lot. Each analysis must be per-
formed with reference standards, all of which are traceable to the same source.
This traceability can be established with:
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1. The original SRM and the original internal standard of the lot;
2. A new SRM and the original internal standard of the lot; or
3. The original SRM and a new internal standard of the lot.
Any SRM used as a reference standard must be within the period of original
certification or within the period of some subsequent recertification by NIST.
It should again be emphasized that the analyst should never proceed with the
analysis of the lot if the imprecision of the analysis or the concentration of
the internal standard is questionable.
The CRM producer's report to NIST should contain information concerning
the imprecision and linearity of the analytical method, the stability of the
internal standard, and the stability and homogeneity of the lot. The
statistical tests to determine the stability and homogeneity of a lot before
its certification are described in appendix A of this document. After
certification, the stability of the reanalyzed cylinders should be determined
as described in section B of this appendix (appendix G).
The CRM producer shall provide a "Certificate of Analysis" and a copy of
the certification authorization letter from NIST to EPA, to any purchaser of a
CRM that is certified from date of sale. The certificate shall include, as a
minimum, the sample number, the cylinder number, the certified concentration of
the CRM, the uncertainty assigned to the certified concentration, the date of
sale, and the period of certification. The certificate also must identify the
SRM used by the producer to establish the traceability of the CRM.
d. Converting from Fixed Period Certification to Date-of-Sale Certification
It should be apparent from the preceding narratives in this appendix that
there is little difference between the analytical requirements for a fixed
period certification (or extension of a certification through recertification)
and a date-of-sale certification. In fact, there is no reason why a lot
originally certified for a fixed period cannot be converted to date-of-sale
certification if the additional necessary analyses are performed and the
resulting data are made available for NIST review. These data must support the
conclusion that all samples in the lot are stable and can be expected to remain
stable for at least 4 years if no changes are observed during the first six
months. If the initial six-month observation period was extended to two years,
for instance, and no changes were observed, then the assumption that the lot
was stable and could subsequently be certified from date of sale would be
equally valid.
Thus, unsold samples remaining from a lot originally certified for a two-
year certification could be certified for subsequent date-of-sale certification
if all samples are analyzed and found to be stable (as specified in section C,
preceding). The results of this analysis would be treated in the same manner
as the third analysis of the date-of-sale certification described in section C
of this appendix. Samples from the lots old before this analysis could not be
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retroactively recertified unless they are reanalyzed at the same time as the
remainder of the lot.
A CRM producer who wishes to convert from fixed period certification to
date-of-sale certification should proceed as follows:
1. Analyze all remaining samples in the lot, being certain that either the
original analytical standards (SRM or internal standard) or acceptable
replacements are available.
2. Forward all data to NIST, together with a letter requesting conversion
of the remaining samples in the lot to date-of-sale certification.
NIST will examine the data for the reanalysis as well as for any previous
analyses, and will advise both the CRM producer and EPA as to whether or not
the request should be granted. If the data indicate that the request is
justified, NIST will inform the CRM producer and EPA by letter of its approval.
If deemed necessary, NIST may require either an additional analysis by the
producer, or that another pair of samples be submitted for an EPA Performance
Audit.
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APPENDIX H
Oxygen in Nitrogen
a. Introduction
This appendix describes the preparation of Certified Reference Materials
(CRM's) consisting of oxygen in nitrogen. The reader should be familiar with
the general description of the preparation and analysis of CRM's given in the
document, and the material in Appendix G for extending the certification of
CRM's and for date-of-sale certification of CRM's.
b. Current SRM's
Following is a list of SRM's of oxygen in nitrogen for which CRM's may be
made. The concentrations shown are in mole percent. The actual concentration
of a particular SRM may differ by more than several percent relative from the
nominal value and it is therefore essential that the exact concentration of the
SRM to be duplicated be known before attempting the preparation of a CRM.
SRM No. Nominal Concentration
2651 5.0 mole percent*
2652 10.0 mole percent*
2657 2 mole percent
2658 10 mole percent
2659 21 mole percent
c. Cylinders
The CRM's must be packaged in new aluminum cylinders equipped with valves
of appropriate material that conform to CGA recommendations for these part-
icular mixtures; alternatively, aluminum cylinders previously used for oxygen
in nitrogen CRM's or which have been used only for oxygen in nitrogen mixtures
may be used for these CRM's, provided that the history of a reused cylinder is
well known, and, in case of a CRM cylinder, an analysis must be performed of
the remaining gases to assure that the contents have not been diluted or con-
taminated. Note: The concentration of the remaining gas mixture must lie
within plus or minus one percent relative of the concentration of the original
CRM, as certified. In the case of reused cylinders, the cylinder must be
evacuated before refilling, and the concentration of the new CRM gas mixture
must lie within one order of magnitude (within 0.1 to 10 times) of the
concentration of the previous CRM or other oxygen mixtures previously
contained.
*SRM Nos. 2651 and 2652 also contain 0.01 mole percent propane.
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d. Preparation and Analysis
Prepare a lot of at least 10 cylinders all at identical concentration.
The concentration of oxygen must lie within ±1 percent relative of the con-
centration of the specific SRM with which the lot is to be compared. Analyze
at least 10 samples from the lot, if the lot contains more than 20 samples. If
the lot containing fewer than 20 samples, it is recommended that all samples be
analyzed. Analyze the samples by direct comparison to the SRM or by comparing
samples in the lot to a single sample selected at random from the lot (internal
standard from the lot). Analyze each sample at least two times. Analyze for
impurities as required in Section F.
After the first analysis, the lot should be put aside to "incubate" for at
least one month after which time all samples in the lot are analyzed and at
which time the impurities are redetermined. If the two sets of analyses are
the same, then it may be assumed that the lot is homogeneous and stable. If
the analyses were performed by direct comparison to the SRM, then the analyti-
cal value for the sample will be the certified value. If the analyses were
performed by comparison to an internal standard for the lot, then it will be
necessary to determine the concentration of the internal standard. This is
done by repeated comparison of the internal standard to the SRM. The con-
centration of the internal standard must be determined at the time of the first
and second analysis by comparing it to the appropriate SRMs. The measured
concentration of the internal standard should be the same for both analyses
within the limits of error of the analyses. The concentration measured for the
internal standard is multiplied by measured ratio of each sample to the
internal standard to obtain the concentration of each sample.
The uncertainty of the CRM includes the error of the SRM and the
uncertainty due to the imprecision of the intercomparison. The error of the
SRM, ESRM, is given on the certificate for the particular cylinder and is
defined as the estimated upper limit of the total uncertainty. The estimated
upper limit of the uncertainty for the CRM should be calculated as follows:
E2(Total Error) = 2
+ a2 + b2 + c
where E2 is the estimated upper limit of the uncertainty of the CRM; a is the
imprecision of the intercomparison of the internal standard with the lot; b is
the imprecision of the intercomparison of the internal standard and the SRM;
and c is the error of the calibration.
e. Analytical Method
Analysis of these CRM oxygen mixtures may be done by gas chromatography,
paramagnetic analyzers, or any other method specified for oxygen determination.
The method chosen should have adequate sensitivity and precision, and the
nature of the instrument response must be known for the region between the SRM
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and the CRM. The precision and sensitivity are evaluated during analysis and
are reflected directly in the "imprecision of intercomparion." The instrument
response characteristic is evaluated over the small range between the SRM and
CRM by observing the response over a wide range using SRMs as calibration
standards.
f. Impurities
The major impurity in SRMs of oxygen in nitrogen is argon. The oxygen
content of an SRM is certified while the concentration of argon is given for
information only. If a method of analysis is used that is specific for oxygen
(paramagnetic analysis for example) then the presence of argon will not affect
the measurement of oxygen. However, if a non-specific method is used, then it
is essential that the concentration of argon be known.
There are several sources of argon contamination in SRMs or CRM's. First,
contamination may be present in oxygen or the diluent nitrogen. In addition,
atmospheric leakage through cylinder valves or transfer system connections and
valves can contribute to argon contamination. The atmospheric concentration of
argon is approximately 0.93 mole percent. The argon in the oxygen, nitrogen,
and CRM's can be measured by gas chromatography. The argon concentration in
the CRM should be consistent with the impurities in the reagent oxygen and the
diluent nitrogen that were analyzed prior to the compounding of the CRM's. If
the concentration of argon in the CRM is consistent with the pure gases, it may
be assumed that the sample cylinders and the transfer systems were adequate.
The maximum concentration of impurities shall not exceed the following:
Hydrocarbons as methane 3 ppm
Water vapor 5 ppm
Total other impurities except argon 15 ppm
Argon in 2% 02/N2 30 ppm
Argon in 5% 02/N2 40 ppm
Argon in 10% 02/N2 50 ppm
Argon in 21% 02/N2 100 ppm
g. Period of Certification
Experience at NIST indicates that oxygen mixtures contained in aluminum
cylinders are stable for periods of more than two years. If no change in
concentration of the CRM's is indicated by the results of the producer's first
and second analysis, then stability for a period of two years from the date of
the second analysis is assured with a high degree of probability.
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h. Recertification (see Appendix G for further details)
CRM's of oxygen in nitrogen that have exceeded the period of certification
may be recertified by the original supplier according to the following
provisions.
1. The pressure remaining in the cylinder must be greater than 500 psi
(3.4 MPa).
2. The ratio of the signal generated by the CRM when compared to the
original internal standard must be the same as when originally measured
within the limits of uncertainty generated by both sets of
measurements.
3. The concentration of the internal standard, determined by comparison to
the SRM with which it was originally compared, must be the same within
the limits of precision of the comparison.
4. The concentration of the original SRM must be known either by
comparison with a new SRM, or the original SRM must have been recently
recertified by NIST.
5. If either the original SRM or the internal standard no longer exists,
then recertification can only be made using a new SRM.
6. The uncertainty to be assigned to the recertified value must include
the added uncertainty, if any, of the recertification process.
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APPENDIX I
Carbon Dioxide in Nitrogen
a. Introduction
This appendix describes the preparation of Certified Reference Materials
(CRM's) consisting of carbon dioxide in nitrogen. The reader should be
familiar with the general description of the preparation and analysis of CRM's
given in the main text of this document and the material in Appendix G which
provides procedures for extending the certification of CRM's and for date-of-
sale certification of CRM's.
b. Current SRM's
Following is a list of SRM's of carbon dioxide in nitrogen for which CRM's
may be made. The concentrations shown are in mole percent. The actual con-
centration of a particular SRM may differ by more than several percent relative
from the nominal value and it is therefore essential that the exact concentra-
tion of the SRM to be duplicated be known before attempting the preparation of
a CRM. The eight SRM's covering 2619a through 2626a listed below were
originally developed for use in calibrating combustion efficiency measurements;
the remaining originally developed for use in calibrating mobile source
emission measurements.
SRM Number Nominal Concentration of C02
1674b 7.0 mole percent
1675b 14.0 mole percent
2619a 0.5 mole percent
2620a 1.0 mole percent
2621a 1.5 mole percent
2622a 2.0 mole percent
2623a 2.5 mole percent
2624a 3.0 mole percent
2625a 3.5 mole percent
2626a 4.0 mole percent
2632 300 ppm
2633 400 ppm
2634 800 ppm
The preparer of carbon dioxide in nitrogen CRM's should note that the C02
concentrations in the eight combustion efficiency SRM's (2619a through 2626a)
are certified to within 0.1 percent relative at the 95 percent confidence
level; the C02 concentrations in the remaining five SRM's are certified to
within ±1" percent relative at the 95 percent confidence level.
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c. Cylinders
The CRM's must be packaged in new aluminum cylinders equipped with valves
of appropriate material that conform to CGA recommendations for these
particular mixtures; alternatively, aluminum cylinders previously used for
carbon dioxide in nitrogen CRM's or that have been used only for carbon dioxide
in nitrogen mixtures may be used for these CRM's, provided that the history of
a reused cylinder is well known, and, in case of a CRM cylinder, an analysis
must be performed of the remaining gases to assure that the contents have not
been diluted or contaminated. Note: The concentration of the remaining gas
mixture must lie within plus or minus one percent relative of the concentration
of the original CRM, as certified. In the case of reused cylinders, the
cylinder must be evacuated before refilling, and the concentration of the new
CRM gas mixture must lie within one order of magnitude (within 0.1 to 10 times)
of the concentration of the previous CRM or other carbon dioxide in nitrogen
mixtures previously contained.
d. Preparation and Analysis
Prepare a lot of at least 10 cylinders all at identical concentration.
The concentration of carbon dioxide must lie within ±1 percent relative of the
concentration of the specific SRM with which the lot is to be compared.
Analyze at least 10 samples from the lot, if the lot contains more than 20
samples. If the lot contains fewer than 20 samples, it is recommended that all
samples be analyzed. Analyze the samples by direct comparison to the SRM or by
comparing samples in the lot to a single sample selected at random from the lot
(internal standard from the lot). Analyze each sample at least two times.
Analyze for impurities as required in Section F.
After the first analysis, the lot should be put aside to "incubate" for at
least one month after which time all samples in the lot are analyzed and at
which time the impurities are redetermined. If the two sets of analyses are
the same, then it may be assumed that the lot is homogeneous and stable. If
the analysis was performed by direct comparison to the SRM, then the analytical
value for the sample will be the certified value. If the analyses were
performed by comparison to an internal standard for the lot, then it will be
necessary to determine the concentration of the internal standard. This is
done by repeated comparison of the internal standard to the SRM. The con-
centration of the internal standard must be determined at the time of the first
and second analysis by comparing it to the appropriate SRM's. The measured
concentration of the internal standard should be the same for both analyses
within the limits of error of the analyses. The concentration measured for the
internal standard is multiplied by measured ratio of each sample to the
internal standard to obtain the concentration of each sample.
The uncertainty of the CRM includes the error of the SRM and the
uncertainty due to the imprecision of the intercomparison. The error of the
SRM, ESRM, is given on the certificate for the particular cylinder and is
defined as the estimated upper limit of the total uncertainty. The estimated
upper limit of the uncertainty for the CRM should be calculated as follows:
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E2(Total Error) = 2
+ a2 + b2 + c2
where E2 is the estimated upper limit of the uncertainty of the CRM; a is the
imprecision of the intercomparison of the internal standard with the lot; b is
the imprecision of the intercomparison of the internal standard with the SRM;
and c is the error of the calibration.
e. Analytical Method
Analysis of these CRM carbon dioxide mixtures may be done by gas
chromatography, paramagnetic analyzers, or any other methods specified for
carbon dioxide determination. Analysis of SRM's 2632-2634 also employed a
non-dispersive infrared technique. The method chosen should have adequate
sensitivity and precision, and the nature of the instrument response must be
known for. the region between the SRM and the CRM. The precision and sen-
sitivity are evaluated during analysis and are reflected directly in the
"imprecision of intercomparison." The instrument response characteristic is
evaluated over the small range between the SRM and CRM by observing the
response over a wide range using SRM's as calibration standards.
f. Impurities
The maximum concentration of impurities shall not exceed the following:
Carbon monoxide 0.1 ppm
Nitrous oxide 0.5 ppm
Hydrocarbons as methane 1 ppm
Water vapor 5 ppm
Total other impurities 10 ppm
g. Period of Certification
Experience at NIST indicates that carbon dioxide in nitrogen mixtures
contained in aluminum cylinders are stable for periods of at least two years.
If no change in concentration of the CRM's is indicated by the results of the
producer's first and second analysis, then stability for a period of two years
from the date of the second analysis is assured with a high degree of probabil-
ity. Any samples not sold within two years of the second analysis should be
recertified (see section h., below) and the period of certification should be
appropriately extended.
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h. Recertification (see Appendix G for further details)
CRM's of carbon dioxide in nitrogen that have exceeded the period of
certification may be recertified by the original supplier according to the
following provisions.
1. The pressure remaining in the cylinder must be greater than 500 psi
(3.4 MPa).
2. The ratio of the signal generated by the CRM when compared to the
original internal standard must be the same as when originally measured
within the limits of uncertainty generated by both sets of measure-
ments .
3. The concentration of the internal standard, determined by comparison to
the SRM with which it was originally compared, must be the same within
the limits of precision of the comparison.
4. The concentration of the original SRM must be known either by
comparison with a new SRM, or the original SRM must have been recently
recertified by NIST.
5. If either the original SRM or the internal standard no longer exists,
then recertification can only be made using a new SRM.
6. The uncertainty to be assigned to the recertified value must include
the added uncertainty, if any, of the recertification process.
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NIST'114A U.S. DEPARTMENT OF COMMERCE
(REV. 3-89) NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY
BIBLIOGRAPHIC DATA SHEET
T PUBLICATION OR REPORT NUMBER
EPA-600/7-81-010
2. PERFORMING ORGANIZATION REPORT NUMBER
NBSIR 81-2227
3. PUBLICATION DATE
MARCH 1990
4. TITLE AND SUBTITLE
A Procedure for Establishing Traceability of Gas Mixtures to Certain National Institute
of Standards and Technology Standard Reference Materials. (Revised November 1989)
3. AUTHOR(S)
Ernest Hughes and John Mandel, NISI
6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)
U.S. DEPARTMENT OF COMMERCE
NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY
OAITHERSBURO. MD 20899
7. CONTRACT/GRANT NUMBER
8. TYPE OF REPORT AND PERIOD COVERED
9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP)
Jointly sponsored by:
National Institute of Standards and Technology, Gaithersburg, MD 20899
U.S. Environmental Protection Agency, AREAL, RTP, NC
10. SUPPLEMENTARY NOTES
| | DOCUMENT DESCRIBES A COMPUTER PROGRAM; SF-185, FIPS SOFTWARE SUMMARY, IS ATTACHED.
11. ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBLIOGRAPHY OR
LITERATURE SURVEY, MENTION IT HERE.)
This procedure includes the specifications and requirements that must be followed by gas
during the preparation of compressed cylinder gas Certified Reference Materials (CRM). A
CRM is a certified gas standard prepared at a concentration that does not exceed ± 1 percent
of currently available NIST Standard Reference Material (SRM) cylinder gases. The procedure
includes specifications and requirements for: (1) preparation of compressed gas samples in
cylinders prepared in lots of ten or more of identical concentration with the average concen-
tration for the lot within ±1.0 percent relative to the concentration of a specific SRM;
(2) tests to verify compressed gas sample stability and within lot homogeneity; (3) simul-
taneous submission by the gas manufacturer of analysis results to NIST and cylinder gas
numbers to USEPA (without analysis results); (4) random selection by USEPA of two cylinders
per lot for a USEPA performance audit analysis; (5) submission by USEPA of audit results
to NIST; and (6) an evaluation by NIST to determine whether the lot satisfies the criteria
for a NIST-traceable CRM. Individual CRM procedures are described for CO in N2 or air
(Appendix C), NO in N2 (Appendix D), S02 in N2 (Appendix E), propane in N2 or air (Appendix F)
02 in N2 (Appendix H), and C02 in N2 (Appendix I). Procedures for CRM recertifications and
for date-of-sale certification are given in Appendix G.
12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITALIZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)
Traceability, Certified Gas Standards, National Standards, Nitric Oxide, Sulfur Dioxide,
Propane, Carbon Dioxide, Carbon Monoxide, Oxygen, Quality Assurance, Air Pollution,
Environmental Measurements, Data Quality
13. AVAILABILITY
UNLIMITED
FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS).
ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE,
WASHINGTON, DC 20402.
ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD. VA 22161.
14. NUMBER OF PRINTED PAGES
75
IS. PRICE
A04
ELECTRONIC FORM
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