Compilation of Various Questions and Answers about
EPA Traceability Protocol for Gaseous Calibration Standards
Question 1-As expected, we've already had a "grey area" occur as we implement the 2012 Green Book.
Recertification of a 16ppm SO2/N2 EPA Protocol Mix. If the recertification passes the TOST 1-percent test
for stability, does it get a new certification period of 4 years or 8 years? I expect it is 4 years, but not sure
if it should be moved up to the next higher level such as is typical for upgrading the 6-month mixes.
Answer 1- The "grey area" arises because a 16 ppm SO2/N2 mixture was classified in the 1997 protocol
as a lower-concentration mixture with a 6-month certification period. Under the 2012 protocol (see Table
2-3), the same gas mixture falls into the 1 to 50 ppm SO2 range with a 4-year certification period. When
an EPA Protocol Gas gets recertified, the 2012 certification period applies if the recertification data pass
the TOST stability test. Do not use an 8-year certification period, which applies for SO2/N2 mixtures
(either new or recertified) greater than 50 ppm. The 2012 certification periods are based on NIST data.
In future years, the certification periods could be revised if longer-period and lower-concentration stability
data become available. If available, please send me the recertification data for this EPA Protocol Gas.
Question 2- What is the certification period for an EPA Protocol Gas that is currently in the field and that
was certified under the 1997 revision of the protocol? What if the certification has expired?
Answer 2- First, the maximum certification periods that are given in Table 2-3 are based on concentration
stability data from NIST and specialty gas producers rather than on the results of the TOST stability test.
Both TOST and Student's t-test concern short-term, catastrophic stability problems, rather than long-term
ones. The maximum certification periods are based on historical NTRM concentration stability data from
NIST and other stability data that have been submitted by specialty gas producers for EPA review. If we
assume that the cylinder passivation techniques being used by specialty gas producers for EPA Protocol
Gases are similar to those used for NTRMs, then the stability for EPA Protocol Gases should be similar to
that for NTRMs. Because the stability data were obtained from NTRMs that were produced in the past,
they should be representative of existing EPA Protocol Gases. Consequently, the new maximum
certification periods are applicable to existing EPA Protocol Gases and to those now being produced and
whose short-term stability is still being tested using Student's t-test. The longer certification periods are
applicable independent of the statistical test that is used to evaluate stability. If the protocol's certification
period for a gas mixture had been 2 years and is now 4 years, then an existing EPA Protocol Gas that
had been certified for 2 years can be used for 4 years and remain in certification.
Second, producers may exercise their own discretion to certify EPA Protocol Gases for less than these
maximum periods if they believe that their standards may not be as stable as EPA believes.
Third, producers may elect to notify their customers that the certification periods for existing EPA Protocol
Gases (both those in certification and those that are expired) have been extended with the beginning of
the now-longer certification period remaining the date of the last assay. For example, an EPA Protocol
Gas containing 50 ppm propane in air was originally certified on January 1, 2009 under the 1997 revision
of the protocol. Its certification ended three years later, on January 2, 2012. Because the new maximum
certification period for this gas mixture is eight years as a result of the 2012 revision, the new certification
expiration date would be January 2, 2017. The EPA Protocol Gas would not have to be recertified by its
producer to receive the longer certification period, but it would have to be recertified after January 2,
2017. The TOST stability test would have to be used during the recertification assay to show that the
certified concentration has remained stable.
For those end users who are required by EPA regulations to use EPA Protocol Gases as calibration
gases, some form of written documentation probably will be needed for the end users' records. For
example, the producer could send a blanket letter or E-mail message to its end users and could offer to
send a new certificate to those users who need written documentation and/or are required to report about
their calibration gases to EPA. I understand that the longer certification periods would have to be
reported electronically to EPA. I appreciate that the notification process will place some burden on
producers and hope that giving them some discretion in this matter will reduce this burden.
Finally, the longer certification period does not protect against concentration shifts due to back diffusion of
oxygen from the regulator into a cylinder through an open hand valve. End users still must be cautious to
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Compilation of Various Questions and Answers about
EPA Traceability Protocol for Gaseous Calibration Standards
prevent such concentration shifts overtime and to properly purge regulators before use and to close hand
valves after use. Given the longer certification periods, they must be more cautious than previously.
Question 3- In the field, an end user has an EPA Protocol Gas which is about to expire. They can contact
their supplier and request a recertification of the calibration gas. How the supplier achieves the
recertification is up to them, either requiring a reanalysis or not, but in any event a new certificate is
required to be in the possession of the end user prior to being able to use the gas beyond its original
certification period. Actual reanalysis is not a requisite for recertification.
Answer 3- First, the producer can exercise its discretion in this matter. The EPA Protocol Gas would not
have to have a recertification assay to receive the longer certification period under the original
certification, but the end user could elect to pay for such an assay if it wanted to be completely sure that
the concentration has not shifted. Producers are not required to send new certificates to all end users
because many end users may not retain the calibration gases for longer than the 1997 certification
periods. That being said, a producer may wish to maintain good customer relations by sending new
certificates to those end users who request them and who need them for regulatory reporting purposes.
Question 4- What about an EPA Protocol Gas that has already expired. I assume the same route as
above but how long can a gas be expired before it is no longer recertifiable?
Answer 4- First, there are DOT requirements for hydrostatic testing of cylinders that may have a bearing
on this topic, but I'm not competent to discuss them. Using the example from Question 1, the end user
could request a new certificate for the propane in air cylinder at any point up to January 2, 2017 (producer
discretion still applicable), but the new certificate would still have an expiration date of January 2, 2017.
However, would one expect that an end user pay continuing demurrage for an "expired" EPA Protocol
Gas if the longer certification period were not known by the end user? In all likelihood, the end user will
return the "expired" EPA Protocol Gas to its producer.
Question 5- Would it be permissible for a third party (e.g., another specialty gas producer) to recertify the
expired, or about-to-expire, EPA Protocol Gas without an analysis, even though they were not the
producer? Absent the actual reanalysis, can the non-producer recertify another producer's EPA Protocol
Gases? The potential third-party recertifier would have PGVP registration and, in fact, be an EPA
Protocol Gas producer, but they just did not produce the calibration gas in question in the example.
Answer 5- The original producer can be the only organization that can send out any new certificates for
expired/expiring EPA Protocol Gases that are not reassayed. An end user can't shop around for another
producer who is willing to send out another certificate. The original producer's discretion in this matter
has to be respected.
Questions 6, 7, and 8- The quality assurance laboratory of (deleted country) conducts certifications of
EPA Protocol Gases using NIST SRMs following the previous EPA traceability protocol. I would like to
know more about the new EPA protocol. In the revised version, EPA has introduced the TOST statistical
test for checking whether the first and second assayed measurements are equivalent, and would
determine the better models (linear, quadratic, etc.) for the multi-point calibration of the analyzers. For us,
we would accept the two assayed measurements if the two concentrations are within 1%. If we also
assume the multi-point calibrations of SO2/NOX/CO analyzers are linear, then we calculate the uncertainty
for the regression-predicted concentration based on Student's t-distribution which should be less than 1%
of the largest concentration used in the multi-point calibration. In this regard, I would like to know:
1 What is the advantage for using TOST acceptance criteria?
2 Should we assume the multi-point calibrations of SO2/NO2/CO analyzers are linear as we find these
characteristics for analyzers deployed in the (deleted government) air monitoring network?
3 Should I assume now that the certification periods of all EPA Protocol Gases (50 ppm in our stock)
can be extended from 2 years to 4 years, though the gas certificates show otherwise? Or only after
we recertify it upon expiration, then the certification period would be extended for another 4 years.
If so, the certification period can be up to 6 years!!!
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Answers 6, 7, and 8- The main advantage of using Schuirmann's TOST is that the analyst is not
rewarded for making measurements with poor precision. Student's t-test determines if there is a
statistically significant difference between two values and large analytical uncertainties can mask a
smaller concentration difference. On the other hand, TOST determines if two values agree to within a
specified acceptance criterion and large uncertainties make it harder to show that the values are
equivalent. Read the statistical papers referenced in the protocol for a statistical discussion of TOST.
Note that the Appendix C statistical spreadsheet in the protocol calculates the uncertainty of the EPA
Protocol Gas based on the uncertainties of the two assays of the candidate standard.
The Appendix A statistical spreadsheet produces polynomial regression equations from linear (i.e., first-
order) up to fourth-order. It also makes a recommendation about which regression best fits (i.e., yields
smallest uncertainty) the assay data from statistical principles alone. The uncertainty of the calibration
equation may be reduced by using higher-order equations, but EPA discourages the use of higher-order
equations unless there are sound theoretical grounds for doing so. The analyst should be the one who
makes the decision about the most proper equation to use. See Section 2.1.4.2 of the protocol for a
discussion of this topic. I surmise from your questions that ambient air quality analyzers are being used
for the assays. If so, I would agree that a linear calibration equation is the best assumption. But look at
the data to be sure.
Please consult with your specialty gas producer regarding extending the certification periods for existing
EPA Protocol Gases without a recertification assay. If the producer elects to do so, new four-year
certificates could be issued for existing EPA Protocol Gases, but the new certification period still begins
on the date of the last assay and ends four years later. The total certification period without a
recertification assay cannot be six years.
Question 9- If a mixture is certified at exactly 50.0 ppm nitric oxide should it receive a 3-year or 8-year
shelf life? Same for 50.0 ppm SO2.
Answer 9- Although I question whether a producer could hit these concentrations exactly, I'll go with the
rather arbitrary principle that the concentration ranges in Table 2-3 start just a smidge above the stated
lower value and end exactly at the stated upper value. Turning the question inside out like a sock,
wouldn't it be better to ask the end user if you could ship a slightly-higher-than-ordered-concentration in
exchange for a longer certification period?
Question 10- Does EPA want to review the concentration limit for propane in nitrogen? The 2012
document requires anything less than 100 ppm to receive a 6-month shelf life. Can this concentration
limit be lowered to 0.1 ppm similar to the air balance propane?
Answer 10- You are correct. The concentration range for propane in nitrogen EPA Protocol Gases in
Table 2-3 should have been changed to 5 ppb to 2 percent because NIST NTRMs are available in that
concentration range as is shown in Table 2-1. During the revision of the protocol, the concentration range
in Table 2-3 was changed but I somehow failed to make the corresponding change in Table 2-1. Please
accept my apologies for this error. Unfortunately, no one found this error when the draft of the protocol
was sent for external review in September 2011. A copy of the 2012 protocol with technical corrections is
posted at https://www.epa.qov/air-research/epa-traceabilitv-protocol-assav-and-certification-qaseous-
calibration-standards
Question 11- How should specialty gas producers handle shelf life for concentrations above the highest
indicated concentration in the protocol (Page 31), for example, 1000 ppm propane in air?
Answer 11- This question about the shelf life for high-concentration standards is largely moot because
such standards cannot be assayed or certified under the protocol. Section 2.1.1 of the protocol
states "A candidate standard having a concentration that is lower or higher than that of the reference
standard may be certified under this protocol if both standards' concentrations (or diluted concentrations)
fall within the well-characterized region of the analyzer's calibration curve". Section 2.1.4.2 states "All
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EPA Traceability Protocol for Gaseous Calibration Standards
measurements of candidate standards must fall within the well-characterized region of the analyzer's
calibration curve, which lies between the largest and smallest measured concentrations of the multi-point
calibration and for which U for the regression-predicted analyzer response is <±1 percent of the measured
response for the largest concentration in the calibration". The heart of the matter is the lack of a high-
concentration NIST-traceable reference standard with which to generate a calibration curve whose well-
characterized region would bracket the high-concentration candidate standard. How would one assay the
standard-in-question (i.e., 1000 ppm propane in air) if the highest concentration reference standard
available from NIST is 500 ppm for that gas mixture? Such a great extrapolation beyond the measured
calibration curve would be extremely questionable.
Question 12- As RGMs are now allowed as reference standards the specialty gas community will, over
time, become more flexible/ adaptable to adding new components and concentrations to our EPA
Protocol portfolio. If we are limited at the top end of the concentration in the document, there will be
confusion of how to apply shelf life rules above the indicated concentration. For example, if we develop
RGMs with NIST and are capable of analyzing 100 ppm ammonia to the protocol requirements, how will
we assign shelf life at this concentration? Does EPA want to consider removing the upper concentration
limit on the concentration range (except for the lower segments of NO and SO2)? Examples:
- nitric oxide 0.5 ppm to 20.0 ppm 3 years
- nitric oxide > 20.0 ppm 8 years
- propane in air > 0.1 ppm 8 years
Answer 12- The concentration ranges in Table 2-3 are based on concentration stability data obtained
from NIST and from specialty gas producers. If producers wish to extend the ranges, they should contact
EPA and provide long-term concentration stability data for the gas mixture that they propose to be
certified as an EPA Protocol Gas. NIST will be consulted regarding this proposal. As appropriate, the
protocol will be revised and producers will be notified of the extended concentration range. It is EPA's
intention that the protocol be based on the best available information and it may be in the interest of
producers to generate such concentration stability data.
Question 13- Methane in nitrogen is not listed on page 31, what will the shelf life be for this combination?
Answer 13- You have found another error that I and the external reviewers missed. Sorry about my
error. Table 2-3 should have included methane in nitrogen from 0.5 ppm to 4 percent with a maximum
certification period of 8 years. A copy of the 2012 protocol with technical corrections is posted at
http://www.epa.gov/nrmrl/appcd/mmd/db-traceability-protocol.html
Question 14- How shall we handle SO2 in air; H2S in air, nitrous oxide in nitrogen, and oxides of nitrogen
in nitrogen?
Answer 14- Table 2-3 has been revised to include SO2 in air based on the following information from
NIST's Frank Guenther, who has since retired:
"It is true that the DOE (i.e., Declaration of Equivalence) does not state that there is equivalence
between VSL and NIST for SO2 in air balance. However, with CCQM-K76 it was declared that the
key comparison results were applicable to a concentration range from 50 ppm to 1 % mol/mol in a
balance of air or nitrogen. The CCQM Gas Analysis Working group could see no reason to exclude
the possibility of using air as a balance gas in reference materials. I have attached the report for
CCQM-K76, which was coordinated by NIST. Therefore, NIST would recognize equivalence in VSL
gas standard in air above 50 ppm to a maximum of 1 % mol/mol. I will seek to include air balance
SO2 in the next DOE with VSL."
Specialty gas producers may use VSL SO2 in air reference standards to assay SO2 in air candidate
standards. At this point in time, SO2 in N2 reference standards may not be used to assay SO2 in air
candidate standards due to possible balance gas interferences (e.g., collisional broadening of absorption
lines) with the sulfur dioxide measurements. Frank Guenther replied to an EPA inquiry as follows:
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Compilation of Various Questions and Answers about
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"We cannot say if there is significant bias due to the balance gas in the wide spread of instruments
used to analyze SO2. The band broadening issue is but one mechanism that can cause biases.
Some instruments draw sample through a capillary tube to control flow. Due to viscosity differences,
air and nitrogen can behave differently. The magnitude of the resulting bias is unknown to us here at
NIST. It would be a relatively easy experiment to set up using one of ourdiluters. We will see if we
can set up a test station, and then test the various instruments we have in our possession. This will
take some time, as we have plenty to do and I am not looking for more projects at this time. But we
will try to fit it in over the next 6 months. It would be something a contractor could do, and we would
be willing to assist with technical advice."
Table 2-3 of the protocol has been revised to show that EPA Protocol Gases containing 10 to 1000 ppm
oxides of nitrogen in nitrogen may be certified for up to two years. VSL sells 10 to 1000 ppm nitrogen
dioxide in nitrogen primary reference materials (PRMs) and that the DOE between NIST and VSL
includes this gas mixture. The 2013 on-line VSL PRM catalog states that the stability period is two years
and that mixtures of nitrogen dioxide in nitrogen also contain approximately 1000 ppm oxygen. NIST-
certified reference standards containing oxides of nitrogen in air cannot be used as reference standards
for the assay of candidate standards containing oxides of nitrogen in nitrogen because of potential biases
from balance gas differences.
There are no available NIST reference standards (or equivalent VSL PRMs) for the other gas mixtures
and no corresponding EPA Protocol Gases can be produced as a result. At such a time as NIST makes
these reference standards available as SRMs, NTRMs, or RGMs, the protocol will be revised and
producers will be notified of the revision
Question 15- We have a 40CFR75 electric utility that wants NOx certified as an EPA Protocol Gas, and
with a certification accuracy assigned. They want the NOx as nitric oxide in nitrogen mixtures. This
electric utility is purchasing single minor component nitric oxide EPA Protocol Gases. I envision that this
could be done if we minimize the NO2 impurity such that the uncertainty in the NO2 and total NOx is not
statistically significant to the nitric oxide certification accuracy.
In doing NOx analysis, I thought that the NOx channel of a chemiluminescent instrument would be better
vs. FTIR. On the chemi, we can measure the total NOx of an SRM or NTRM, and correlate it to NIST's
reported NO and NO2 concentrations. Of course on an FTIR, one obtains separate peaks for NO vs.
NO2, but the resolution and accuracy of the ppm to sub-ppm NO2 comes more into question on an FTIR.
Also, the only NO2 SRM has air balance gas, and we do everything that we can to keep air out of our
FTIR delivery train. What I would like your ruling on:
1 Does the 2012 EPA Protocol Document allow a NOx EPA Protocol with the composition of nitric
oxide, and total NOx certification?
2 Would you agree that the calculations in the attached work aid would allow this composition?
Answer 15- You can directly assay the NO and NOx concentrations of the nitric oxide in nitrogen
candidate standard using the certified NO and NOx concentrations of the NIST nitric oxide in nitrogen
reference standard and a chemiluminescent instrument as discussed below.
Mike Kelley of NIST's Gas Metrology Group says "NIST has been certifying both the NO and NOx content
in their SRMs for years. As far as NTRMs are concerned, if the producer analyzes NOx and submits the
results to NIST, we will certify that as well. Some producers analyze the NO only".
EPA's Part 75 Emissions Monitoring Policy Manual (see https://www.epa.gov/sites/production/files/2015-
05/documents/part 75 emissions monitoring policy manual.pdf) states:
"Question 9.34
Topic: Use of EPA Protocol Gas Components for Calibration
Question: Should the NO or the NOx concentration on an EPA Protocol gas cylinder be used for NOx
analyzer calibrations and linearity checks?
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Compilation of Various Questions and Answers about
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Answer: Prior to 2004, only the NO component of EPA Protocol gas cylinders was certified as
traceable to the National Institute of Standards and Technology (NIST); the NOx concentrations
shown on calibration gas certificates were for informational use only. However, since then, NIST has
been certifying both the NO and NOx concentrations of Standard Reference Materials (SRMs) and
NIST Traceable Reference Materials (NTRMs). Therefore, it is now possible for specialty gas
companies to produce EPA Protocol gas cylinders in which both the NO and NOx concentrations are
NIST traceable. In view of this:
(1) When both the NO and NOx concentrations of an EPA Protocol gas cylinder are certified NIST-
traceable:
(a) If you have an analyzer that measures total NOx, you may use either the certified NO
concentration1 or the certified NOx concentration when conducting calibration error tests or linearity
checks, or when calibrating a reference analyzer for a Part 75 NOx RATA or an App E NOx test; or
(b) If your analyzer measures only NO, rather than total NOx, use the certified NO concentration for
calibration error tests, and linearity checks.
(2) If only the NO concentration of the EPA Protocol gas cylinder is NIST-traceable but the NOx
concentration is not, use the certified NO concentration for calibration error tests and linearity checks,
and for calibrating a reference analyzer1 for a Part 75 NOX RATA or an App E NOx test.
1 Note: An N02 EPA Protocol gas must also be used when calibrating a reference analyzer that
measures NO and NO2 separately without a converter.
References: Appendix A, § 6.2 and 6.3; Appendix B § 2.1.1 and 2.2.1
Key Words: EPA Protocol gas, calibration gas, calibration error test, linearity check, NOx monitoring
History: New"
Question 16- A refinery needs an EPA Protocol Gas that is 4.9% methane, 2% propane with balance
nitrogen. The highest NIST SRM for methane is 100 ppm; however (deleted producer) has a natural
gas NTRM with methane at 90%; and propane at 3%. Plus we can use pure methane for the calibration
curve. We want to use a GC/FID for the analysis so dynamic dilution of pure methane is not a good fit
into a GC sample valve (per section 2.1.3.2 on page 11). Since method G2 allows static dilution which
could be quantitative gravimetric dilution of non-reactive gas components, could we use static, gravimetric
dilution of the pure methane to generate the calibration curve?
Answer 16- In response to EPA's inquiry, NIST's Frank Guenther addressed this question:
"To analyze a 5% methane properly you need to construct a calibration curve with standards near
5%. I would use two or more standards that bracket the analyzed cylinder, but no lower than 1 % and
no higher than 10%. The larger the standard range the more standards needed. If you have a
standard you trust that lies extremely near the candidate cylinder in concentration, you can even just
use the one trusted standard. NIST can certify methane standards up to 10 % in nitrogen, and thus
can issue NTRM certs or RGM certs for this concentration."
Question 17- (deleted producer) uses the following statement on EPA Protocol Certificates of Analysis:
"This certification was performed according to EPA Traceability Protocol for Assay and Certification of
Gaseous Calibration Standards September 1997, using procedure G1 or G2."
(deleted producer) will continue to practice the 1997 EPA Protocol document while we update the
uncertainty calculations. However, we want to implement the new shelf lives immediately. Is the
following statement acceptable?
"This certification was performed according to EPA Traceability Protocol for Assay and Certification of
Gaseous Calibration Standards September 1997, using procedure G1 or G2. The certification
expiration date is assigned using the May 2012 revision of the EPA Traceability Protocol document."
Answer 17- The proposed statement is acceptable during the one-year interim period while the producer
updates its uncertainty calculations.
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Question 18- We currently provide a significant amount of end users with EPA Protocol Gases and we
procure these standards from producers on the PGVP. My question is can a distributor who procures
EPA Protocol Gases from a producer on the PGVP register and participate in the audit program and have
their name added to the PGVP? Any help or clarification in this matter would be greatly appreciated.
Answer 18- This question was forwarded to EPA's John Schakenbach (schakenbach.john@epa.gov)
who ran the Emission PGVP and Mike Papp (Papp.michael@epa.gov), who ran the Ambient Air PGVP.
John's response is shown below and Mike agreed with John's position on this question:
"40 CFR Part 75 only allows production sites (i.e., sites that actually assay an EPA Protocol gas
cylinder) to participate in the Emission PGVP. The main reason is we don't want to end up with
duplicate cylinders from the same production site because a distributor happens to sell cylinders from
that production site. For our audit, NIST needs to end up with the same number of cylinders from
each production site. Our rule allows a distributor to sell unaltered cylinders from an Emission PGVP
participant, and those cylinders would have a PGVP Vendor ID associated with them. So the
distributor could claim in their advertising that they sell cylinders assayed by an Emission PGVP
participant."
Because John and Mike ran the two PGVPs, their decisions are definitive. Please contact me if you have
any other questions about the protocol. John Schakenbach has retired and responsibility for the emission
PGVP has shifted to Travis Johnson (202-343-9018 orjohnson.travis@epa.gov). Responsibility for the
ambient air PGVP has shifted from Mike Papp to Solomon Ricks (919-541-5242 or
ricks.solomon@epa.gov).
Question 19- Subsection 2.1.5.3 Assay/Certification of Multicomponent Candidate Standards: Data
from the interference study must be evaluated using multiple-variable least-squares regression analysis.
The analyst should consult with a statistician before beginning the study or evaluating its data. The
regression analysis must produce an interference correction equation and an estimate of the standard
uncertainty (ucorrected) associated with the corrected concentrations for the assayed components. The
interference correction equation will be valid for the range of concentrations covered in the study for
which the uncertainty of the corrected concentration is <1 percent of the corrected concentration. The
analyst must add the interference correction uncertainty to the total uncertainty of the standard. The
certification documentation must include a statement that the certified concentration of a specified
component has been corrected for interferences from other specified components. An interference study
is not needed if the assay analyzer is interference free. In your opinion, who will be qualified as a
statistician? The one with a statistic or math degree or have some kind of statistical training? Does our
six-sigma black belt person qualified as a statistician?
Answer 19- The person who needs to analyze the assay data that are needed to develop an interference
correction equation needs to have well-developed and appropriate statistical skills, rather than any
specific educational degree or professional certification. Many individuals with statistical or mathematical
degrees would not have the particular set of statistical skills that are needed for this task. A six-sigma
black belt certification does not appear to require any knowledge of statistics as applied to metrology.
The preface to Harry and Schroeder's book, Six Sigma: The Breakthrough Management Strategy
Revolutionizing the World's Top Corporations, states: "What is Six Sigma? It is a business process that
allows companies to drastically improve their bottom line by designing and monitoring everyday business
activities in ways that minimize waste and resources while increasing customer satisfaction." The
individual that you need should have experience in performing multi-variant linear regressions and in
calculating the uncertainty associated with a correction factor that is predicted from the resulting
regression equation. This individual also should have experience in the statistical design of experiments
so that you can be advised of the optimum combination of gas mixtures and concentrations that will yield
the smallest uncertainty estimates for the multi-component gas mixtures that you anticipate assaying.
These uncertainty estimates must be used in the Appendix C statistical spreadsheet to determine the
expanded uncertainty of the candidate standard. It would also be helpful if this individual has a good
understanding of BIPM's Evaluation of measurement data — Guide to the expression of uncertainty in
measurement (see http://www.bipm.org/utils/common/documents/icqm/JCGM 100 2008 E.pdf), which
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is abbreviated as GUM, or NIST's Technical Note 1297: Guidelines for Evaluating and Expressing the
Uncertainty of NlST Measurement Results (see http://www.nist.gov/pml/pubs/tn1297/index.cfm 1
Question 20- The effect of instrument interference could cause a positive influence (response) and
contribute the concentration of an analyte such as NDIR due to the overlap of IR regions. It could cause a
negative influence (response) and reflect a lower concentration of the analyte such as the quenching
effect of NOx chemiluminescence analyzer. From my own observation, interference in NDIR could be
expressed in one equation. However, the interference of NOx chemi was more complicated and may
require multiple equations in different conditions. Does EPA allow us to tabulate the interference
correction instead of using just one equation?
Answer 20- Al Dageforde of Horiba Instruments, Inc. wrote a paper in 1980 on the determination of the
carbon dioxide quench effect on chemiluminescence NOx analysis using a standard gas divider. He used
a graphical approach to correct for interference based on measurements of two diluted NO calibration
gases; one with CO2 and one without CO2. In 2010, NIST's Lyn Gameson (lyn.gameson@nist.gov) made
a presentation on the accurate quantification of multi-component EPA Protocol Gases. He used linear
regression equations to correct for interference based on measurements of NO, SO2 or CO2 calibration
gases diluted either with balance gas or with interferent gas. He found that the interference correction for
chemiluminescence NOx analyzers can be represented by a single equation that is a function of the CO2
concentration and that is independent of the NO concentration. Finally, a group of Chinese researchers
published a 2012 article on interference correction for a multi-gas (CO2, CO, and NO) NDIR analyzer.
They used linear regression equations for interference correction. The calculation of calibration curves
was based on least-square fittings with third-order polynomials. The interference correction equations
were approximated by linear curves. They state that after the interference correction, the signal detected
at each filter channel only depends on the absorption of the intended gas. The citation for their research
is Sun YWet al., "Cross-interference correction and simultaneous multi-gas analysis based on infrared
absorption" Chinese Physics B, vol. 21, no. 9, pg. 090701, 2012.
The advantages of using linear regression equations for interference correction are (1) the ability to obtain
an interference correction for any combination of gases and concentrations, (2) the ability to obtain an
uncertainty estimate for the interference correction, and (3) the ability to incorporate the equations in the
software that is used to calculate the certified concentration and the expanded uncertainty estimate.
While one could use a graphical or tabular approach to determine the interference correction, it would be
difficult to obtain an uncertainty estimate using either of these approaches. Additionally, the tabular
approach would introduce an additional error that is associated with interpolating between the values in
the table and that would have to be added to the expanded uncertainty estimate. Unless your procedures
for calculating the certified concentration and the expanded uncertainty estimate rely heavily on manual
techniques, linear regression is the best and most direct method to perform the interference correction
calculations and to estimate the uncertainty of the interference correction. Also note that the range
restriction (i.e., uncertainty is <1 percent of the corrected concentration) means that the interference
study's data must be analyzed using least-squares regression analysis.
Question 21- Subsection 2.1.4.3 Uncertainty of the Calibration Curve: This third component of
uncertainty does not exist if the concentrations of the reference and candidate standards are equal. The
assumed calibration equation and the true calibration curve will pass through the data for the reference
standard regardless of whether they diverge elsewhere and the equation will be accurate for that single
concentration. However, the uncertainty does exist if the concentrations of the reference and candidate
standards differ. Does EPA allow a point-to-point analysis? If yes, how close the standard and sample
concentrations need to be? (0.5%, 1%?)
Answer 21- There are many analytical procedures for assaying gas mixtures and the EPA traceability
protocol represents just a few of them. Nevertheless, these few procedures have been developed since
1978 and there is consensus acceptance of them by specialty gas producers and end users. This
acceptance is partly due to the external review of the protocol by producers as it has been revised in
1987, 1993, 1997, and 2012. The protocol has evolved incrementally, rather than radically, to minimize
disruptions of the specialty gas industry. New procedures have been added to the protocol over the
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years and they have been reviewed by interested parties. Although a point-to-point comparison
procedure might have some utility in some limited circumstances, the current procedures were designed
to allow for a lot of analytical flexibility to assay a continuous range of concentrations. Restricting EPA
Protocol Gases to only the same concentrations as NIST certifies would not allow end users much
choice. A new procedure cannot be established casually, but it would have to be formally added to the
protocol, which might take several years to accomplish. The time to have proposed such a procedure
was while the protocol was being revised between 2005 and 2012. You are free to make a more
comprehensive technical proposal to add a new procedure to the protocol and to justify the need for the
new procedure. Supporting technical data would strengthen the case for the new procedure.
Question 22- I have a customer that requires a protocol mix that is at a higher concentration than my
SRM. May I dilute my candidate cylinder down to the concentration of my existing SRM and be in
compliance with G2? My dilution system is in current calibration, traceable to NIST by a NVLAP
accredited ISO 17025 metrology lab.
Answer 22- Three aspects of this question need to be addressed. First, EPA Protocol Gases cannot be
prepared at concentrations that exceed the ranges of reference standards that are available from NIST or
VSL as are shown in Tables 2-1 and 2-2 of the protocol. If the customer specifies a calibration gas that
exceeds these ranges, offer the customer a non-protocol calibration gas that is traceable to your own
primary standards. Second, if the concentration of the specified calibration gas is within the range of
NIST and VSL reference standards, purchase one of these reference standards from NIST, VSL or an
NTRM from another specialty gas producer or offer the customer a non-protocol calibration gas. Third,
the lack of a NIST-traceable reference standard of the appropriate concentration does not justify assaying
candidate standards using casually-designed modifications of the analytical procedures included in the
protocol. As indicated in the previous answer, the trust that the regulated community places in EPA
Protocol Gases is partly due to the producers' external review and consensus acceptance of the
procedures included in the protocol. The publication of PGVP results also generates trust in EPA
Protocol Gases. If other procedures are needed, they must be developed in a formal fashion and must
be published in a revised version of the protocol.
The proposed procedure is a combination of elements of Procedures G1 and G2 in that the reference
standard would be assayed without dilution and the candidate standard would be assayed with dilution.
While it is physically possible to perform this procedure, the big issue is whether assays performed using
it would be considered by the regulated community to be traceable to NIST reference standards. The
NIST Policy on Metrological Traceability (see https://www.nist.qov/nist-poIicv-traceabiIitv) states that
NIST adopts for its own use and recommends for use by others the definition of metrological traceability
provided in the most recent version of the International Vocabulary of Metrology: "property of a
measurement result whereby the result can be related to a reference through a documented unbroken
chain of calibrations, each contributing to the measurement uncertainty." The proposed procedure would
need to include measurements and statistical calculations for quantifying the uncertainties associated
with the analyzer's calibration curve and with the dilution flow rates and for including them in the
expanded uncertainty of the candidate standard. Estimating the expanded uncertainty is a component of
the protocol that cannot be ignored.
Questions 23, 24, and 25- (deleted distributor) imports gas mixtures from (deleted producer) and a
significant part of these mixtures are EPA Protocol Gases. In the last 2 years, (deleted country) requires
an ISO 17025 for environmental mixes. As a result of this, our customers request the same. I would
appreciate your help with some questions I have:
1. Is there a list of all the companies who authorized to produce EPA Protocol Gases?
2. Are there any EPA regulation regarding analyzing tests?
3. If the company who produce the mixture is not certified/ have ISO 17025 certification, and they
send the mixture to third-party accreditation services for testing it, would the mixture have the
ISO 17025 certification?
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Answer 23- EPA monitoring regulations require that vendors advertising certification by the protocol and
distributing calibration gases as "EPA Protocol Gases" must participate in the EPA Protocol Gas
Verification Program (PGVP) or not use "EPA" in any form of advertising. The participants are posted at
http://www.epa.aov/ttn/amtic/aapgvp.html and https://www.epa.gov/airmarkets/protocol-qas-verification-
proqram-pqvp along with PGVP results and other information about the PGVP. All that being said, EPA's
regulations do not extend beyond the United States' borders. Your country's requirements are applicable
to air pollution monitoring in your country.
Answer 24- The EPA traceability protocol and other information about EPA Protocol Gases are posted at
https://www.epa.qov/air-research/epa-traceabilitv-protocol-assav-and-certification-qaseous-calibration-
standards
Answer 25- The EPA traceability protocol is somewhat different from ISO Standard 17025 (General
requirements for the competence of testing and calibration laboratories) although both documents
concern traceability. The EPA document is a general analytical procedure with associated statistical
calculations for assaying and certifying gaseous calibration standards. The ISO document establishes
technical requirements for calibration and testing laboratories to demonstrate their competence, to
document and implement their quality management system, and to produce valid measurement results in
technical fields of their own choosing. In other words, the EPA document concerns the gaseous
calibration standards while the ISO document concerns the laboratories that might assay and certify
these standards. ISO Standard 6143 (Gas analysis - Comparison methods for determining and checking
the composition of calibration gas mixtures) more closely parallels the EPA document. Now with all this
background discussion finally out of the way, your question can be addressed. A third-party accreditation
service would not be capable of assaying and certifying gaseous calibration standards. It can only
determine the competence of the laboratories that perform such assays and certifications. The specialty
gas producer would have to obtain ISO 17025 accreditation to meet your customers' specifications.
Question 26-1 have a few questions about the following paragraph in Section 1:
"15. A new procedure has been written and a new spreadsheet has been prepared for the assay and
certification of dynamic gas dilution systems (see Section 4). At this time, EPA does not require the
regulated community to use NIST-traceable dynamic gas dilution systems for the calibration of
ambient air or continuous emission monitors that are required by 40 CFR Parts 50, 58, 60, and 75.
However, end users may elect to use these systems for calibrations."
Does the regulated community include both end users and EPA Protocol Gas producers?
Answer 26- The regulated community is comprised of those organizations that are required to monitor
ambient air quality and air pollution emissions under Parts 50, 58, 60, 72, and 75 of Title 40 of the Code
of Federal Regulations (CFR). Examples of such organizations are state and local air pollution control
agencies, electrical utilities, and industrial facilities. The term does not include specialty gas producers
unless they are covered by these EPA regulations.
EPA does require NIST-traceable calibrations of dynamic gas dilution systems that may be used under
Appendices A and C of Part 50 to calibrate ambient air quality monitors for SO2 and CO. 40 CFR Part 75
only allows use of compressed gas calibration standards when calibrating CEMSs that are being used for
purposes of Part 75 and when calibrating Test Methods 3A, 6C, and 7E when these methods are used for
Part 75 testing.
Question 27- Appendix F is for the calculation of uncertainty due to a gas dilution system based on mass
flow control. Does EPA allow other types of dilution techniques such as gravimetric dilution or gas divider
based on capillary tube? If so, will it be acceptable to EPA for Protocol gas producers to calculate the
uncertainty of the dilution system? (Obviously we cannot use prot12appendf.xls)
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Answer 27- Section 4 of the protocol is intended to be used for the direct calibration of ambient air quality
and air pollution emission monitors by the regulated community, rather than for the preparation of
calibration gases by specialty gas producers for such monitors.
Please note that the use of gas dilution systems (e.g., capillary-tube-based gas dividers) for multipoint
calibrations in the preparation of EPA Protocol Gases is already allowed under Section 2.1.3.2 of the
protocol:
"The reference standards for the multipoint calibration must be diluted or undiluted SRMs, RGMs,
PRMs, CRMs, NTRMs, or GMISs (see Subsection 2.1.3) or dynamically diluted pure gases. Pure
gases may be dynamically diluted to prepare gas mixtures for use in multipoint calibrations, but such
mixtures may not be used as the reference standards for the span gas check or for the assay of the
candidate standard. Pure gases may not be diluted by more than a factor of 100."
Any significant change to the protocol such as the use gravimetric dilution in the preparation of EPA
Protocol Gases or alternative statistical procedures would require a formal revision to the protocol, which
could take several years to complete. In addition to the actual writing of the revised document, internal
and external reviews are needed before the final revisions are made. New statistical spreadsheets would
have to be developed. This revision is not something any one specialty gas producer can undertake
independently because the entire specialty gas industry would have to have access to and benefit from
this revision. The revision would have to be acceptable to all interested parties, including NIST. Any
producer may request that the protocol be revised again. The probability of accepting a suggested
revision would be improved by the submission of suggested text and supporting technical data
demonstrating the accuracy and precision of the suggested revision. Both technical and statistical
revisions will be considered.
Question 28- Question about prot12appendc.xls. I input some test data and wonder if the cell A110
format was wrong. Should the cell A110 be divided by 100?
Answer 28- The Appendix C statistical spreadsheet has been revised to address minor calculation errors.
A note has been added stating that all relative uncertainties (in Cells A15, A23, A31, A108, A110, and
A112) are in the Excel format for percentages. If the relative uncertainty is 0.5 percent, then it should be
keyed in as 0.005 and the cell will display 0.50%. It's a recognized quirk of Excel.
Question 29- Will it be required for a facility that only produces H2S/N2 EPA Protocol Gas to get a PGVP
vendor ID? H2S was not listed in 40 CFR 75.21(g)(6) and (7).
Answer 29-1 spoke with both Mike Papp (ambient air PGVP) and John Schakenbach (emission/acid rain
PGVP), who said that the facility would not be required to participate in the PGVP because that facility
does not produce the specific EPA Protocol Gases that are verified by their respective PGVPs.
That being said, there may be some business advantage to have the facility listed as participating in the
PGVP because this action may reduce customer confusion if that facility sells and ships non-H2S EPA
Protocol Gases that are produced at other facilities. That is, the sales reps at that facility wouldn't have to
explain to potential customers why the facility doesn't participate and yet still sells EPA Protocol Gases in
apparent conflict with EPA regulations. Participation by the facility would not cost your firm any money if
the facility's H2S products are never verified by EPA.
Of course, it would be solely your business decision for that facility to participate or not participate in the
PGVP. If you decide that you want the facility to participate, the application should include a note to the
effect that the facility only produces H2S EPA Protocol Gases. In this manner, EPA will not attempt to
procure ambient air or emission/acid rain EPA Protocol Gases from that facility for verification purposes.
Your contact for the ambient air PGVP is Solomon Ricks (919-541-5242 or ricks.solomon@epa.gov).
The ambient air PGVP web page is http://www.epa.gov/ttn/amtic/aapqvp.html. Your contact for the
emissions/acid rain PGVP is Travis Johnson (202-343-9018 orjohnson.travis@epa.gov).
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The emission/acid rain PGVP web page is https://www.epa.gov/airmarkets/protocol-qas-verification-
proqram-pqvp
Question 30- We had a new request come in to provide a 6-pack (6 cylinders connected into a common
manifolded pallet with a common outlet) of EPA Protocol Gases. Essentially, the 6 cylinders are to be
dynamically blended at one concentration as a homogenous batch and individually certified as EPA
Protocol Gases while in the manifolded assembly. The previous supplier to the customer did this with
one COA covering the entire 6 cylinders, which I don't believe is correct. The previous supplier argues
that they are filled simultaneously, must be identical, and since the cylinder valves are all open that they
constitute one "container" and can be covered with one certificate and label. I proposed to the customer
via our sales people that we manufacture through dynamic blending, which does not require rolling to
homogenize, and individually test each cylinder and individually certify each cylinder.
Answer 30-Your interpretation of the protocol is correct. The protocol has always been intended to be
used on a cylinder-by-cylinder basis. As was stated in the 1978 version of the protocol, "analyze each
cylinder gas directly against the nearest SRM (or GMPS) by alternate analyses of the SRM and cylinder
gas in triplicate (three pairs)". In general, EPA Protocol Gases are prepared, assayed, and certified
individually, rather than in multiple-cylinder batches as is the case with SRMs and NTRMs. This approach
allows producers to sell individual cylinders containing user-specified gas mixtures at user-specified
concentrations on demand. The protocol does not require producers to maintain a large inventory of
cylinders having identical compositions, which helps to reduce the cost of these standards. The protocol
was never intended for the bulk assay and certification of multiple EPA Protocol Gases, either manifolded
together or used individually. Any specialty gas producer who has misunderstood this aspect of the
protocol should immediately stop bulk assays and certifications of EPA Protocol Gases and should assay
and certify them individually. Any user of bulk-assayed and -certified EPA Protocol Gases should stop
using them. The protocol is silent regarding production techniques and producers may employ their own
cylinder-filling procedures, such as dynamic blending. Multiple cylinders may be filled simultaneously, but
must be assayed individually. A technical correction will be added to Page 5 of the corrected version of
the protocol to clarify this point.
Question 31-1 just want to make sure that the paragraph at the bottom of Page 33, "Standards having
certified...the last assay", only applies to compositions below the bottom end with an initial 6-month cert
period, and does not imply that a 3-year NO mix (or 4-year SO2 mix) can move up to 8 years.
Answer 31- Thanks for spotting something that I had not considered when revising the protocol. The
corrected version of the protocol will include the following technical correction on Page 33: "The maximum
certification periods for recertified, low-concentration standards containing nitric oxide and sulfur dioxide
are 3 years and 4 years, respectively."
Question 32-1 continue to get challenged by customers as well as other Protocol producers concerning
whether or not a SO2 in air can be labeled as an EPA Protocol mix. I have been referring to your answer
to question #13 in your e-mail dated 8/7/12 ("Various Questions and Answers about EPA Traceability
Protocol for Gaseous Calibration Standards"). In this answer it seems clear to me that, in the absence of
a SRM or NTRM in a balance gas of air there can be no SO2 in air EPA Protocol mix. It still appears that
some producers are making the mix as EPA Protocol and they defend their decision by saying that they
can demonstrate that there is no bias in their readings on their instrument between S02 in air vs. S02 in
nitrogen. Are they correct that this is acceptable? Please let me know if that is the case. Thanks for your
help on this.
Answer 32-As is indicated in Answer 14, Table 2-3 has been revised to include S02 in air. When NIST
and VSL sign the next Declaration of Equivalence (DoE) in the summer of 2014, a specialty gas producer
will be able to purchase a VSL reference standard containing SO2 in air to use in assaying candidate
standards containing SO2 in air.
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Question 33- Since EPA invested a significant amount of time in writing Procedure G3 for the assay and
certification of zero air materials, I was surprised to find out from NIST that no standard exists for air or
N2. I would like to start monitoring our zero gas. What do you recommend that I use as a standard?
Answer 33-1 understand that NIST is developing zero air reference standards, which could be used to
assay zero air materials using Procedure G3 in the EPA traceability protocol. Until such time as NIST-
certified standards become available, I suggest that you use the best available zero air cylinders as your
reference standards. Any assay of zero air cylinders using these reference standards would not be NIST-
traceable, but they would help you to identify contaminated cylinders. It's the best that you can do for the
moment.
While the protocol was being revised in the past few years, several specialty gas producers had pointed
out the need for NIST-traceable, commercially-available zero air cylinders. Three things were needed to
allow such cylinders to be produced. First, EPA air pollution monitoring regulations or some other driver
was needed to create wide user demand for such cylinders. Second, an analytical procedure for
assaying and certifying the zero air materials using NIST-certified reference standards had to be
developed. Third, NIST had to see enough commercial demand for NIST-certified zero air reference
standards to justify expending government funds to develop the standards. A different government
organization (EPA's regulatory office, EPA's research office, and NIST, respectively) was responsible for
each one of the three necessary steps. The inclusion of Procedure G3 in the revised protocol was just
one of these steps. NIST's current work is another of these steps.
The International Organization for Standardization (ISO) Technical Committee TC 158 (gas analysis) is
currently working on specifications and a standard for zero gas. I suggest that you keep track of TC 158's
progress regarding zero gas. Contact VSL's Annarita Baldan (abaldan@vsl.nl) for information about their
progress. See http://www.macpoll.eu/zero-aas
Question 34-1 have heard folks in our industry referring to what they call an "EPA Protocol Blue List",
which evidently refers to those PGVP participants that should be avoided. These comments typically
originate from competitors in the field. Is there such a list and where may I find it on the web-site?
Answer 34- I've not heard about this blue list, either as an EPA-written document or an industry-written
document. A PGVP participant is a PGVP participant, period. The underlying philosophy behind the
protocol and the PGVP has been that EPA does not certify specialty gas producers as EPA Protocol Gas
vendors. Rather, EPA publishes the verification results and lets the end users make the decision about
what producer to buy from. The 2003 publication ("The Role of the Accuracy Assessment Program in the
EPA Traceability Protocol for Gaseous Calibration Standards") about the old audit program states:
"The protocol does not provide a blanket certification of a specialty gas producer and EPA has not
established a list of producers who are qualified or certified to produce EPA Protocol Gases. The
protocol may be used by any producer, standard user, or other analytical laboratory to establish the
traceability of a gaseous calibration standard to NIST SRMs or NTRMs."
This philosophy has continued with the PGVP. The emission PGVP web site
(https://www.epa.qov/airmarkets/protocol-qas-verification-proqram-pqvp) states:
"The PGVP has four main objectives: (1) to ensure that EPA Protocol gases meet the accuracy
requirements of 40 CFR Part 75; (2) to assist calibration gas consumers in their purchasing decisions;
(3) to provide an incentive for gas vendors that perform well in the audits to continue to use good
practices; and (4) to encourage gas vendors that perform poorly in the audits to make improvements."
The ambient air PGVP web site contains an annual report for 2012
(http://www.epa.oov/ttn/amtic/files/ambient/qaqc/aaqvp2012report.pdf). which states:
"This program is considered a verification program because its current level of evaluation does not
allow for a large enough sample of EPA Protocol Gases from any one specialty gas producer to yield
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a statistically rigorous assessment of the accuracy of the producer's gases. It will not provide end
users with a scientifically defensible estimate of whether gases of acceptable quality can be
purchased from a specific producer. Rather, the results provide information to end users that the
specialty gas producer is participating in the program and with information that may be helpful when
selecting a producer."
Question 35- We are a (country deleted)-based company with a production facility in (country deleted).
We produce and calibrate ISO 17025 emission gases. Our clients in (country deleted) have a
requirement to use EPA Protocol Gases. We would like some information on how to become registered
as a provider of EPA Protocol Gases and what criteria we need to meet.
We use the analytical technique of bracketing according to ISO CD 12963, with mass flow controllers for
dynamic dilution (ISO 6145: Part 7). We use a robust statistical method that takes into consideration both
the uncertainty on the measurement and the uncertainty on the calibration standard. We have traceability
back to NPL (UK). We are also accredited to ISO 17025 for this calibration. As part of the accreditation
we are required to do regular linearity checks and participation in annual PT schemes (round-robin).
Would it be acceptable to put a note in the description "conforms to EPA Protocol methods" on the
certificate?
Answer 35- The EPA traceability protocol does not provide a blanket certification of a specialty gas
producer to assay and certify EPA Protocol Gases. EPA has not established a list of producers who are
qualified or certified to produce EPA Protocol Gases. The protocol may be used by any producer,
standard user, or other analytical laboratory to establish the traceability of a gaseous calibration standard
to NIST SRMs or NTRMs. Because you are not selling EPA Protocol Gases in the US, you do not need
to participate in EPA's PGVP.
In order for your firm to certify a calibration gas as being an EPA Protocol Gas, the assay procedures,
NIST-traceable gaseous reference standards, and statistical analysis procedures that are defined in the
EPA traceability Protocol (see https://www.epa.gov/air-research/epa-traceabilitv-protocol-assav-and-
certification-qaseous-calibration-standards ) must be followed. The certificate of analysis for the
calibration gas must contain all the information that is specified in the protocol. Alternative procedures
are not acceptable for EPA Protocol Gases. Your traceability must be to US NIST gaseous reference
standards. Traceability to other national metrology institutes' reference standards or to NIST mass
reference standards is not acceptable for EPA Protocol Gases (see note below). If your firm is not able to
follow the EPA traceability protocol, you should contact your client and inform them that your firm cannot
supply EPA Protocol Gases. Of course, this is an opportunity to educate your client regarding alternative
traceability routes for calibration gases. If the client is required by EPA air pollution monitoring
regulations to use EPA Protocol Gases, then the calibration gases must be purchased from a specialty
gas producer that can follow the protocol. If the client is not required to use EPA Protocol Gases, you can
offer calibration gases having an alternative traceability route (e.g., to NPL) that can meet their needs.
Note that NIST and VSL have signed a Declaration of Equivalence (see
https://www.vsl.nl/sites/default/files/rtf/DoE%202016-
2018%20siqned%20bv%20NIST%20and%20VSL O.pdf). which allows specific VSL gaseous reference
standards to be used for the assay of EPA Protocol Gases. VSL gaseous reference standards may be a
traceability route that you may wish to consider.
Question 36- My client wants to sell EPA Protocol Gases to countries in (deleted global region), which
has adopted 40 CFR Part 75 regulations and which requires EPA Protocol Gases as calibration gases. I
want to know how EPA Protocol Gas production relates to participation in the PGVP.
Answer 36- This question was forwarded to EPA's John Schakenbach (schakenbach.john@epa.gov),
who ran the Emission PGVP and who is now retired. John's response is shown below:
"Our international law expert said that if the country is a participant in one of our international free
trade treaties, we cannot discriminate against them based on foreign nationality. Therefore, we'll
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need to know first what country is asking to participate. If the country is not planning to sell the
cylinders in the U.S., what is the benefit to us and the U.S. in allowing them to participate in the
program? If none, then there might be an appropriations problem.
"I just want to be clearer about the possible participation in the Emission PGVP of a non-U.S. EPA
Protocol gas production site. 40 CFR 75.21 (g)(1)(iii) requires a valid address for an Emission PGVP
participant. A valid address must include the country where the production site is located. Therefore,
if the production site that you represent does not provide the country where it is located, it cannot
participate.
"In the Emission PGVP, the National Institute of Standards and Technology (NIST) analyzes a blind
sample of EPA Protocol Gas cylinders collected from specialty gas companies. NIST provides the
results to EPA; EPA posts the results on our web site. Calibration gas customers can use our web
site to make buying decisions. The Emission PGVP is based on economic incentives - - production
sites that do well in the audit will presumably be rewarded by gaining customers; those that do poorly
may lose customers.
"However, for the PGVP economic incentives to work, we need to ensure that potential buyers have
"equal" access to all participants. If a production site has significantly higher shipping costs, or
refuses to sell EPA Protocol gas cylinders to U.S. customers, that production site no longer has the
same economic incentives to maintain or improve the quality of its calibration gases as the other
PGVP participants have, and would not be a good candidate for the Emission PGVP."
Question 37- (name deleted) facility has an infrared continuous emission monitor system (CEMS) for
hydrogen sulfide (H2S) permitted under 40CFR Part 60. Part 60 Appendix F requires cylinder gas audits
using NIST-traceable or EPA Protocol Gases. After reviewing your EPA traceability protocol, I still have
several questions concerning the composition of these gases and meeting traceability requirements.
1. (name deleted) has determined that the CEMS produces optimal data if the H2S cylinder
composition matches the digester gas matrix which is predominantly composed of methane (60%)
and carbon dioxide (40%). Gas vendors are not able to provide EPA Protocol Gases with this mix
but can provide a gravimetric NIST traceable gas mix. Attached is an example certification sheet for
a 700ppm H2S gas with 40% CO2 and 60% ChU. Does a gas traceable by weight, such as this
example, fulfill the Part 60 requirements?
2. In conjunction with #1 above, (name deleted) has found it difficult to obtain these gases at high H2S
concentrations such as 1600ppm and 2600ppm either in balance nitrogen or in matrix CH4/CO2.
Again, these are available as gravimetric NIST-traceable gases, are these acceptable?
3. The CEMS is currently set up to perform cylinder gas audits and also sealed cell audits. The
CEMS has several sealed cells containing different concentrations of H2S. Attachment #2 is a copy
of a certificate the vendor has supplied for one of the sealed cells which contains 5.1% H2S and, in
this case, balance N2. In your opinion is this a NIST-traceable standard as the certificate
indicates? If not, do you have any guidance as to what criteria are necessary to demonstrate that
the sealed cells would meet EPA protocol or NIST traceability?
4. Part 60 does not include a provision for using sealed cell technology, (name deleted) is in the
process of evaluating if the sealed cells could be used instead of the cylinder gas audits. We
propose performing side-by-side audits of the cylinder gas and sealed cells at a frequency that
would produce a statistically robust data set to compare and present to regulators for approval of
this technology. Do you have any advice on putting together that study or know of any other utility
or business that has done this that I could contact? If not, do you know anyone else at EPA I could
contact about this?
Answer 37- This question was forwarded to Ray Merrill (merrill.raymond@epa.gov, 919-541-5225), who
works in EPA's Office of Air Quality Planning and Standards. Ray's response is shown below:
"As I understand your procedure involves using an extractive system based on White cell - tunable
diode laser (TDL) technology to monitor H2S on a continuous basis. You did not mention a specific
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subpart of 40 CFR Part 60 or 63 that applies to the requirement to measure H2S so my answer will be
general. Our Performance Specification 7 is generic for continuous H2S monitors. The ongoing
QA/QC that are required for monitoring should be identified in the specific regulatory requirement in
your permit or regulatory rule.
"You are correct that we consistently require EPA Protocol Gases when you are required to perform
Cylinder Gas Audit (CGA) as part of the QA/QC from 40 CFR Part 60 Appendix F. (e.g., from
Procedure 1 intended for fixed gas analysis:
"Section 5.1.2, (3) Use Certified Reference Materials (CRM's) (See Citation 1) audit gases that have
been certified by comparison to National Institute of Standards and Technology (NIST) or EPA
Traceability Protocol Materials (ETPM's) following the most recent edition of EPA's Traceability
Protocol No. 1 (See Citation 2). Procedures for preparation of CRM's are described in Citation 1.
Procedures for preparation of ETPM's are described in Citation 2. As an alternative to CRM's or
ETPM gases, Method 205 (See Citation 3) may be used. The difference between the actual
concentration of the audit gas and the concentration indicated by the monitor is used to assess the
accuracy of the CEMS.")
"There seem to be two questions in your correspondence that need to be addressed and a third that I
bring to your attention:
1 If H2S cylinder gas audit material that meets NIST traceability or EPA's protocol gas traceability
standard, can you use a vendor certified standard that is traceable gravimetrically to NIST?
2 Are sealed cell H2S standards that you insert into your instruments optical path equivalent to flow
through cells?
3 Since your system is an extractive White cell technology, one question you did not ask, but we
should consider is: Does a calibration type cell inserted into the optical path meet the requirements of
a CGA?
"In my opinion, the intent of the CGA is to test the entire CEMS system not just the measurement
path in the instrument. (As a related example, direct measurement of H2S using Method 15 requires
a recovery test executed by spiking a certified H2S standard gas at the probe.)
"We've received several inquiries regarding CGA requirements when the facility or test firm believed
there was no qualifying NIST or EPA Protocol Gas available. I'm consistently recommending that
facilities or test firms review and follow the guidance (3 pages) for requesting an alternative method
request found on our website at https://www3.epa.gov/ttn/emc/quidlnd/qd-022.pdf. By submitting a
request that has the information summarized in this document we can review and provide a formal
response that you will have for your records. Supporting data that demonstrates the effectiveness of
your approach will streamline the process. Once you've had a chance to review the alternative
request guidance, call me if you have questions."
Question 38-1 received an e-mail from the owner/operator of a cement plant in (location deleted). He
indicated that he could no longer purchase EPA Protocol Gas for CO2 in a concentration over 20%. Our
current guidance is that cylinder gases must be in accordance with the requirements specified in the
"Reference Gases" section of 40 CFR, Part 75, Appendix A or as specified in an applicable Federal
regulation. Could you provide a recommendation on what requirements the vendor should follow for CO2
gas in concentrations above 20% assuming that EPA Protocol Gas is no longer available in such
concentrations?
Answer 38- This question was forwarded to Ray Merrill (merrill.raymond@epa.gov, 919-541-5225), who
works in EPA's Office of Air Quality Planning and Standards. Ray's response is shown below:
"To help in this instance, I need to know if the testing requirement comes from 40 CFR Part 75 or
from one of the other stationary source requirements like 40 CFR Part 60, or 63 (i.e., NSPS or PSD).
For Part 60, and 63 we recognize many of the higher concentration gases are not available as "EPA
Protocol Gases" because there is no comparable NIST standard gas (NTRM/RGM/etc.) for
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traceability. In some cases, we've been asked to approve an alternative to the protocol gas
requirement when, for instance, cylinder gas audits are required for our instrumental methods.
"For example, see Alternative 102 on our website which says in part: We acknowledge that NIST-
certified reference gases are not available for TRS gases at the applicable 40 CFR 60 Subpart Ja
instrument span values, and that alternative gases must be permitted. We believe that gases
certified to 2 percent of the manufacturer's listed concentrations are a reasonable alternative for the
CGA in this case.
"If formally asked, we'd most likely follow the same intent in response to similar requests for
alternative approvals when protocol gases are not available to meet Part 60 or 63 requirements.
Formal requests for alternative methods can be submitted following Guidance Document 22 on our
website, https://www3.epa.gov/ttn/emc/quidlnd/qd-022.pdf
"Subpart 75 falls under the jurisdiction of EPA's Clean Air Markets Division (CAMD). You'd have to
ask Travis Johnson (202-343-9018, johnson.travis@epa.gov) in the Emissions Monitoring Branch of
CAMD for his recommendation or opinion on this issue in regard to Subpart 75."
Question 39- Has there been a determination of the acceptability of CO2 in air, and C3H8 in air gas
mixtures above the 500-ppm limits found in Table 2-3 of the 2012 EPA traceability protocol? In addition
to the balance air mixtures, our company also continues to receive requests for CO2 or C3H8 or SO2
mixtures each containing different concentrations of %-level O2 in a balance of nitrogen. We have been
successfully producing these gas mixtures for decades under the older EPA Protocol Gas rules. Are
these acceptable under Paragraph 2.1.5.3 of the 2012 EPA traceability protocol? Again, our experience
is that all these gas mixtures are no different than any other multi-component candidate standard and
they are stable and the O2 does not alter the stability of either compound.
Answer 39- Calibration gases containing gas mixtures and concentration ranges that are not listed in
Table 2-3 cannot be certified as EPA Protocol Gases. As indicated in previous answers, end users can
submit an alternative method request to EPA for the use of other calibration gases in the place of EPA
Protocol Gases. In addition to concerns about the potential instability of such calibration gases, there are
also concerns about potential measurement bias if the candidate standard and the reference standard are
not closely-matched. NIST's Frank Guenther, who has since retired, responded to EPA's question about
NIST's definition or specification for the composition of balance air in SRMs, NTRMs, and RGMs as
follows:
"The composition of the balance gas is a very important issue that relates to biases in certain
instrumentation. Even gas chromatography, where the balance gas is largely separated from the
peaks of interest, these issues can cause small biases. The bottom line is that the calibration gas
must match the balance gas of the flow being analyzed as much as possible. Air is a fuzzy definition
that I do not think solves anything, as the flow being analyzed may differ from normal air composition.
Close matching of the balance gases would be my suggestion, and if greater accuracy is required, a
study of composition biases in the analytical system should be done. As far as our definition of "air" it
is:
Oxygen: (20.95 ± 0.05) % mol/mol
Argon: (0.93 ± 0.01) % mol/mol
Nitrogen: (78.08 ± 0.02) % mol/mol
Carbon dioxide: (490 ± 10) jjmol/mol
"This mixture will eliminate composition biases in most instrumentation."
Question 40- EPA Protocol Gases have an initial certification period of 6 months if they fall below the
lowest concentration that is listed in Table 2-3 (e.g., 4 ppm ammonia in N2). If a low-concentration
standard is returned, re-assayed, and meets the TOST test for stability, can it then be recertified for the
next level above, i.e. 12 months?
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Answer 40- Your question is covered by Section 2.1.11 of the protocol, which states:
"Standards having certified concentrations that are lower than those given in Table 2-3 may be
recertified for the period given in Table 2-3 provided at least 6 months have elapsed between the
initial certification and the recertification. The maximum certification periods for recertified, low-
concentration standards containing nitric oxide in nitrogen and sulfur dioxide in nitrogen are 3 years
and 4 years, respectively. The corresponding maximum certification period for sulfur dioxide in air
standards is 2 years. For example, a 0.5-ppm sulfur dioxide in nitrogen standard will have an initial
certification period of 6 months. After a successful recertification, this standard will have a maximum
recertification period of 4 years. The certification date is the date of the last assay."
Question 41-1 understand "Calibration gases containing gas mixtures and concentration ranges that are
not listed in Table 2-3 cannot be certified as EPA Protocol Gases."; however, according 2.1.3.2, "Pure
gases may be dynamically diluted to prepare gas mixtures for use in multipoint calibrations, but such
mixtures may not be used as the reference standards for the span gas check or for the assay of the
candidate standard." Taking 22% CO2/N2 as an example; even though the top range listed in Table 2-3 is
at 20%, one could use pure CO2 dynamically diluted to 25% to extend the curve, then use 20% SRM as
span gas to certify this 22% CO2 EPA protocol gas. Please advise if my understanding of protocol is
wrong.
Answer 41- The procedure that you propose would be acceptable under the protocol as is discussed
below:
Section 2.1.1 of the protocol states "A candidate standard having a concentration that is lower or higher
than that of the reference standard may be certified under this protocol if both standards' concentrations
(or diluted concentrations) fall within the well-characterized region of the analyzer's calibration curve."
As you found, Section 2.1.3.2 states "Pure gases may be dynamically diluted to prepare gas mixtures for
use in multipoint calibrations, but such mixtures may not be used as the reference standards for the span
gas check or for the assay of the candidate standard. Pure gases may not be diluted by more than a
factor of 100."
Section 2.1.4.2 states "All measurements of candidate standards must fall within the well-characterized
region of the analyzer's calibration curve, which lies between the largest and smallest measured
concentrations of the multipoint calibration and for which U for the regression-predicted analyzer
response is <±1 percent of the measured response for the largest concentration in the calibration.
Section 2.1.4.2 also states: "If a gas dilution system is used in the assay apparatus, it must have a
specified accuracy of no worse than 1.0 percent of the undiluted reference standard concentration.
Additionally, the gas dilution system must be checked by the analyst at monthly intervals to verify that its
calibration has not drifted significantly since its last calibration or recertification. Use an NIST-traceable
flow rate reference standard to check at least one flow rate setting for each pollutant and dilution gas
stream in the assay apparatus."
Your experimental procedure would be to first check the calibration of the gas dilution system to make
sure that it is functioning correctly. Second, dilute the pure CO2 to generate a multipoint calibration curve,
whose minimum concentration must be less that the concentration of the reference standard. Don't
skimp on the number of measurements and the number of concentrations because more measurements
and concentrations will give you a wider and tighter well-characterized region. It may be a good idea to
limit the range of the multipoint calibration to the region immediately around the concentrations of the
candidate standard and reference standard. Third, measure the reference standard and predict its
concentration using the multipoint calibration curve. If the actual and predicted concentrations for the
reference standard match to within the uncertainty of the curve, then you good to go ahead with the assay
of the candidate standard. If not, something has gone wrong, probably with the gas dilution system. Stop
until you figure out what caused the disagreement in the predicted and actual concentrations.
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This approach should work if the concentration of the candidate standard is not much greater than the
reference standard. The protocol's uncertainty constraints will make it difficult to assay if the
concentration separation is too great.
Question 42-1 just want to confirm my understanding of the EPA Protocol, Section 2.1.3.1 about GMIS:
"A candidate GMIS must be assayed on at least three separate dates that are uniformly spaced over
at least a 3-month period."
One could interpret this to mean that we need three assays which are separated by a month. I don't think
that was the intention. The way I read the sentence is that we need three assays (of at least three
measurements each) that are spread over 90 days. To clarify...Interpretation A: If I assayed the
candidate GMIS on 1/10/2017 and again on 2/10/2017 and again on 3/10/2017. (Three separate months
-total elapsed time = ~60 days). Interpretation B: If I assayed the candidate GMIS on 1/10/2017 and
again on 2/25/2017 and again on 4/10/2017. (Three calendar months - total elapsed time = ~90 days). I
think Interpretation B is correct. Am I correct???
Answer 42- You are correct. The point of requiring three separate assays over a three-month period is to
detect any instability in the gas mixture. The uniform spacing of the assays is not that rigid of a
requirement, but would help in the statistical analysis of the data to quantify any trend. There is nothing
magical about what specific dates are chosen in that period. Perhaps I should substitute the words
"approximately uniformly spaced" to clarify the purpose of this requirement.
Question 43- We are looking into producing methane EPA Protocol blends, but the NIST website is
limited as far as the SRM's offered. Currently, there are 10 ppm, 50 ppm, and 100 ppm methane/air
SRM's available as well as a 1 ppm methane/air SRM that is out of stock. Since the recommended 5
points for the curve is not available from NISY, would we be able to start with 3 points from the 10 ppm,
50 ppm, and 100 ppm SRM's and build up to a 5-point curve by analyzing 25 ppm and 75 ppm GMIS's
against the SRM curve? We want to know if there is a way we can get started with what is available.
Answer 43- Section 2.1.3.2 of the protocol states: "The reference standards for the multipoint calibration
must be diluted or undiluted SRMs, RGMs, PRMs, CRMs, NTRMs, or GMISs (see Subsection 2.1.3) or
dynamically diluted pure gases." So it is acceptable to do the multipoint calibration using a combination
of undiluted SRMs and undiluted GMISs as you have proposed.
Section 2.1.4.2 of the protocol states: "The multipoint calibration must consist of one or more
measurements of the analyzer responses to at least five different concentrations. The use of an NIST-
traceable zero air material in the calibration is recommended, but is not required (see Section 2.1.3.3)."
The correct section about the zero gas is Section 2.1.3.5, which states: "Zero gas used for multipoint
calibrations, zero gas checks or for dilution of any candidate or reference standard must be clean, dry,
zero-grade air or nitrogen containing no detectable concentration of the pollutant of interest." These
sections mean that you can do a multipoint calibration using a zero gas and four undiluted SRMs, RGMs,
PRMs, CRMs, NTRMs, or GMISs. You can't do a multipoint calibration with less than five different
concentrations.
Question 44-1 was reviewing the EPA Protocol again this morning and found this section:
"2.1.3.2 Reference Standards for Multipoint Calibrations—
The reference standards for the multipoint calibration must be diluted or undiluted SRMs, RGMs,
PRMs, CRMs, NTRMs, or GMISs (see Subsection 2.1.3) or dynamically diluted pure gases. Pure
gases may be dynamically diluted to prepare gas mixtures for use in multipoint calibrations, but such
mixtures may not be used as the reference standards for the span gas check or for the assay of the
candidate standard. Pure gases may not be diluted by more than a factor of 100. Information
concerning this standard (e.g., cylinder identification number, certified concentration, expanded
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uncertainty, certification expiration date, cylinder pressure, etc.) must be recorded in the laboratory's
records."
This can help us a lot for those cases where the concentration is in the percent level range and no SRM's
exist (e.g. 5% methane in nitrogen). This section would allow us to linearize using pure methane and a
suitable dilution system. This allows us to linearize but not for the span check or use for the assay of the
candidate standard.
Question: If we use a pure gas and dilutions to linearize and do the assay on the same day of
linearization must we also have an SRM, RGM, PRM, CRM, NTRM, orGMIS in the percent range (which
does not exist) as a reference standard for the assay of the candidate standard?
Follow-up question: If the answer above is "Yes" - If we must have an additional standard in order to
perform the assay on the same day as the linearization by dilution of a pure gas, may we use a standard
developed under ISO 6143 as the standard? (FYI - We are ISO 17025 and Guide 34 accredited.)
Answer 44-1 don't think that your proposed procedure will work because error of the diluted pure gas is
unknown. You will need to use a NIST-traceable reference standard for the assay of high-concentration
calibration gases and you cannot use the multipoint calibration of diluted pure gas as the reference
standard.
Question 45-1 .For assay of GMISs.... If we identify and correct a specific problem with an instrument
that made the third assay invalid, may we discard that third assay and perform a 4th assay after another 7
days?
Answer 45- The situation is analogous to that of a candidate standard which has been found to be
unstable by TOST after two assays. The paragraph at the top of Page 24 in the protocol covers this
situation:
"If a candidate standard's concentration is not found to be stable (i.e., not within the TOST
acceptance criterion), the analyst may elect to discard the candidate standard or may elect to conduct
a third assay of the candidate standard to assess whether stability has been achieved. The analyst
must add the additional data to the Appendix C spreadsheet. The analyst must wait an additional 7
days or more and conduct the third assay. If the data for the third assay is found to be equivalent to
the data for either of the two previous assays, the candidate standard can be certified using the data
from the two equivalent assays, which will be used to calculate the overall estimated concentration
and the total uncertainty. Data from a nonequivalent assay should be discarded. The analyst must
disqualify the candidate standard if none of the three sets of data are found to be equivalent. The
analyst is expected to investigate and document the cause of the lack of agreement among the three
assays and is expected to correct any problems that are discovered. Record the equivalent data and
any discarded data in the laboratory's records."
If the fourth assay of the GMIS agrees with two of the three previous assays, probably the oddball assay
was a blunder for some reason. Discard the data for the blunder and report the data for the three good
assays. That being said, I would tear apart the assay apparatus and the experimental procedures to
determine how the blunder occurred. If a blunder happens once, it may happen again if something isn't
corrected.
Question 46- 2.On the bottom of Page 24, we read the following, "A value of one-half of U for the
reference standard should be used in calculating U of candidate standards that are certified under this
protocol (see Appendix C).". But, when we go to Spreadsheet C, cell A104, we see the following,
"Expanded uncertainty (the two-sigma uncertainty) of the reference standard". Am I reading this right?
Should we report U (k=2) or u (k=1) for the reference uncertainty in cell A104 of spreadsheet C?
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Answer 46- Report U. The protocol should not be your only source of information regarding uncertainty
calculations. NIST Technical Note 1297 (https://www.nist.gov/pml/nist-technical-note-1297') is the basis
of the ISO Guide to the Expression of Uncertainty in Measurement (GUM) (See
https://www.bipm.org/utils/common/documents/icqm/JCGM 100 2008 E.pdf), which is the international
standard for metrology uncertainty calculations. There are also several text books that explain error
propagation calculations. Two of these books on my bookshelf are John R. Taylor's "An Introduction to
Error Analysis. The Study of Uncertainties in Physical Measurements (1997)" and Coleman and Steele's
"Experimentation and Uncertainty Analysis for Engineers (1999)". I bought both books on Amazon.
You need to become familiar with the standard uncertainty (u, small u) for one measurement variable, the
combined uncertainty (Uc, small U-sub-c) for multiple measurement variables, the coverage factor (k,
usually equal to 2), and the expanded uncertainty (U, big U). As NIST's David Duewer et al. (2006)
http://cenam.mx/memsimp06/Trabaios%20Aceptados%20para%20CD/Qctubre%2027/Blogue%20E/E2-
QUIMICA%20IV-lncertidumbre/E2-1 .pdf state:
"Most results were derived using the propagation of uncertainty (PoU) approach recommended in the
Guide to the expression of uncertainty in measurement (GUM). For the product of a series of quantities
such as Result = A * B * C, the PoU model for the combined uncertainty is
"where u(A), u(B), and u(C) are the standard uncertainties for the quantities A, B, and C."
Now go back to the Appendix C spreadsheet and play around with some numbers. Then check your
calculations with a calculator. For example, if your NIST reference standard has an expanded uncertainty
(Urs) of+/-1 percent and your assay of the candidate standard has a combined uncertainty (uc) of+/- 0.5
percent, then the expanded uncertainty of the certified concentration of the EPA Protocol Gas should be
(calculating in relative percentages):
U = 2 x V((1 percent/2)2 + (0.5 percent)2)) = 2 x V (0.5) = 2 x 0.707 = 1.4 percent, which would go on the
certificate of analysis. Urs was divided by the coverage factor before it was used in the error propagation
calculation.
I hope that this discussion will help you put the correct values into the Appendix C spreadsheet. Let me
know if you still have questions about the uncertainty calculations. I will check Page 24 and see if the
explanation of the uncertainty calculations has to be clarified. Thanks for bringing the problem to my
attention.
Question 47- A specialty gas producer approached EPA with a question about assaying and certifying
high-concentration EPA Protocol Gases. Its customers requested EPA Protocol Gases with
concentrations that are higher than NIST reference standards. The producer proposed to use a variation
of Procedure G2, which allows for dilution of reference standards and candidate standards as presented
below:
"Most importantly, the use of Procedure G2 using an ISO17025 annually certified NIST-traceable gas
dilution system for analytical testing of compositions above available NTRMs. Example:
1) Customer orders 40% CO2/N2 Protocol
2) We monthly generate the 10+ point curve on a CO2 NDIR covering 5% to 25% CO2 through use
of the gas dilution system and a nominal 25% CO2/N2 NTRM, and then drop undiluted nominal 20%
CO2/N2 and 10% CO2/N2 NTRMs to validate and develop the characterization statistics,
3) Then dilute a 25% NTRM down to 20.0% and the candidate Protocol mix (at nominal 40%
CO2/N2) also down to 20.0% through the gas dilution system
4) Run using the normal 3 triads (zero/NTRM/sample)
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5) Calculate total uncertainty incorporating: measurement uncertainty, curve uncertainty, NTRM
uncertainty, and dilution system uncertainty at 95% CI using propagated uncertainties
"We field a significant number of requests and orders for mixes at higher concentrations than listed in
revised Table 2-3. We've produced NTRMs up to 25% CO2/N2, 23% O2/N2, 2.0% propane/ISh, 1.0%
methane/N2 to capture most requests while using Procedure G1. Plus have PRMs up to 300ppm
NH3/N2 that permit Procedure G1."
Answer 47- In response to EPA's inquiry, NIST's Frank Guenther addressed this question:
"I think the procedure is sound as long as the gas dilution system is shown to have linear dilution
characteristics, and the uncertainty of the dilution is properly considered. Extending outside the
validated region as outlined below, (diluting a 25% into the range thus validating the dilution, and then
diluting a 40% into the range which effectively is outside the validated range) is the problematic part
of the procedure. It would need proper uncertainty estimation, and perhaps a larger uncertainty. For
normally stable compounds, I do not see stability issues unless it condenses at high concentrations.
Some compounds, NO for instance, is less stable at high concentrations due to the complex nitrogen
chemistry, so some caution and chemical knowledge is required. I would think these concentrations
would be of less interest to EPA."
EPA unsuccessfully tried to develop an analytical procedure that was similar to the proposed procedure.
The main difference would have been that the reference standard and the candidate standard would be
diluted equally. This procedure would have been a modification of the G2 procedure as outlined below:
1- Measure your highest-concentration NIST-traceable reference standard without dilution;
2- Use a gas dilution system to dilute your highest-concentration NIST-traceable reference standard
to generate a multipoint calibration curve;
3- Enter the calibration data (undiluted and diluted) in the Appendix A spreadsheet;
4- Use an undiluted check standard (also an NIST-traceable reference standard) to verify the
accuracy of the multipoint calibration curve;
5- Use the same gas dilution system to dilute the highest-concentration NIST-traceable reference
standard such that its analyzer response falls somewhere in the well-characterized region of the
calibration curve;
6- Calculate the dilution ratio using the analyzer responses for the undiluted and diluted reference
standard measurements;
7- Wthout altering the settings of the gas dilution system, dilute the candidate standard such that its
analyzer response also falls somewhere in the well-characterized region of the calibration curve;
8- Enter the data for the diluted reference standard and the diluted candidate standard in the
Appendix A spreadsheet;
9- Use the Appendix A spreadsheet to determine the concentration and uncertainty of the diluted
candidate standard; and
10- Multiply the concentration and uncertainty by the dilution ratio from Step 6 to obtain the
concentration and uncertainty of the undiluted candidate standard.
The beauty of this approach would have been that the uncertainty of the gas dilution system would not
need to be considered in determining the uncertainty of the undiluted candidate standard concentration.
The procedure is shown graphically below. The analyst is using 15.85- and 9.51-percent carbon dioxide
reference standards to assay a 30-percent carbon dioxide candidate standard. All three standards are
diluted by a factor of two to bring their diluted concentrations into the well-characterized region of the
pollutant gas analyzer's quadratic calibration curve.
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• I
• I
• I
Multipoint Calibration
Undiluted Check Standard
Diluted Reference Standards
.1
• 1
• 1
Diluted Candidate Standard
Undiluted Candidate Standard
Quadratic Regression
y-=q
j.0026xz +1
R2 = 1,
,0359x-0.0054
.0000
/V /
i
y" j
*
-d
0 5 10 15 20 25 30
Concentration (percent)
Assay Procedure for Higher-Concentration Candidate Standard
This proposed procedure failed to account for the fact that the physical and thermal characteristics of
high-concentration carbon dioxide gas mixtures vary significantly as the concentration varies significantly.
This variation has significant effects on the k-factors for mass flow controllers and viscosity effects for gas
dividers, which prevent mass flow controllers and gas dividers from diluting the gas mixtures accurately.
The measurement principle for thermal mass flow controllers involves measuring the temperature
difference across two temperature sensors that are mounted on a capillary equidistant upstream and
downstream of a constant-power heater. When no gas is flowing through the capillary, the temperatures
of the two sensors are equal. When gas is flowing, the upstream sensor will be cooler, and the
downstream sensor will be warmer. For very low gas flow rates, the heat transfer equation is given by
QmCp = CtempAT
where
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Qm = mass flow rate of the gas through the capillary;
Cp = specific heat capacity of the gas at constant pressure;
Ctemp = constant depending on the geometry and design of the capillary; and
AT = temperature difference across the two sensors.
For a specific pure gas, Qm can be determined by measuring AT. Different pure gases have different
values of Cp at standard temperature and pressure [STP (i.e., 0°C, 1 atmosphere)]. As a consequence, a
mass flow controller that is calibrated to deliver a specific mass flow rate of nitrogen will deliver another
flow rate for another pure gas at the same setting. To obtain the correct flow rate, the measured value
must be multiplied by the gas correction factor (or K-factor) for the pure gas, which is the ratio of the
specific heats of the pure gas and nitrogen.
Gas correction factor = Cp(pure gas)/Cp(nitrogen)
Because most of the gas passing through a mass flow controller goes through a bypass laminar flow
element rather than through the capillary, real controllers may not follow theoretical models. Gas
correction factors for individual controllers may vary significantly across brands, models, devices, and flow
rates. To avoid errors associated with diluting different pure gases or high-concentration gas mixtures,
the user manual for the mass flow controller being used must be consulted for the device-specific gas
correction factors to be used. Published gas correction factors are approximations possessing an
unknown level of uncertainty. For example, gas correction factors for various gases were measured for
three identical mass flow meters at The Department of Energy's Oak Ridge National Laboratory. In the
case of carbon dioxide, the published gas correction factor was 0.74. The experimentally determined
factors ranged between approximately 0.705 and 0.73 over flow rates between 0 and 2 liters per minute
(L/min). At one flow rate, the factors for the three flowmeters ranged between approximately 0.705 and
0.725.
The measurement principle for capillary-tube gas dividers involves passing the pollutant gas and diluent
gas through variable numbers of capillary tubes with identical pressure drops. Different dilution ratios are
obtained by changing the numbers of capillaries passing pollutant gas and diluent gas. To a first
approximation, the laminar flow through a capillary is determined by the Hagen-Poiseuille equation:
m= ("npr4 (Pi-P2))/8r|L
where
m = mass flow of a compressible fluid through the capillary;
p = density of the fluid;
r = radius of the capillary;
Pi = upstream pressure;
P2 = downstream pressure;
r| = dynamic viscosity of the fluid; and
L = length of the capillary.
The flow rate of gas through a capillary is a function of the viscosity of the gas, which is a measure of the
resistance of a fluid to the transport of momentum. It is a characteristic of the internal motion of individual
gaseous molecules and their collisions with other molecules and the walls of the channel. The viscosities
of pure gases are a function of the molecule itself, temperature, and pressure.
The flow rate must be corrected if the gas being used differs from the gas used in the calibration of the
capillary. In the case of pure gases, the flow rate correction is straightforward using the Hagen-Poiseuille
equation. The uncorrected flow rate through all diluent capillaries and the corrected flow rate through all
pure gas capillaries yield the correct pollutant concentration (in units of percent) using the following
equation:
Concentration = 100 (corrected pollutant flow rate)
(corrected pollutant plus diluent flow rates)
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Different pure gases have different values of density and dynamic viscosity at normal temperature and
pressure [NTP (i.e., 20°C, 1 atmosphere)].
For high-concentration gas mixtures that are diluted by capillary-tube gas dividers, obtaining the same
dilution ratio may be complicated by differences in the viscosities of the reference and candidate
standards. If the two standards have nearly the same concentrations, there is little need to correct the
flow rates for viscosity differences. However, these differences must be considered for high-accuracy
assays of standards with significantly different high concentrations. For example, EPA evaluated the
characteristics of a gas divider used to generate calibration data for exhaust emission gas analyzers.
Calibration curves that were generated by gravimetric calibration gases [ i.e., C3H8 (0 to 100 ppm), CO2 (0
to 15 percent), CO (0 to 2500 ppm), and O2 (0 to 24 percent)] were compared with gas mixtures using the
gas divider. The gas divider was found to provide acceptable results for C3H8 and CO. However, the
viscosity of the CO2 and O2 at the higher concentrations affected the gas flow, which caused a 1 to 2
percent error relative to the gravimetric calibration curve.
The technical literature contains numerous publications about theoretical calculations and experimental
measurements of the viscosity of gas mixtures. Various theories are used to predict the viscosities, which
become more complex as one departs from room temperature and atmospheric pressure to higher levels.
Considerable variation exists in the predicted values.
Question 48-1 have a request from one of my customers for both carbon monoxide in nitrogen and
oxygen in nitrogen EPA protocols in steel cylinders. I looked through my copy of the EPA protocol 2012
revision and didn't see any guidance regarding this. I know from personal experience we can do the
oxygen ones (not that we have), but the carbon monoxide ones might be a little tricky in steel because of
stability issues. Can you provide me some guidance regarding EPA protocols in steel cylinders?
Answer 48- The relevant sentence in the protocol can be found in Section 2.1.9 on Page 30:
"In general, the certification period for standards that are contained in nonaluminum cylinders is 6
months."
If your customer can live with a 6-month certification period, you can prepare the CO in N2 cylinder and
certify it as an EPA Protocol Gas.
The late Robert Denyszyn worked for Scott Specialty Gases and later Praxair. In 1985, he got a PhD
from Drexel University. His thesis, Stability Studies of Low Part Per Million Carbon Monoxide Standards
in Steel Cylinders may be of interest to you. The abstract states:
"This stability study was based on the potential reaction of low ppm carbon monoxide with the various
oxides found on steel cylinder surfaces. Analytical and sampling methods were developed to
measure the stability of low ppmv CO standards so that CO measurements at the 20 ppmv level
could be made with a precision of 0.1% at the 0.95 confidence interval. To confirm the changes in CO
concentration, a cryogenic sampling system was developed to concentrate the CO2 formed during the
reaction. The concentrated samples were analyzed on a GC methanization FID system. Oxygen,
water and other variables which were known to play a significant role in the overall reaction were also
monitored."
"Carbon monoxide stability was evaluated under five different surface cylinder treatments. One
treatment was found to significantly improve the stability of carbon monoxide from a batch
concentration change of-1. 0% relative/year to -0.1% relative/year. Twenty-one additional CO
stability experiments were performed to evaluate the influence of the three predominant iron oxides
found on steel surfaces, FeO, Fe203, Fe304. At 14,000 kPa, all three oxides were also found to be
active at room temperature. Oxygen was found to play a significant role in the rate of CO change. In
some cases, increasing the 02 content from <0.02 ppmv to only 5 ppmv changed the rate of CO
change/year by an order of magnitude. Gas phase H2O concentration significantly decreased the
reactivity of all oxides."
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You be the judge, but I think that there would be fewer stability hassles if your customer used aluminum
cylinders, which would have a longer certification period than steel cylinders.
Question 49- In Appendix A, we input the data for zero/span in cell C264 to F273. Shall we enter the
same zero/span values in B307 to C326? The table shows it is for "Reference Standards" only. Are the
assay data input to H307 to H316?
Answer 49- Section 2.2.6.4 on Page 41 of the EPA traceability protocol for gaseous calibration standards
states: "The gas mixtures to be used during the zero and span gas checks need not be, but can be, the
reference standards used for the assay of the candidate standard or for the multipoint calibration." The
protocol gives the analyst considerable latitude in the gas mixtures that are used for the multipoint
calibration, for the zero and span gas checks, and for the assay of the candidate standard. There are two
purposes of the zero and span gas checks. The first purpose is to verify that the analyzer's precision is
acceptable and to determine how many replicate measurements are needed during the assay of the
candidate standard. The second purpose is to verify that excessive calibration drift in the analyzer has
not occurred since the multipoint calibration.
The same measurements of the zero and span gases can be used for determining the estimated
concentration of the candidate standards. Measurements of different zero and span gases may also be
used. The analyst may chose the alternative that makes the most sense for the assay of the candidate
standard. If the same measurements are used, then the values that are entered in Cells C264 to F273
should also be entered in Cells B307 to C326. Measurements of the candidate standard are entered in
Cells H307 to H316.
Question 50- We have to conduct two assays for new reactive gases (SO2, NO) and thus we have two
sets of results for Appendix C. How about the non-reactive gas such as CO? From the guideline, we only
need to conduct one assay for the new gas cylinder. In our old practice, we used to compare the set of
assay result with the certificate from manufacturers. If so, we do not have multipoint/zero/ span data to
the Appendix A and thus not able to apply the TOST method. Could you please advise?
Answer 50- In the assay of a nonreactive gas mixture that is purchased from a specialty gas producer,
the producer's value for the candidate standard should not be used for determining the certified
concentration of the candidate standard. I presume that the gas mixture was not purchased as an EPA
Protocol Gas from the producer and that the producer did not use NIST-traceable reference standards in
the assay of this gas mixture. I also presume that the producer's certificate of analysis for the gas mixture
does not have a statistically-valid estimate of the uncertainty of the certified concentration of the gas
mixture. If so, it would be an apples-to-oranges comparison to try to compare the producer's value to the
concentration that is determined by the protocol. The former value is not traceable in a metrological
sense, but the latter value is NIST-traceable traceable.
You may wish to compare the two values for informational purposes. Because you have some statistical
estimates from the protocol assay, but none from the producer's assay, Student's t-test for means or
TOST are not applicable. I suspect that you will want to do some statistical test having to do with
conformance to specification, which is a topic that I have only limited knowledge. The simplest approach
would be to see whether the producer's value falls within the expanded uncertainty of the protocol's
value. If so, there's probably no problem. If no, there may be some problem with the producer's
measurements, the protocol's measurements, or unexpected decay in the cylinder. I suggest that you
consult with a local statistician for a more sophistical approach to address the conformance question.
Question 51- We have a low CO cylinder, approximately 100 ppb, that we just received. We were able to
certify it and will use it for a CO trace audit. I looked in the green book for recert info and it has CO in air
40 to 500 ppb has TBD for cert period. Do we just say what the recert period will be in our QAPP or has
there been some determination on when this low concentration cylinder needs recertified?
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Answer 51- Here is the reply from the Van Swinden Laboratorium (VSL), the Dutch national metrology
institute, about the appropriate certification period for sub-ppm CO in air standards. VSL is a European
equivalent of NIST. They agree with NIST that a six-month certification period is most appropriate for
these standards. In the future, longer certification periods may be reasonable provided better treatments
or cylinder materials become available. Luxfer in England has developed a new SGS cylinder treatment
that looks promising (see http://www.luxfercylinders.com/press-releases/luxfer-exclusive-sgs-aluminum-
cylinder-internal-surface-for-specialty-gas-applications), but I am not aware of any long-term stability data
for sub-ppm CO in air mixtures in the SGS cylinders.
Question 52- 2.On the bottom of page 24, we read the following, "A value of one-half of U for the
reference standard should be used in calculating U of candidate standards that are certified under this
protocol (see Appendix C).". But, when we go to Spreadsheet C, cell A104, we see the following,
"Expanded uncertainty (the two-sigma uncertainty) of the reference standard". Am I reading this right?
Should we report U (k=2) or u (k=1) for the reference uncertainty in cell A104 of spreadsheet C?
Answer 52- First, take a look at Appendix C on Page 139 in the protocol. The portion of the expanded
unceriainty2 (i.e., variance) due to calibration is shown as a decimal number between 0 and 1 (in this case
0.5) showing that one-half of the variance is due to the multipoint calibration. Second, take a look at Cells
K234 and S234 in the spreadsheet associated with Appendix A. It's best to get an unaltered copy of the
spreadsheet from the protocol's web page (https://www.epa.gov/air-research/epa-traceabilitv-protocol-
assav-and-certification-qaseous-calibration-standards). These cells also show decimal numbers. One or
another of these numbers is reproduced in Cell D 251 as a percentage. Third, take a look at Cells A14,
A21, and A29 in the spreadsheet associated with Appendix C. Again, the portion of the expanded
unceriainty2 due to calibration is represented as a decimal number (in this case 0.5 again).
So the units to use in Cells A14, A21, and A29 are not in the same units as the measurand because they
are not associated with the assay of the candidate standard. Rather, they are associated with the
uncertainty of the linear regression parameters arising from the multipoint calibration(s). If you are trying
to assess the stability of a candidate standard from two assays that were made within the same multipoint
calibration, you don't have to take the uncertainty associated with the multipoint calibration into
consideration. However, if the two assays were made during separate multipoint calibrations, then the
calibration uncertainty has to be taken into consideration. The portion of expanded uncertainty
associated with calibration is used in the former case, but not in the latter case. That is the reason why
you have to input the dates of the multipoint calibration in the Appendix C spreadsheet. If the expanded
uncertainties are small, the dates of the multipoint calibration won't be much of a factor in determining
whether the candidate standard is stable. However, if the expanded uncertainties are larger the dates of
the calibration may be a deciding factor in the stability determination.
Question 53- We are looking into producing methane EPA Protocol blends, but the NIST website is
limited as far as the SRM's offered. Currently, there are 10 ppm, 50 ppm, and 100 ppm methane/air
SRM's available as well as a 1 ppm methane/air SRM that is out of stock. Since the recommended 5
points for the curve is not available from N.I.S.T., would we be able to start with 3 points from the 10 ppm,
50 ppm, and 100 ppm SRM's and build up to a 5-point curve by analyzing 25 ppm and 75 ppm GMIS's
against the SRM curve? We want to know if there is a way we can get started with what is available.
Answer 53- Section 2.1.3.2 of the protocol states: "The reference standards for the multipoint calibration
must be diluted or undiluted SRMs, RGMs, PRMs, CRMs, NTRMs, or GMISs (see Subsection 2.1.3) or
dynamically diluted pure gases." So it is acceptable to do the multipoint calibration using a combination
of undiluted SRMs and undiluted GMISs as you have proposed.
Question 54- I am getting requests for HCI mixes as EPA Protocols. I am sure this is related to the
issuance ofEPA's PS-18. I am not sure how to respond. HCI mixes appeared in the 2014 Letter of
Equivalency in the 10-300 ppm range from VSL. However, the relative difference was +/- 5%. I assume
that this is the accuracy of the primary standard from VSL. If we were to use that standard per G1
methodology there is no way to satisfy the 2% maximum expanded uncertainty requirement in the
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Protocol. So am I correct in thinking that we cannot certify as EPA Protocol? I want to be clear on this
issue before I respond to my customers.
Answer 54-1 just spoke with Ray Merrill (919-541-5225, merrill.raymond@epa.gov) of EPA's Office of
Air Quality Planning and Standards. He suggested that you check
https://www3.epa.gov/ttn/emc/approalt.html for (ALT-114) Approval of Alternative Method for Preparation
ofHCI Gas Standards for PS 18 and Procedure 6, which I have also attached. This alternative method
will allow you prepare and certify alternative HCI calibration gases until such time as NIST-traceable
reference standards become available and HCI EPA Protocol Gases can be assayed and certified. After
reading these documents, contact Ray Merrill if you have further questions about the alternative HCI
calibration gases.
Question 55- I'm building a EPA spreadsheet for a mix that I'm trying to produce and on my TOST page
it says "Case 18", what does that mean? I have also seen "Case 12" what does that mean?
Answer 55- When you are doing the TOST calculations in Appendix C of the EPA traceability protocol,
you must properly handle the uncertainty associated with the assay itself and the uncertainty associated
with the corresponding multipoint calibration. If two assays of a candidate standard are performed under
the same calibration, you don't have to double-count that portion of both expanded uncertaintyies2 that is
due to the same calibration. However, if the two assays have two different calibrations, you do have to
consider those portions of both expanded uncertaintyies2 that are due to the different calibrations.
Appendix C determines which case applies depending on the dates that are entered in Cells A15, A23,
and A31. The case that is shown in Cell G69 tells you which situation exists for the data that you have
entered in Cells A11 through A31. If you go to Page 140 of the protocol, you will see the following table
of the various possible combinations of assays and calibrations:
Case
Cal,
Cal.
Cal. f>
No..
No
4*
1
1
—
6*
1
2
—
9
= 1
1
1
12
= 1
1
2
15
1
2
2
18
= 1
2
3
So Case 18 means that each of the three assays is associated with a different calibration. Case 12
means that the first two assays are associated with the same calibration and the third assay is associated
with a different calibration. This Excel spreadsheet was put together by a PhD statistician who is cleverer
than me in statistical bookkeeping and who deserves all the credit for making the spreadsheet simple
enough for non-statisticians like the two of us to use and to get the correct uncertainty estimates.
Question 56- One of our cement kiln customers has requested the same 14 ppm ammonia, balance N2
EPA Protocol Gas, but this time he wants an SF6 tracer added. My question to you is whether a standard
protocol made using a NIST standard remains a protocol if you add something for which there is no NIST
standard.
Answer 56- Your basic question is whether an EPA Protocol Gas can contain a noncertified component.
I think so. It strikes me that this situation is the same as a NIST NOx SRM 2660a for which the NO2
concentration is provided for informational purposes only, not as a NIST-certified value. One question
that still nags me is the possibility that the SF6 component might interfere with the ammonia
measurement. If you are using FTIR to measure ammonia, be sure that the SF6 infrared absorption
peaks don't overlap with the NH3 peaks. If you do have more information about your customer's
application, I think that the EPA regulatory folks would appreciate learning whether the NH3 and SF6
measurements are being required by a state or local air pollution control agency.
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Question 57-1 downloaded the Appendix A spreadsheet from the protocol web site. However, the 1997
version of the protocol is cited in its reference section. Is this citation correct?
Answer 57- You have sharp eyes to have noticed this minor error in the Appendix A spreadsheet, which
was originally constructed for the 1997 version of the EPA traceability protocol. The spreadsheet was
slightly modified by a statistician for the 2012 version of the protocol, but we did not notice that the now-
obsolete reference citation was retained. The citation should have been changed to the 2012 version of
the protocol at that time. The Appendix A spreadsheet as is found on the protocol website is correct and
can be used to calculate the certified concentrations of candidate standards. At some point, I will need to
fix this error. Thank you for pointing it out to me.
Question 58-1 have a question regarding assaying a previously-certified GMIS on a different day from
that of the initial multipoint calibration. Step 6 of the Appendix A spreadsheet deals with the expanded
uncertainty of the estimated concentration. There are lower and upper limits for the uncertainty. Does
the previously-certified value of my GMIS have to fit in between these limits?
Answer 58- The estimated concentration of the candidate standard (Cell B333 in the Appendix A
spreadsheet) does not have to match the previously-certified value of the GMIS. This value always will,
by definition, lay within lower and upper limits for the expanded uncertainty (Cells B340 and C340). I
suspect what you are doing is remeasuring your GMIS to confirm that its just-assayed value matches the
originally-certified value of the GMIS. That's a good thing and confirms that your measurement system is
under control. In essence, what you are doing is a recertification of your GMIS whether or not the GMIS
is out of its certification period. The relevant section of the traceability protocol is Section 2.1.11 (on Page
32 of the protocol), which covers the recertification of standards. This section states:
"Use the spreadsheet described in Appendix C (or an EPA-approved equivalent statistical technique) to
compare the measured concentrations from the recertification assay with the measured concentrations
from the previous assays. If the TOST acceptance criterion is attained, the standard can be recertified.
The recertification period is the same as that given in Table 2-3. If the measured concentrations are
shown to be not equivalent, the standard must undergo a full certification (e.g., Procedure G1) before it
can be used again."
I would not use the Appendix A spreadsheet to verify the concentration of your GMIS. Use the Appendix
C spreadsheet for this purpose.
Question 59- Recently, we are getting many requests for nitric oxide and nitrogen dioxide at certified
concentrations in the same mix per the EPA Protocol. We have always had a policy of not mixing those
two species in the same mix. My first question is, "does the Protocol allow this"? And second, "what are
your thoughts on this combination as relates to stability and shelf life"?
Answer 59- Dave Worton of the National Physical Laboratory (NPL), which is NIST's equivalent in the
United Kingdom, addressed this question:
"I oversee our reference materials of NO and NO2 and am currently coordinating a large consortium
project to improve the underpinning metrology for ambient measurements of nitrogen dioxide, for
more information see http://empir.npl.co.uk/metno2/.
"Due to the reactivity of NO2, we prepare our NO2 primary reference materials (PRMs) from NO and
reaction with O2 in cylinder at high amount fractions to form NO2 (as opposed to starting with pure
NO2) which we then dilute to lower amount fractions. This reaction (2NO+O2 <--> 2NO2) is reversible
so to drive the equilibrium towards NO2 providing the required stability we maintain an excess of O2,
typically more than 1000 umol/mol, in our NO2 PRMs making them incompatible with the presence of
additional NO. As a result, we do not provide PRMs that contain both NO and NO2 and therefore
provide NO PRMs in N2 and NO2 PRMS in N2 or air.
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"Our experience based on a previous project (CERMATAIR 2001-2004,
https://cordis.europa.eu/project/rcn/58201_en.html) some years ago discussed preparing high
pressure cylinders containing 300 nmol/mol (ppb) of NO and 100 nmol/mol of NO2 in a balance of N2.
A provisional aim was to see if the amount fraction would at least stabilize enough to be useful for
field checks. However, there were reportedly difficulties in the initial preparation but eventually 6
mixtures were prepared but further analyses revealed that these mixtures were unstable. The results
indicated that there were substantial and variable losses of NO2 within the cylinders, which would be
consist with our current understanding of the need for excess 02 to ensure stability for NO2.
"Additionally, previously (producer name deleted) offered NO/NO2 mixtures for sale and colleagues
had attempted to purchase them. However, those cylinders never passed the internal stability testing
and were thus never delivered."
Given that the concentrations are quite low and given that the work was done in 2002-2003, I can't
completely slam the door shut on preparing EPA Protocol Gases containing NO and NO2. However, I
would remain skeptical in the absence of newer data showing that such mixtures are stable, particularly at
higher concentrations. The burden of proof would be on specialty gas producers to demonstrate that
such mixtures could be assayed accurately and that they remain stable in the long term.
I suggest that you take two steps that you can take that could help to clarify the situation. First, contact
your customers to see if their end users' regulatory counterparts are willing to accept environmental
monitoring data from analyzers that were calibrated with such calibration gases. If the end users are just
trying to save money by obtaining multi-component calibration gases, they may wiser to purchase
multiple binary mixtures. If the end users believe that their governmental regulators will accept
NO/NO2/N2 calibration gases, sell them certified standards that are not EPA Protocol Gases. Tell your
customers that EPA does not have confidence in such mixtures based on currently available information.
Second, if you really believe that there is a market for such multi-component calibration standards,
prepare several in-house standards and study their long-term stability. I have heard that NO2 gas
mixtures are stable in Luxfer SGS cylinders for VSL. If these standards can be shown to be stable for a
reasonable period of time, I would be happy alter the protocol to allow such gas mixtures. In the past,
EPA has followed NIST's practice regarding the certification periods for EPA Protocol Gases because
EPA doesn't have the facilities to do such stability studies itself. This case is analogous to the situation
with NO2 EPA Protocol Gases for which EPA is seeking long-term stability data.
Question 60-1 have a question about the use of interference corrections for analysis of a multi-
component EPA Protocol Gas. Could you define what a "statistician" is in Section 2.1.5.3 of the protocol?
Answer 60- Over the 25-odd years during which I have been involved with the EPA traceability protocol
for gaseous calibration standards, I have had the privilege to work with several PhD statisticians with
degrees from North Carolina State University and the University of North Carolina at Chapel Hill. They
possess the statistical skills that were needed to construct and modify the Excel spreadsheets that are
used to calculate the certified concentrations and expanded uncertainties of gas mixtures that are
assayed and certified under the protocol. Their abilities far exceed my meager statistical skills.
Perhaps the best way to assess the statistical skills that are needed is to review the Excel spreadsheets
that are used with the protocol. See https://www.epa.gov/air-research/epa-traceabilitv-protocol-assav-
and-certification-qaseous-calibration-standards. Whoever is going to construct the interference correction
equation should have sufficient statistical skills to understand the uncertainty calculations that are
performed in these spreadsheets. More complicated calculations are needed for the interference
correction calculations. Whereas the existing spreadsheets use bivariant regression analysis of pollutant
concentration versus analyzer response, multivariant regression analysis of pollutant concentration
versus analyzer response versus interferent concentration would be needed for the interferent correction
calculations. The uncertainty that is associated with the interferent correction must be added to the
uncertainties of the assay itself, of the multipoint calibration, and of the reference standards. That
uncertainty may change with pollutant concentration and with interferent concentration. Some
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experimental design work will be needed to determine the proper number and concentration ranges for
the pollutant and interference measurements that are needed to keep the added uncertainty within
acceptable limits.
To make things even more complicated, one also needs to consider the statistical software that would be
involved in the interference correction calculations. Proper statisticians prefer to use proper statistical
software like R. However, mastering and using such sophisticated software for routine production
analysis may be beyond the computing and data handling capabilities of most specialty gas producers.
EPA made the decision a long time ago to use Excel to perform the statistical calculations that are
associated with the protocol because Excel is a software package for which a producer could be
reasonably expected to have some mastery. I am ever grateful that the statisticians were willing to humor
me and to get their hands dirty by using Excel. Excel has a number of problems with doing statistical
calculations correctly and gets by with approximations that generally doesn't cause problems, but may do
so for low-uncertainty calculations such as are used for the protocol. I expect that the integration of the
interference correction calculations with the existing protocol uncertainty calculations will not be a trivial
exercise.
All this long discussion is meant to convey to you that (producer name deleted) would need a high level of
statistical talent to implement the interference correction calculations in a production environment. It
certainly can be done, but it won't be easy or cheap
Question 61- We are in the process of acquiring a new SRM to replace the one that we could not receive
an extended shelf life from NIST. I was wondering what we need to do with our GMISs that we certified
using this SRM. Are they still good to use because we need to reference the expiration date of the SRM
that we used to certified them?
Answer 61- Section 2.1.3.1 of the EPA traceability protocol covers gas manufacturer's intermediate
standards (GMISs) and Section 2.1.11 covers the recertification of standards. YourGMIS is OK to use to
the end of its original certification period regardless of whether the reference standard that was used in its
assay is still in its certification period. At the end of the GMIS' certification period, it can be recertified
using your new SRM as the reference standard. The results of the GMIS' recertification assay should be
compared to the results of its original assay. If Schuirmann's two one-sided test (TOST) show that the
estimated concentrations from the two assays differ by less than 1.0 percent, the GMIS can be recertified
for the same period as its original certification. If the TOST acceptance criterion is not attained, a second
recertification assay may be conducted to establish the new concentration for the GMIS. Hopefully, a
second recertification assay would not be needed.
Your question is not off-the-wall. Recently, I fielded a question from (user name deleted) concerning a
multi-component EPA Protocol Gas' certificate of analysis that listed an in-certification propane GMIS and
an out-of-certification propane SRM. This certificate listed the concentrations and the expiration dates for
the GMISs that were used for the EPA Protocol Gas' assay and for the SRMs that were used in the GMIS
assay. I explained that this calibration standard was OK because the propane assay was performed with
an in-certification reference standard. Perhaps, the certificate could have more explicitly identified which
reference standards were used for the assay of the EPA Protocol Gas and which were used for the assay
of the GMISs. The end user would then have a clearer picture of the traceability chain from NIST to their
calibration standard.
Question 62-1 have a question with G1 vs G2 and labeling. Let say I have a mix that has two
components, one component I do by G1 analysis and the other by G2 analysis. On my COA and label I
already have a statement that says "analysis preformed in accordance with G1 or G2 analysis of the
EPA...." Do I need to specifically state which component I did by G1 and what component I did by G2?
Answer 62- It's OK to put "Analysis was performed in accordance with Procedure G1 or G2 of the EPA
Traceability Protocol for Gaseous Calibration Standards" on the certification documentation. The fact that
the assay was performed according to the protocol is more important than the specific procedure that was
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used. EPA monitoring regulations specify that calibration gases be traceable to NIST or be EPA Protocol
Gases, but they do not mention the assay procedures.
Question 63-1 am looking into the options for recertifying EPA Protocol Gases that have expired in our
inventory. Instead of sending them back to the manufacturer and paying about the same cost as a new to
recertify, I want to take advantage of (name deleted) in-house FTIRs to potentially recertify our recently
expired gases.
Is it acceptable to reference the EPA Traceability Protocol document Section 2.1.11 to perform the
recertification process using the Appendix B spreadsheet and include a copy of the spreadsheet
(assuming it meets criteria) in an emissions test report with the original "expired" gas certificate? Is this
satisfactory for a source emissions test report?
If there is some other alternative to sending the gas back to the original manufacturer for recertification,
can you please point me in the right direction? As I mentioned, (name deleted) has several in-house
FTIR analyzers and hundreds of EPA Protocol Gases to reference in house. I am sure we can follow the
technical aspects of the procedures. I just want to make sure that it will be acceptable in the eyes of EPA.
Answer 63- The EPA traceability protocol is a general analytical procedure that any competent laboratory
can use to assay and certify gaseous calibration standards as being traceable to NIST reference
standards. If your laboratory has the appropriate analytical instrumentation (such as FTIRs), it is
theoretically possible to recertify expired EPA Protocol Gases. However, the key to the recertification
assay is having the NIST-certified reference standards (see Section 2.1.3) or gas manufacturers
intermediate standards [GMISs (see Section 2.1.3.1)] on hand to serve as the analytical reference
standards for the assay. Other EPA Protocol Gases cannot serve as the analytical reference standards
for the recertification assay. As stated at the beginning of Section 2.1.3, "The EPA monitoring regulations
define a "traceable" standard as one that has been compared and certified, either directly or via not more
than one intermediate standard, to a primary standard such as an SRM or a Certified Reference Materials
(CRM)". Because each link in a calibration chain adds more uncertainty to the certified value of the last
standard in the chain, the chain must be kept as short as possible relative to the uncertainty requirements
of the measurement being calibrated.
NIST-certified reference standards are very expensive and are in high demand when they are even
available for sale from NIST. Specialty gas producers prepare batches of NIST-Traceable Reference
Materials (NTRMs), which are then assayed and certified by NIST as a means of increasing the
availability of NIST-certified reference standards (see https://www.nist.gov/proqrams-proiects/nist-
traceable-reference-material-proqram-qas-standards). Even NTRMs are closely guarded by their
producers and are unavailable for sale (to the best of my knowledge).
In my estimation, sending your EPA Protocol Gases back to their original producer is probably the
cheapest way to get them recertified. I have no doubts that your laboratory would have the technical
capability to perform the recertification assay. The trick is having the analytical reference standards to
perform the assay.
Question 64- Can a specialty gas company use the EPA Protocol, Section 2.4 to produce air and use
suitable detector tubes to certify that the SO2 and NOx is below 0.1 ppm? If so, do they really need
calibration gases for the SO2 and NOx?
Answer 64- The acid rain monitoring regulations in Section 5.1.6 of Appendix A of 40 CFR Part 75 state:
"5.1.6 Zero Air Material. Zero air material is defined in § 72.2 of this chapter."
Section 72.2 (Definitions) of 40 CFR Part 72 states:
"Zero air material means either:
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(1) A calibration gas certified by the gas vendor not to contain concentrations of S02, NOX, or
total hydrocarbons above 0.1 parts per million (ppm), a concentration of CO above 1
ppm, or a concentration of C02 above 400 ppm;
(2) Ambient air conditioned and purified by a CEMS for which the CEMS manufacturer or vendor
certifies that the particular CEMS model produces conditioned gas that does not contain
concentrations of S02, NOX, or total hydrocarbons above 0.1 ppm, a concentration of CO above
1 ppm, or a concentration of C02 above 400 ppm;
(3) For dilution-type CEMS, conditioned and purified ambient air provided by a conditioning
system concurrently supplying dilution air to the CEMS; or
(4) A multicomponent mixture certified by the supplier of the mixture that the concentration of the
component being zeroed is less than or equal to the applicable concentration specified in
paragraph (1) of this definition, and that the mixture's other components do not interfere with the
CEM readings."
See also Questions 9.1 and 9.31 of EPA's Part 75 Emissions Monitoring Policy Manual:
https://www.epa.gov/sites/production/files/2019-
10/documents/part 75 emissions monitoring policy manual 10-18-2019.pdf
There is no requirement in these regulations that the zero air material must be assayed and certified
according to the EPA traceability protocol. Specialty gas producers do not appear to have to do very
much at all to produce an acceptable zero air material as far as 40 CFR Part 75 is concerned. Your idea
to use SO2 and NOx detector tubes to assay zero air materials seems to be fairly reasonable given the
modest requirements of Section 72.2. However, the total hydrocarbons, CO, and CO2 concentrations in
the zero gas would also have to be measured.
Back in 2012, I wrote Procedure G3 of the protocol in anticipation that NIST would be producing zero air
Standard Reference Materials (SRMs) and that there would be some regulatory demand for NIST-
traceable zero air materials. NIST has never produced a zero air SRM, no regulatory demand emerged,
and Procedure G3 has never been used to my knowledge. Lacking a NIST-traceable zero air reference
standard, there is no current way to assay and certify zero air EPA Protocol Gases.
NIST's European counterparts are working on standards for zero gas (see https://www.macpoll.eu/zero-
gas). This information may not be of any direct help to you, but it may give you some idea about the
current state of zero gas technology. If NIST and the European national metrology institutes (like VSL)
ever include zero gas under a declaration of equivalence (DoE), then there would be a pathway for
assaying and certifying zero gas EPA Protocol Gases.
Question 65-1 get frequent calls from labs asking me what is required to produce both zero air and zero
grade nitrogen and specifically ask about "CEMs Grade" nitrogen or air. Section 2.4 addresses the
procedure's for producing "Zero Air Materials". The text is vague in my opinion as it raises a few questions
for clarification.
1) Does the Document term "Zero Air Materials" become synonymous with what the regulated
industry refers to as "CEMs Grade"?
2) Is the regulated industry required to procure these materials from PGVP vendors?
3) Must the products be labeled as "EPA Protocol Zero Air Materials"?
There are many non-PGVP producers simply selling UHP nitrogen and standard grades of Zero Air into
the regulated industry. As a point of SOPs at (name deleted), when our customers ask for CEMs Grade
nitrogen or air, we certify the gases to meet the requirements of 40CFR 1065 750. These requirements
were designed to meet vehicle emission standards, but seem to best represent the criteria for NIST
Traceability for zero gases. I value your opinion and interpretation on these issues and will welcome your
reply.
Answer 65- Procedure G3 of the protocol has been a disappointment to me because It never got off the
ground. I've always seen the EPA traceability protocol as being one leg of a three-legged stool to
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generate NIST-traceable calibration gases. The other two legs are the availability of NIST reference
standards and the existence of EPA regulatory drivers to require the use of NIST-traceable calibration
gases. Neither of these two other legs currently exist. Back in 2012 when I was revising the protocol,
Frank Guenther of NIST made some noises about preparing NIST zero air, but nothing ever came of it.
The EPA regulations for the Acid Rain Program (40 CFR Part 75) never required that the zero air be NIST
traceable. The attached definition of "zero air material" in 40 CFR Part 72.2 states:
"Zero air material means either:
(1) A calibration gas certified by the gas vendor not to contain concentrations of S02, NOX, or
total hydrocarbons above 0.1 parts per million (ppm), a concentration of CO above 1 ppm, or a
concentration of C02 above 400 ppm;
(2) Ambient air conditioned and purified by a CEMS for which the CEMS manufacturer or vendor
certifies that the particular CEMS model produces conditioned gas that does not contain
concentrations of S02, NOX, or total hydrocarbons above 0.1 ppm, a concentration of CO above
1 ppm, or a concentration of C02 above 400 ppm;
(3) For dilution-type CEMS, conditioned and purified ambient air provided by a conditioning
system concurrently supplying dilution air to the CEMS; or
(4) A multicomponent mixture certified by the supplier of the mixture that the concentration of the
component being zeroed is less than or equal to the applicable concentration specified in
paragraph (1) of this definition, and that the mixture's other components do not interfere with the
CEM readings."
No one is forcing end users to buy NIST-traceable zero air, which would most likely cost more than
vendor-certified zero air. Producers can sell self-certified products with their own specifications.
The situation is different in Europe, where there is an ongoing effort to produce zero gas that is traceable
to national metrology institutes and where European monitoring directives require the use of such zero
gas. See https://www.macpoll.eu/zero-aas. Your best point-of-contact is Annarita Baldan of VSL in the
Netherlands (abaldan@vsl.nl).
To directly answer your questions:
1) "Zero air material" is air that has been certified by its producer to meet the requirements of 40
CFR Part 72.2;
2) The regulated industry is not required to procure these materials from PGVP vendors; and
3) There is no current way to produce "EPA protocol zero air materials" until such point that NIST
or VSL start to sell zero air reference standards that producers can use to assay the zero air that
they would sell to end users.
If your customers are selling zero gas for mobile source testing applications, I suggest that you continue
to reference 40 CFR Part 1065.750. However, if they have ambient air or stationary source applications, I
suggest that you reference 40 CFR Part 72.2 because this regulation is probably more familiar to them.
Question 66-1 have a customer inquiring about EPA Protocol sulfur dioxide balance air. I see in the
protocol itself it talks about balance nitrogen and not air, is this something that hasn't come up much and
that is why it isn't mentioned in the protocol? The mix is definitely in my wheelhouse for testing, but just
wondering why SO2 balance air doesn't have certification period.
Answer 66- At long last, I can now respond to your request for information about the appropriate
certification period for EPA Protocol Gases containing sulfur dioxide in air. I consulted a number of
international metrology organizations for any information that they could share about the long-term
stability of these gas mixtures. I did receive responses from them, which I will summarize below.
Unfortunately, I received only anecdotal stability data and no information about appropriate cylinder
passivation techniques. Based on the limited information that is available, I am reluctant to make a
pronouncement about a reasonable certification period for these gas mixtures until (name deleted) or
another specialty gas producer develops its own stability data that can be shared with EPA.
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The first piece of information comes from a 2004 international key comparison among national metrology
institutes (NMIs) (see https://www.bipm.org/utils/common/pdf/final reports/QM/K26/CCQM-K26.b.pdf).
Thirteen travelling standards containing approximately 280 ppb of sulfur dioxide in air were analyzed by
the NMIs. The standards were obtained from BOC Gases and were contained in their Spectra-Seal
cylinders. The coordinating laboratory was the National Physical Laboratory (NPL) in the United Kingdom
(UK). Concentration drift was seen in the standards. The median drift corresponds to a decay of 2.2
percent over 6 months (see Page 5 of report).
I next contacted NPL for any additional stability data that it might have and received the following
response from NPL's Nick Allen (nick.allen@npl.co.uk):
"I am pleased to be able to inform you that NPL now has Declaration of Equivalence following on
from the memorandum of cooperation with NIST which would cover sulphur dioxide mixtures. We
are currently coordinating a CCQM key comparison for 300 nmol mol-1 of sulphur dioxide in an
air matrix, which will assess the global state of the art and provide additional stability data. For
this comparison we are also trialling a new type of cylinder with a different passivation."
"At NPL we can provide mixtures from 100 - 1000 nmol/mol with a stability of greater than two
years above 200 nmol/mol or a stability of greater than three years above 300 nmol/mol
(depending on the uncertainty required).
"If you are interested in discussing this further or would like to hear more about NPL's sulphur
dioxide mixtures then I am happy to provide further information."
In "A high accuracy dilution system for generating low concentration reference standards of reactive
gases" (see Measurement, Volume 47, January 2014, Pages 607-612), NPL's Paul Brewer etal. state
that "the rate of change in amount fraction for each of the mixtures is similar, with gradients ranging from -
0.0006 to -0.0117 nmol per mol per day". My own regression calculations indicate that the SO2
concentrations decayed between 0.03 and 0.62 percent over 6 months, which is less that that seen in the
2004 key comparison. Note that I haven't seen a Declaration of Equivalence (DoE) between NPL and
NIST and expect that one is planned (see https://www-n ew. n p I. co. u k/q etattach me nt/prod u cts-
services/Gas/NPL-NIST-MOC.pdf?lanq=en-GB).
I tried to get information from BOC, whose webpage stated: "Parts per billion mixtures of sulphur dioxide
in air are now routine and parts per trillion mixtures of some components have been developed". BOC's
Andrew Deighton (Andrew.Deighton@boc.com) provided the following response:
"Many thanks for your enquiry regarding SO2 in air mixtures. We have been manufacturing ppb
level SO2 in air mixtures in the UK for many years, with a 450-ppb mixture being a UK standard
product for the ambient air monitoring sector. Our version of this mixture comes with a 36-month
shelf life, which is obtainable due to our patented Spectra-Seal process. The 36-month shelf life is
not typically available market wide, as our competition tend to have shorter shelf life based on
their different passivation techniques.
"Unfortunately, the Spectra-Seal IP is confidential, so there isn't any data we can share at this
time. Presumably what can be achieved in the US will depend on the passivation technologies in
use by US gas suppliers and the stability these can provide at low SO2 levels in air."
I then contacted Ricardo Energy & Environment, which provides quality assurance support for the UK's
Automatic Urban and Rural Network (AURN) (see https://ee.ricardo.com/air-aualitv/case-studies/aurn).
including certifying AURN calibration gas mixtures. Ricardo's Stewart Eaton
(Stewart.Eaton@ricardo.com) provided the following response:
"Yes-we have quite a bit of data on SO2 cylinder stability. I'm not sure how best to collate it, but
we have two sources of such information:
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"1. A quality control cylinder, analysed every time we calibrate cylinders for the AURN. This gives
regular measurements over a period of a few years
2. Concentration checks of site cylinders, giving a six-monthly value over the life of the gas
mixture. This will have a higher uncertainty but gives a very useful check of the accuracy of the
analyser calibrations.
"I attach the spreadsheets of the last two QC cylinders, the first D337164 is notionally 150ppb,
and the second 108007 450ppb. There are step changes when we start using a new reference
cylinder, calibrated by NPL. While D337164 seems acceptably stable, we got higher numbers of
outliers at intercalibration exercises on the AURN, so we switched to 450ppb.
"Cylinder 107585 was at the Marylebone Road monitoring site in London from mid-2014 to late
2019, and the concentration was reassessed against freshly calibrated audit cylinders every 6
months. I attach the results from that cylinder. It appears our uncertainty on this check is typically
3-5%, although this is not part of our ISO17025 accreditation. There are a couple of missing
results where the audit result was not reliable due to site issues.
"All these cylinders are commercially obtained from Air Liquide, calibrated against NPL calibrated
gas mixtures of typical uncertainty ±2% I have no knowledge of their cylinder preparation
methods. We have certainly seen a few unstable mixtures over the years. In the last year we
have switched to BOC for our AURN and QC cylinders, and Air Products for the reference
cylinder; we have no useful stability data on these at present."
AURN's 150-ppb QC cylinder D337164 does not show SO2 decay, but it shows a lot of scatter. I don't
think that these data are as good as Brewer's regarding stability. I did not receive any data for the 450-
ppb QC cylinder! 08007.
The final piece of information about the stability of sulfur dioxide in air mixtures came from Annarita
Baldan (a.baldan@vsl.nl) of VSL, which is the Dutch NMI equivalent to NIST. Her reply is interleaved
with my questions as follow:
Baldan: "I have consulted my colleagues, specialist in production of sulfur dioxide reference
standards and I will now try to answer your questions. For your information, note that sulfur in air
mixtures were also included in the previous DoE for the same range."
Wright: "First, do your two organizations believe that stable gas mixtures can be prepared by
American producers using aluminum cylinders that have been passivated by their normal
passivation techniques? Alternatively, are special passivation techniques needed for sulfur
dioxide in air mixtures?"
Baldan: "We have no knowledge of the passivation techniques for aluminum cylinders used by
American gas producers, so we cannot make assumptions. VSL uses for the production of sulfur
dioxide primary standards a special passivation treatment which improve the stability of the gas
mixtures (within the stated uncertainties)."
Wright: "Second, what would be an appropriate certification period for such EPA Protocol Gases?
VSL's current catalog of primary gas standards shows a stability period of 18 months for 10
umol/mol mixtures, 2 to 3 years for 11 to 100 umol/mol mixtures, 3 years for 101 to 1000
umol/mol mixtures, and 4 years for 0.11 to 1 percent mol/mol mixtures. Are these certification
periods reasonable for commercially-produced EPA Protocol Gases or should different periods be
specified? As the following message from BOC-UK indicates, their certification period is 36
months for 450 nmol/mol mixtures, which is longer than VSL's period for a PRM that has a 20
times greater concentration. Unfortunately, BOC-UK would not provide any technical data to
support their certification period."
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Baldan: "Again, we cannot suggest or advice on an appropriate certification period for the sulfur
dioxide in air under the EPA Protocol Gases. Being our preparation method based on gravimetry,
we must guarantee that the gravimetric value remains stable along the certified period. Our
research results have shown that the stability of gas mixtures at nmol/mol level are strongly
depending on the quality of the cylinder batch. We see great variations from batch to batch and
even within the same batch. For this reason, our PRMs are sold starting jjmol/mol and preferably
from 10 jjmol/mol. Commercial gas producers use a certified analytical value. This may help
improving the certification period, being the largest losses of sulfur dioxide in the first period after
preparation."
Wright: "Third, do you know what analytical reference standards that BOC-UK uses to assay their
sulfur dioxide in air mixtures? Are NPL gas reference materials now (or will be) recognized as
equivalent to NIST reference standards? See attached Memorandum of cooperation. NPL's
catalog includes 50 nmol/mol to 10 mmol/mol mixtures containing sulphur dioxide in nitrogen or
air (see https://www.npl.co.uk/products-services/gas/air-quality-monitoring)."
Baldan: "We are not informed of the activities carried out under the NIST-NPL cooperation."
Wright: I believe that US EPA will need to explicitly allow sulfur dioxide in air EPA Protocol Gases
under the traceability protocol and the new DoE. By doing so, a reasonable certification period
for these mixtures can be specified in the protocol (rather than letting producers choose whatever
they wish). I'm just at a loss to know what to specify. I would appreciate whatever advice and/or
stability data that you could provide."
Baldan: "As stated above, VSL cannot recommend a certification period for sulfur in air reference
standards. This is much depending on the passivation technology used by specialty gas
producers."
I hope that this long discussion demonstrates that I have taken your question seriously and that the
certification period for sulfur dioxide in air mixtures is still a hard question to answer. The big question is
finding the passivation techniques that are needed to attain acceptable stability.
Question 67-1 have a question related to the Green Book (EPA Traceability Protocol for Gaseous
Calibration Standards). In order to recertify an EPA Protocol #1 dual blend cylinder (i.e. NOx ~2.5 ppm/
CO 5.0 ppm) is there a limitation on the amount the NO2 may change from the initial certification? Our
cylinder vendor indicates their standard procedure limits the NO2 change to no more than 0.3 ppm. In
reviewing the Green Book, I unable to locate this requirement/ limitation. As such, I'm thus seeking your
clarification to better my understanding.
Answer 67- My best guess (without looking at the certificates of analysis for the cylinder) is that the
vendor has an internal policy against recertifying NO calibration gases as EPA Protocol Gases if there is
evidence of excessive NO-t0-NO2 conversion between when the cylinder was originally assayed and
when the cylinder was reassayed. The EPA traceability protocol (see https://www. e pa. q 0 v/a i r-
research/epa-traceabilitv-protocol-assav-and-certification-qaseous-calibration-standards) does not have
any such limitation. My own nonexpert review of 40 CFR Part 60, Appendix F did not reveal any
concentration shift limitations for calibration standards that are used for cylinder gas audits.
I suspect that the pressure regulator for your EPA Protocol Gas was not properly purged when it was put
into use and that some residual oxygen (and perhaps water) in the regulator diffused into the cylinder and
caused the NO-t0-NO2 conversion. I would be worried that water might have gotten to the cylinder's
interior wall and that nitric acid might have formed and might form more in the future. I would be
suspicious of the EPA Protocol Gas. In the long run, it may be less hassle to buy a new EPA Protocol
Gas rather than to risk failing the cylinder gas audit because of a bad calibration standard.
Section 2.1.11 of the protocol covers the recertification of standards. The basic procedure is for the
specialty gas producer to reassay the EPA Protocol Gas using NIST-traceable reference standards,
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although not necessarily the same as was used in the original assay. Then Schuirmann's two one-sided
tests (TOST, see Section 2.1.5.2 and Appendix C) is used to determine if the measured concentration
from the recertification assay is within 1.0 percent of the certified concentration from the original assay. If
the TOST acceptance criterion is attained, the EPA Protocol Gas can be recertified. If the criterion is not
attained, a second recertification assay may be conducted. If the two recertification assays attain the
TOST acceptance criterion, the EPA Protocol Gas can be recertified. If not, the calibration standard must
be disqualified for further use under the protocol.
I talked with Ray Merrill (919-541-5225 or merrill.raymond@epa.gov) of EPA's Office of Air Quality
Planning and Standards (OAQPS) because he has expertise in source measurement technology. He is
not aware of any EPA regulatory limitations on NO-t0-NO2 conversion in calibration gases. He also
suspects regulator purging problems were the cause of the conversion.
Question 68- A tank technically doesn't have to be EPA Protocol, just traceable to one of the highlighted,
correct?
2.6 Gaseous and Flow Rate Audit Standards.
2.6.1 Gaseous pollutant concentration standards (permeation devices or cylinders of
compressed gas) used to obtain test concentrations for CO, S02, NO, and N02 must be
traceable to either a National Institute of Standards and Technology (NIST) Traceable Reference
Material (NTRM) or a NIST-certified Gas Manufacturer's Internal Standard (GMIS), certified in
accordance with one of the procedures given in reference 4 of this appendix. Vendors advertising
certification with the procedures provided in reference 4 of this appendix and distributing gases as
"EPA Protocol Gas" for ambient air monitoring purposes must participate in the EPA Ambient Air
Protocol Gas Verification Program or not use "EPA" in any form of advertising. Monitoring
organizations must provide information to the EPA on the gas producers they use on an annual
basis and those PQAOs purchasing standards will be obligated, at the request of the EPA, to
participate in the program at least once every 5 years by sending a new unused standard to a
designated verification laboratory.
Answer 68- The traceability requirements are different for different EPA air pollution monitoring methods.
For ambient air monitoring, calibration gases must be EPA Protocol Gases. For conventional source
sampling methods, calibration gases must be traceable to NIST standards or to a producer-certified
standard. OAQPS Guidance Document 039 allows for a less rigorous assay procedure that the protocol
for daily CEM calibrations where EPA Protocol Gases are not required. For 40 CFR Part 75 acid rain
monitoring, the calibration gases must be EPA Protocol Gases and must also have a vendor-certified
uncertainty of+/- 2.0 percent. Mobile source (i.e., motor vehicles and aircraft engines) testing regulations
only specify +/- 2 percent uncertainty without any specific traceability requirement. EPA NVFEL has its
own separate gas naming procedure for calibration gases. Therefore, you must look at the specific
monitoring regulation to understand the applicable traceability requirements for calibration gases.
In the case of 40 CFR Part 58, Appendix A, the calibration gas has to be an EPA Protocol Gas because it
has to be certified in accordance with cited Reference 4, which is the 1997 traceability protocol that has
been replaced by the 2012 protocol (see https://www.epa.gov/air-research/epa-traceabilitv-protocol-
assav-and-certification-qaseous-calibration-standards).
It's always bothered me that different parts of EPA have different requirements for calibration gases, but
in the 26-odd years that I've worked with EPA Protocol Gases no harmonization of the requirements has
occurred. Maybe someday.
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