EPA-600/7-81-010

Revised November 1989

A PROCEDURE FOR ESTABUSHING TRACEABILITY OF GAS MIXTURES TO

CERTAIN NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY* SRM’S

by

EE. Hughes

Center for Analytical Chemistry

and

J. Mandel

National Measurement Laboratory

National Institute of Standards and Technology

Gaithersburg, Maryland 20899

Contract No. EPMAGOBE684

EPA-1AG-DW-13932911

Project Officer

D. von Lehmden

US. Environmental Protection Agency

Quality Assurance Division

Atmospheric Research and Exposure Assessment Laboratory

Research Triangle Paris NC 27711

Revised by

WLZielinski, Jt Center for Analytical Chemistry

National Institute of Standards and Technology

Gaithersburg, Maryland 20899

NBSiR 81-2227

Formerly, National Bureau of Standards

—QC

100

.U56

81-2227

1989

U.S. DEPARTMENT OF COMMERCE

Robert A. Mosbacher, Secretary Lee Mercer, Deputy Under Secretary

for Technology

NATIONAL INSTITUTE OF STANDARDS

AND TECHNOLOGY

John W. Lyons, Director NIST
II
EPA-600/7-81-010

Revised November 1989

A PROCEDURE FOR ESTABUSHING TRACEABILITY OF GAS MIXTURES TO

CERTAIN NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY* SRM’S

E.E. Hughes

Center for Analytical Chemistry

and

J. Mandel

National Measurement Laboratory

National Institute of Standards and Technology

Gaithersburg, Maryland 20899

Contract No. EPAJA&D8-E684

ERA4A&DW-13932911

Project Officer

D. von Lehmden

US. Environmental Protection Agency

Quality Assurance Division

Atmospheric Research and Exposure Assessment Laboratory

Research Triangle Parte, NC 27711

Revised by

WL L Zielinski, Jt Center for Analytical Chemistry

National Institute of Standards and Technology

Gaithersburg, Maryland 20899

NBSIR 81-2227

Formerly, National Bureau of Standards

U.S. DEPARTMENT OF COMMERCE

Robert A Mosbachef, Secretary Lee Mercer, Deputy Under Secretary

for Technology

NATIONAL INSTITUTE OF STANDARDS

AND TECHNOLOGY

John W. Lyons, Director
III
ABSTRACT

A procedure is described by which the concentration of commercially

produced gas mixtures may be related to certain gas Standard Reference Materials (SRM's) currently offered by the National Institute of Standards and

Technology. The concentration of the gas mixture as a Certified Reference

Material (CRM), must, by definition, lie within one percent relative of the

concentration of a particular SRM to reduce the error involved in the comparative analysis. Statistical treatment of the traceability process is given by

an example, as is the process by which a body requiring traceability can evaluate the quality of the CRM. Procedures also are included for recertification of CRM's, and for initiating the certification period from the date of

sale rather than from the date of CRM approval by NIST. Appendices are included for the preparation of CRM's related to carbon monoxide, nitric oxide,

sulfur dioxide, propane, oxygen, and carbon dioxide SRM's.

DISCLAIMER

This' revised report has been reviewed by the Environmental Monitoring

Systems Laboratory, U.S. Environmental Protection Agency, and approved for

publication. Approval does not signify that the contents necessarily reflects

the views and policies of the U.S. Environmental Protection Agency, nor does

mention of trade names or commercial products constitute endorsement or recommendation for use

.

i
FOREWORD

One important role of the National Institute of Standards and Technology

(NIST) is to provide those services necessary to assure data quality in

measurements being made by Federal, state, local, and industrial laboratories

participating in environmental measurement programs. The work at NIST is

conducted in appropriate disciplinary Centers (the Center for Analytical

Chemistry, the Center for Radiation Research, and the Center for Chemical

Technology) . NIST activities address data quality assurance needs of air and

water monitoring programs. NIST efforts in support of data quality assurance

inc lude

:

• Studies of the feasibility of production of Standard Reference Materials (SRM's) which could be used for the verification of performance

audit samples for quality control programs or used for the calibration

of field and laboratory instruments.

• The development and demonstration of new or improved measurement

methods, particularly when needed for the certification of SRM's.

• The evaluation and dissemination of data on the physical and chemical

properties of effluents, products and raw materials of environmental

significance in energy production.

• The provision of reference materials for the evaluation and validation

of monitoring methods.

This document was initially issued in May 1981, as one of the Interagency

Energy/Environmental Research and Development Series Reports prepared to

provide detailed information on the development of a NIST measurement standard

or method. The document provides a procedure by which commercial specialty gas

companies could produce gas mixtures as Certified Reference Materials (CRM's)

that would be directly traceable to certain NIST gas SRM's. These CRM's would

be accepted by the U.S. Environmental Protection Agency (EPA) as reliable

substitutes for SRM's in environmental monitoring programs required by EPA

regulations . EPA published an announcement in the Federal Register that such

CRM's could be used in place of SRM's. The traceability procedure described in

the May 1981 issue of this document was jointly reviewed and approved by NIST,

EPA, and the U.S. Compressed Gas Association.

The original issue of this document in May 1981 contained appendices for

the production of four types of gaseous mixtures, covering 43 different NIST

SRM's: CO in N2 or air (Appendix C) ; NO in N2 (Appendix D) ; S02 in N2 (Appendix E) ; and propane in N2 (Appendix F) . This revision to the document contains

appendices for two additional types of gaseous mixtures, covering 18 additional

NIST SRM's: 02 in N2 (Appendix H) ; and C02 in N2 (Appendix I). In addition,

this revised document includes a procedure (Appendix G) for extending the

certification period for existing CRM's and a mechanism by which a CRM producer

can certify a CRM from the date of sale, rather (as required in the May 1981

document) from the date of NIST approval of sale. The advantage of the

procedure for extending this certification period is that an approved CRM can

be recertified for additional incremental periods (each period usually being

ii
two years), indefinitely, as long as the certified concentration of original

gas mixture in the cylinder has not changed, supported by reanalysis data

developed for each additional certification period. The advantage of the

mechanism for date of sale certification is that a producer of an approved CRM

can certify the concentration of the mixture for the full certification period

authorized for the CRM, at the time of its sale to a customer. This mechanism

eliminates the problem in selling a two-year certified CRM a year after its

approval with only a one-year certification period, as required in the May 1981

document. The three additional appendices included in this revised document

(Appendices G, H, and I) have been reviewed and approved by NIST, EPA, and

representatives of the U.S. specialty gas industry. Questions relating to this

revised document and its procedures and specifications, including the new

appendices, should be addressed to Walter L. Zielinski, Jr., William D. Dorko, or George C. Rhoderick at NIST, or to Darryl J. von. Lehmden at EPA.

Ranee A. Velapoldi

Chief

Gas and Particulate Science Division

Center for Analytical Chemistry

National Institute of Standards and Technology

iii
.
CONTENTS

1 . INTRODUCTION

2. CERTIFIED REFERENCE MATERIALS

2.1 Description

2.2 Preparation

2.3 Analysis

3. SUGGESTED ANALYTICAL PROCEDURE

3.1 Description

3.2 First Analysis

3.3 Second Analysis

3.3.1 Stability and Homogeneity

3.3.2 Estimation of the Uncertainty of the CRM

4. AUDIT OF THE CRM

4.1 Introduction

4.2 Environmental Protection Agency Audit Progress

4.2.1 Selection of Samples

4.2.2 Analysis

4.2.3 Report of Results

4.3 Examination of Results

4.4 Certification

ACKNOWLEDGEMENTS

APPENDICES

Appendix A.

Appendix B.

Appendix C.

Appendix D.

Appendix E.

Appendix F.

Appendix G.

Statistical Analysis of Data

Report Forms

Carbon Monoxide

Nitric Oxide in Nitrogen

Sulfur Dioxide in Nitrogen

Propane

Extension of Certification (Recertification) and

Date-of-Sale Certification

a. Introduction

b. Procedures to Extend the Certification of a CRM

c. Analytical Procedures for Date-of-Sale Certification

d. Converting from Fixed Period Certification to

Date-of-Sale Certification

Appendix H. Oxygen in Nitrogen

Appendix I . Carbon Dioxide in Nitrogen

v

1. INTRODUCTION

This, procedure is intended to increase the availability of accurate gas

standards by the creation of a series of secondary standards, to be referred to

herein as "Certified Reference Materials" (CRM's). The CRM's prepared according to this procedure will be related, within known limits of uncertainty, to

specific gaseous Standard Reference Materials (SRM's) offered by the National

Institute of Standards and Technology (NIST). The CRM's are intended to

supplement the supply of existing SRM's, and are not intended to offer traceability for gas mixtures at concentrations other than those of the complementary SRM's, nor for components of such mixtures that are not certified in the

SRM's.

The procedures described are based on experience at NIST relative to the

production and certification of gaseous SRM's, and are intended to assure the

development of reliable CRM's with reasonable effort, but are not intended to

reduce the effort to a minimum. The acceptance of the CRM's by regulatory

agencies, by industry, and by other users, will depend on the reliability of

the CRM's. The CRM's, in turn, will depend on the integrity of the gas

supplier relative to the methods, conditions, and limitations of the procedure

described here and on the reliability and extent of any complementary audit

program.

2. CERTIFIED REFERENCE MATERIALS

2 . 1 Description

The CRM's will consist of compressed gas samples in cylinders prepared in

lots of ten or more of identical concentration. The average concentration for

any lot must lie within 1.0 percent relative to the concentration of a specific

SRM, but preferably the concentration should be as close as technically possible to the SRM to reduce to a minimum any errors that might arise in subsequent analyses of the CRM's. The concentration for a specific CRM will be

determined by analysis with a specific SRM and the uncertainty assigned to the

concentration of the CRM will consist of the uncertainty of the SRM and the

added uncertainty resulting from the intercomparison of the CRM to the SRM.

The period of time during which the concentration of the CRM is certified, and

the presence of any impurities of consequence will be reported by the supplier.

2 . 2 Preparation

Each CRM will be prepared as a homogeneous lot containing at least ten

(10) cylinders, all filled from a bulk mixture prepared in a single container

or in several containers ganged together in such a manner that delivery occurs

simultaneously from the several containers, or by use of a dynamic blending

system. The reagent gases with which the bulk mixture is prepared must be

analyzed for the major component (s) and for minor constituents of interest.

The concentration of the component of interest in the bulk mixture must lie

within 1 percent relative to the particular SRM which is being duplicated. The

cylinders into which the bulk mixture is transferred and which will become the

CRM's must be clean, of known compatibility and history, and must be treated to

1
assure stability of the mixtures which will be contained. The cylinders will

be equipped with valves of appropriate material which conform to Compressed Gas

Association (CGA) recommendations for the particular gas mixture.

After filling, an "incubation" period must be allowed before final

analysis. This period will vary depending on the nature of the gas mixture and

is described in the appendices of this document.

2 . 3 Analysis

The analysis consists of a comparison of each CRM to the appropriate SRM.

The requirement that the CRM have a concentration within 1 percent relative of

the SRM reduces the error arising from the intercomparison and simplifies the

calibration procedure for the analytical instruments. In most cases, the

characteristics of the analyzer are such that the response of the instrument

over a small concentration range (1 percent relative or less) can be described

by the linear equation

y

= mx + b

where y is the signal, m is the instrument sensitivity in signal units per unit

of concentration, x is the concentration, and b is a constant in signal units.

Consequently, it is only necessary to determine that the sensitivity does not

change significantly over the interval between the concentration of the CRM and

the SRM to compare the two signals directly. It follows then that the closer

in concentration that the CRM is to the SRM, the less possibility for error exists in assigning a value to the CRM.

The general characteristics of the analyzer response relative to con- centration is determined by constructing a calibration curve using the SRM

against which the CRM is to be compared as one calibration point, and at least

two other gas mixtures for a minimum of three calibration points. These

additional calibration gases should be SRM's, one higher in concentration and

one lower in concentration, as compared to the SRM which is being used to

analyze the CRM. Obviously, this is not possible when the CRM is being

compared to either the highest or lowest concentration SRM in a particular

series. In this case, the CRM should preferably have a concentration slightly

lower than the SRM, if it is the highest SRM in a series, or slightly higher if

the SRM is the lowest in a series. If the SRM is the lowest in a series, the use of a "zero" gas may be feasible. However, either the use of a zero gas or extrapolation beyond the calibration curve established with SRM's is not

recommended without technical justification.

The ideal case where the instrument response over a wide range is directly

proportional to concentration can be readily recognized by measuring the signal

generated by each of the three SRM calibrating gases and dividing each by its

certified concentration to obtain the sensitivity (m) at the three concentrations. If the response is linear and if the calibration line passes through

the origin, the value for m will be essentially the same at all three con- centrations, and the equation reduces to y

= mx. If m is constant over a wide

range of concentrations, then it should follow that it will be constant over a smaller range. In this case, the concentration of the CRM is simply

2
^CRM *CRM ^SRM

^crm = csrm or ^crm = since m =

IsRM m CSRM

where I is the signal generated by either the SRM or the CRM and C is con- centration.

In the somewhat less ideal case where the response is linear but does not

pass through the origin, the value of the signal divided by the concentration

will not be equal at the three concentrations but a straight line of slope m

and intercept b can be fitted to the data. It again follows that if m is

constant over a wide range of concentrations, it should be constant over a small interval, and the concentration of the CRM is expressed as:

Icrm

£crm = Csrm or CCRM

Isrm

b

^crm

k

m

since m

*SRM 'b

Csrm

A third case is that of an analytical instrument having a slight non-linear

response. In this situation, a calibration curve is constructed using three or more (preferably, five) SRM's, and the concentrations are determined using the

derived equation for the curve. However, in most cases, the component in a CRM

that is being analyzed will be measured with an instrument which is essentially

linear over small ranges of concentration. In other words, the sensitivity of

the instrument - the response in signal units per unit of concentration - will

not change within the limits of the precision of the instrument over short

intervals of concentration. If the sensitivity is found to be essentially

constant over the internal of 1 percent of the concentration of the SRM that is

to be reproduced by the CRM, then the sensitivity measured for the SRM can be

used to calculate the concentration of the CRM. In this case, the closer the

CRM is in concentration to the SRM, the smaller will be the error introduced by

assuming short range linearity.

It is essential in analyzing the CRM's that the analyses be made with the

highest of degree of precision possible so that the uncertainty of the con- centration of the CRM not be appreciably larger than the total uncertainty of

the concentration of the SRM with which it is compared.

The principal elements that contribute to the imprecision of this

comparison include instrument sensitivity and instability. The sensitivity of

the instrument should be high enough so that differences of concentration of

0.1 percent relative easily can be measured. Instruments are available that

are capable of measuring all current SRM's with this sensitivity. However,

most instruments exhibit drift to one degree or another. The drift has two

components, short term drift (or noise), and a long term drift characterized by

a slowly changing signal under constant operating conditions. The effect of

short term drift can be minimized by a number of techniques including the

multiple analyses of each sample, and signal averaging. The effect of long

term drift can be minimized by frequent calibration of the instrument. The

characteristics of each analytical system should be considered individually to

3
determine the optimum frequency of calibration and the approach to compensation

or control of the effect of noise. However, it should be noted that the effort

expended in minimizing these effects will be reflected in the precision of the

analyses relating the CRM to the SRM. In general, if duplicate analyses of

single samples consistently agree within 0.1 percent relative, then the effect

of noise is at an acceptable level, and if repeated analysis of the same sample

performed at intervals during the analysis of a lot agree within the same limits, then the long term drift is at an acceptable level.

3. SUGGESTED ANALYTICAL PROCEDURE

3 . 1 Description

The analytical procedure recommended here is considered to be the minimum

effort necessary to adequately reduce the possibility of accepting an unstable

CRM. All CRM's must be stable, and each batch should be homogeneous. An

unstable batch must be rejected, while an inhomogeneous batch may be acceptable, providing that the reason for the inhomogeneity is known and further

providing that the cause of the inhomogeneity will not result in a future

change in concentration not revealed by the test for stability.

The object of the analysis is to compare a lot of at least ten samples of

presumably identical concentration to an SRM and to express the concentration

of each sample in the lot with error limits defined by the uncertainty of the

SRM and the uncertainty added by the analytical procedure. Obviously, the

simplest procedure would involve direct analysis of the CRM with the SRM, but

with most analytical methods this approach would be unduly extravagant in terms

of SRM consumption. Consequently, it is recommended that the concentration be

determined on a relative basis by comparing one sample from the lot of all

others in the lot. Thus, only the one sample is compared to the SRM and the

concentration of all others in the lot can then be determined from the measured

ratio of each to the one sample. An additional uncertainty is introduced in

this procedure but the size of this uncertainty can be reasonably small, and

the final uncertainty in the CRM will not be increased significantly.

The sample from the lot with which the rest of the lot is to be compared,

the lot "internal standard", is selected at random from the lot. The signal

generated by the internal standard is compared to the signal generated by each

sample. The internal standard is repeated throughout the sequence of analysis

so that any long term instrument drift can be compensated for, if necessary.

The resulting data can be expressed as the ratio of the signal generated

by individual CRM's to that generated by the internal standard. The ratio in

all cases should be very close to 1.000 if the lot is homogeneous.

3 . 2 First Analysis

An initial analysis of the batch of CRM's should be performed as soon as practical after the preparation of the CRM's. A sample is selected at random

from the lot to serve as the lot "internal standard" to which a number of

samples from the lot are compared. At least ten samples selected at random

4
from each lot should be analyzed during this first analysis. However, if the

lot is small (<20 samples) it is recommended that all samples be analyzed.

The internal standard should be analyzed against the appropriate SRM

during the first analysis to confirm that its concentration lies within 1 percent relative to the SRM and to establish a reference for later analyses

intended to define the stability of the lot. The internal standard should be

analyzed at least ten times using SRM's to calibrate the instrument. Each of

the remaining nine or more samples selected is then analyzed in duplicate,

using the internal standard as a reference.

Some preliminary conclusions regarding the homogeneity and stability of

the batch may be drawn from the results of this first analysis. For instance,

if the batch was inhomogeneous when transferred and if no subsequent reactions

have occurred randomly in the cylinders, the concentration of all samples

analyzed should be the same within the limits of precision of the analytical

method. The appropriate statistical treatment at this step is given in the

appendices. If the statistical test shows lack of homogeneity among the

samples, it is not possible at this time to determine whether such inhomogeneity is due to improper preparation or to reaction in the cylinders. If the

test shows reasonable closeness among the results for all samples, then there

is some assurance that the batch is homogeneous.

If the analyses at this point indicate that no serious problem exists, the

entire batch is set aside for the required "incubation" period. If, however,

the batch appears to be somewhat inhomogeneous the analyses should be continued

to include all samples in the batch. An analysis of each sample at this time

may become critical later in differentiating inhomogeneity resulting from

preparation and inhomogeneity arising from instability.

3 . 3 Second Analysis

After the requiring incubation period has passed, all samples in the lot are analyzed, including those analyzed during the first analysis. The first

analysis served to define the concentration of the CRM and to test the

stability and homogeneity of the lot on a qualitative basis. Final decisions

regarding these factors depend on the second and more extensive analysis.

Sufficient time will have elapsed so that reactions in individual cylinders,

which were of too low a rate to be recognized by the distribution of concentrations obtained in the first analysis, will be revealed by this subsequent

analysis. Decisions regarding homogeneity, which may have been based on analysis of a portion of the batch, will now be based on results obtained on

the entire batch.

3.3.1 Stability and Homogeneity

The stability of the lot is assumed if the concentration in each CRM

cylinder included in the first analysis has not changed appreciably at the time

of its second analysis (see appendix A) . An inspection of the data of the

first and second analyses may reveal gross instability, and, if such is found,

the lot should be discarded and remade.

5
In a lot consisting of cylinders prepared by transfer of a single bulk

mixture to a number of smaller containers, the composition of the gas in all

the smaller cylinders should be identical. Differences observed among

cylinders should reflect only the random error in the analytical process.

Differences greater than this random error arise from two sources. First,

there are the differences that result from faulty mixing or from dilution by

residual gases left in the cylinder prior to transfer of the bulk mixture. The

second and more critical source arises from reactions, chemical or physical,

that occur in the cylinder after transfer. Because these reactions are largely

due to wall effects and because no two cylinder walls are alike, the reactions

generally proceed at different rates in different cylinders. The two effects

may be differentiated by sets of measurements made over a period of time

sufficiently long so that the extent of reaction is greater than the error in

the analysis. For instance, if a lot is analyzed immediately after preparation

and individual samples are found to have concentrations that vary more than can be accounted for statistically by the analytical imprecision, then the mixture

is either inhomogeneous or some or all samples in the lot are unstable. If the

lot is reanalyzed after some time and each sample has retained essentially its

previous value, then the samples are probably stable, but the lot is inhomogeneous. If the concentration of one or more of the samples has changed

significantly, then it is plausible to conclude that the lot is unstable.

An inhomogeneous lot need not necessarily be discarded, but an unstable

lot must be discarded. An inhomogeneous but stable lot in which the range of

values does not exceed 1 percent of the value of the SRM with which it is to be

compared may be used, providing that the reason for the inhomogeneity is

determined and that the cause does not otherwise affect the integrity of the

CRM. If the bulk container consists of several large cylinders ganged together

to allow simultaneous delivery, then some inhomogeneity may arise if the

concentrations in the several bulk containers differ slightly. In the special

case of large scale dynamic dilution systems where two components are mixed

under constant flow conditions and are then compressed into the cylinders in

the lot, some inhomogeneity may result, depending on the design of the delivery

system. If variations in concentration are observed in a lot prepared by this

method, the variation may be related to the position of individual cylinders on

the manifold, and it is therefore important to record the manifold position of

each cylinder for possible future reference.

It should be reemphasized at this point that all samples in a batch which

is not entirely homogeneous should be analyzed both during the first and second

analysis. It is essential in cases where the samples are stable but not

entirely identical in terms of concentration, that each sample should be

certified separately.

The nature of the distribution of the concentration of samples in a lot

may give significant information concerning both the inhomogeneity and stability of the lot. A lot which has concentrations that vary no more than can be

accounted for by the error of the analytical method, is probably homogeneous

and stable. A lot in which the range of values exceeds significantly what

might be expected on the basis of analytical error and in which several samples

are considerably below the average, probably is unstable. A lot in which the

range of values is large but all concentrations are essentially symmetrically

6
distributed around the average, was probably not prepared or transferred in

such a way that homogeneity was assured. A lot may be found with concentrations grouped around two or more average values (bimodal or multimodal distributions) . This usually results from errors in the gas blending or transfer

operations. It should be noted, however, that the detection of such situations

is, except for very large number of cylinders, statistically very uncertain.

Occasionally, a single sample in a lot will be found with a concentration

appreciably below the average for the lot. A single such sample probably

represents a fault in the preparation or filling of the cylinder and may be

omitted from the lot, providing the lot is otherwise stable and homogeneous.

The presence of more than one outlier in a lot or the occurrence of a single

outlier in several lots should be cause for concern, and, in the absence of a reasonable explanation for the multiple outliers, the lots should be remade.

All of the evidence at this point must support the assumption that the lot

is stable., reasonably homogeneous, and that it meets all other requirements for

a CRM. If there is any suspicion that the lot may be unstable or excessively

inhomogeneous, then further analytical work should be conducted before a final

decision is made.

3.3.2 Estimation of the Uncertainty of the CRM

The estimated uncertainty of the CRM is composed of at least three

components: (1) the uncertainty of the SRM with which the CRM is analyzed; (2)

the imprecision of analysis of the internal standard with the SRM; and (3) the

imprecision of analysis of the CRM's with the internal standard. A discussion

of the statistical procedure for estimating and combining these three sources

of error will be found in the appendices. Here we merely mention that the

second component is evaluated on the basis of ten or more measurements of the

concentration of the internal standard, and that the third component is

evaluated from the variability of the results of all samples in the lot. From

the combined variability of these three sources, an upper limit for the

uncertainty of the concentration can be calculated for each individual CRM

sample, for use in the certificate accompanying the CRM.

4 . 1 Introduction

4. AUDIT OF THE CRM

At this point the producer should have sufficient evidence to make a decision as to whether or not the batch will qualify as a CRM. This decision

must be based on the measured stability and homogeneity and on the fact that

the measured concentration of any sample in the batch lies within 1 percent

relative to the complementary SRM. The evidence for compliance with the

requirements for a CRM may be so overwhelming that a visual examination of the

data may convince the producer that all is well. However, the final decision

must await the results of an independent examination of the data and of an independent analysis of at least some of the samples. This independent

analysis will be referred to as the "audit program". Upon its completion, the

data of the CRM producer and those resulting from the audit will be

7
intercompared, using appropriate statistical procedures of data analysis which

are described in the appendices.

4 . 2 Environmental Protection Agency Audit Program

The purpose of the Audit Program is to determine whether or not the CRM's

in a particular lot are of the concentration claimed by the producer. This

will be determined by an analysis of two or more samples from the lot. The

analysis will be performed by a laboratory chosen by the Environmental Protection Agency (EPA) . Analytical results obtained by the auditor and the CRM

producer will be submitted to NIST for evaluation. If the results of the

analysis by the auditor confirm the analysis claimed by the producer, the EPA

will be so informed by NIST. If the results do not agree, either the auditor

or the producer (or perhaps both), will be asked to repeat the analysis. If

the additional analysis demonstrates the validity of the producer's value, the

EPA will be so informed by NIST. If, however, there is still disagreement, the

samples used in the audit will be submitted to NIST for analysis. The auditor

may be required to analyze the CRM's for substances other than that for which

they are certified, depending on the requirements specified in the appendix

concerning the particular substance.

4.2.1 Selection of Samples

The producer will submit a list of samples identified by sample number and

cylinder number to the auditor. The auditor will select two samples at random

which the producer will then send to the auditor. The producer will inform the

auditor of the particular SRM which the CRM is intended to duplicate. This SRM

will be identified by SRM number, sample number, cylinder number and date of

purchase

4.2.2 Analysis

The analysis of the CRM will be performed with an instrument whose

response characteristics are known at the concentration of the CRM. The

auditor will calibrate the instrument using SRM's, one of which must be of the same nominal concentration as the CRM. The number of other SRM's used in the

calibration will depend on the characteristics of the instrument but should

include at least two others. The auditor must be able to demonstrate the

validity of the calibration of the instrument in the concentration range

between the SRM and CRM.

Each sample will be analyzed ten times with the calibrated instrument.

The procedure and sequence of analyses will be planned by the auditor and

should be such that a high degree of precision is obtained.

The average concentration for each sample and the standard deviation are

to be reported. In general, the precision of the methods used in the analysis

of gas mixtures of a type represented by the CRM should yield standard deviations which do not exceed 0.5 percent relative. If this value is exceeded,

then the procedure should be carefully examined for sources of imprecision and

the analyses should be repeated. In calculating the average and the standard

deviation of the ten measurements for each sample, it is not permissible to

8
omit or change any values (possible "outliers") except where experimental

conditions were known to be inappropriate during the measuring process. 4.2.3

Report of Results

Both the producer and the auditor will submit a comprehensive report of

their respective analyses to NIST. Each individual analysis result for each of

the samples analyzed should be reported. The producer will also report the

SRM's, identified by SRM number, sample number, cylinder number, concentration,

and purchase date, which were used in the calibration of the instrument. A

brief description of the method, the calibration procedure and the measured

instrument sensitivities at the calibration points will also be submitted.

The auditor will submit the individual result of each analysis on each of

the two samples, with information concerning the SRM's used, the instrument

employed, the calibration procedure, and any other analytical results obtained

as may be required for the particular mixture.

The report may be made on the forms shown in the appendices , or by a letter containing the same information.

4 .

Examination of Results

The analytical data submitted by the producer will be examined by NIST to

assess the homogeneity and stability of the batch. The results reported by the

auditor will be used to confirm or reject the concentration claimed by the

producer. It is assumed that an unstable lot would be readily evident to the

producer and would not have been retained for processing as CRM's. It is

further assumed that an inhomogeneous lot would be recognized and would not be

retained except in the very special case where the reason for the inhomogenei ty

was known and where the stability was unequivocally demonstrated. Such batches

will be individually considered and further analyses may be requested of the

producer

4.4

Certification

If the particular batch of CRM's is reasonably homogeneous and definitely

stable, and if there is no significant difference between the concentration

claimed by the producer and that found by the auditor, both the EPA and the

producer will be notified by NIST. The producer may then certify the samples

in that particular batch as having met the requirements of a Certified

Reference Material that has traceability to the respective SRM.

To assure the purchaser of a CRM that it has been prepared according to

the procedures described herein, and that the results of the EPA audit and the

NIST analysis of the analytical data confirm the producer's concentration

within acceptable limits , the producer will supply the purchaser with the

following:

1 . A copy of the letter from NIST to EPA and to the producer stating the

particular lot meets the requirements for CRM's. The NIST letter will

list each cylinder in the lot, identified by cylinder and sample

9
number, and will show the producer's value for the concentration of

each sample in the lot. In addition, the letter will identify the

samples audited by EPA and will give the concentration determined by

the audit.

2. A producer's "Certificate of Analysis" will include, as a minimum, the

cylinder and the sample number, the concentration for the particular

sample, and the uncertainty assigned to that concentration. In

addition, the period of certification and the date of the beginning of

the period will be given.* Finally, the certificate will identify the

SRM used by the producer to establish traceability of the CRM to that

NIST SRM.

*The date of the beginning of the period of certification normally is the date

of the NIST letter to EPA and to the producer. However, the producer may

elect to follow the procedure given in appendix G for date-of-sale certification. If appendix G is strictly followed, the date of the beginning of the

period of certification is the date of sale of the CRM by the producer to a purchaser of the CRM.

10
ACKNOWLEDGMENTS

Special acknowledgment is made to Ernest E. Hughes, the principal

scientist responsible for the development of the NIST gas SRM program, who led

the development and approval of the May 1981 document, and who recently passed

away. It is to his outstanding technical efforts that this revised document is

dedicated.

Appreciation is given to Robert Wright of Research Triangle Institute and

to Darryl J. von Lehmden of EPA for their thorough review of this revised

document

Acknowledgment also is made to Robert Wright who has conducted the

analytical audits for EPA for the certification of CRM's since the initiation

of the CRM traceability program in 1981.

11
APPENDIX A

Statistical Analysis of Data

I . Introduction

The data submitted to NIST by the producer and the auditor will be

examined in detail to determine that the candidate lot of CRM's is of the

concentration claimed and that the lot is stable and homogenous. The statistical treatment is illustrated in this appendix by means of an example based, for

the most part, on real experimental data. However, it was necessary to synthesize some' data, particularly in describing the auditor results, to produce an example illustrative of the whole process. It should be noted that this

example does not define the degree of measurement precision required of a CRM,

and, in many cases, the precision shown may not necessarily be attainable, due

to such factors as instrument sensitivity and the chemical and physical

properties of the gases involved.

II . Illustrative Example and Statistical Analysis

1 . Producer's Calibration Data :

The producer provided the following data on the calibration of his

instrument

Table 1

SRM No. Concentration* Signal or Sensitivity

1677 9.67 ± .09 ppm 11.51 mV (signal)

1678 44.9 ± . 5 ppm 54.10 mV (signal)

1679 97.1 ± .9 ppm 114.86 mV (signal)

*The ± values are the total uncertainties given in the SRM certificates.

A linear regression of signal vs. concentration provides the equation:

signal = .464 + 1.181 (concentration)

The fit to the straight line is good, and is only minimally affected by

making the intercept zero. This leads to the modified equation:

signal = 1.187 (concentration)

12
The fit provided by this equation is shown in the following table where

the fitted values in the last column should be compared with the observed

values of signal:

Table 2

Signal

Observed Fitted

11.51 11.48

54.10 53.30

114.86 115.26

We conclude that the instrument gives essentially a linear response and is in

good state of calibration. However, since the samples to be analyzed have a concentration close to that of SRM 1678 (about 45 ppm) , a sensitivity value can be adopted that is based on a calibration line going through the origin and

through the point whose abscissa is the certified value of SRM 1678 and whose

ordinate is the measured value for this SRM. This gives the calibration

equation which will be used for all further analyses:

signal = 1.205 (concentration)

2 . Analysis of Internal Reference Standard

One of the cylinders was randomly selected as the lot "Internal Reference

Standard." The measurements,* in mV are shown in the second column of Table 3 The third column shows the concentration values, in ppm, obtained by using the

sensitivity 1.205 mV/ppm for the conversion.

The average concentration for the Internal Reference Standard is:

x = 44.6139 ppm

with a standard deviation among single replicate measurements of

sx = 0.018 ppm

The standard error of the average value for this cylinder is:

0.018

s- = = 0.0057 ppm

yio

13
Table 3

Measurement No.

Signal

(mV)

Concentration

(ppm)

Measurement No.

Signal

(mV)

Concentration

(ppm)

1 53.7623 44.616 6 53.7683 44.621

2 53.7382 44.596 7 53.7454 44.602

3 53.7454 44.602 8 53.7237 44.584

4 53.7948 44.643 9 53.7707 44.623

5 53.7719 44.627 10 53.7767 44.628

*Note: It is not required that either producer or auditor provide the actual

measured value of the signal. However, the value of the concentration

calculated from the signal should be expressed with sufficient digits to

reflect the magnitude of the signal. In other words, don't round off the

calculated concentration when submitting the data for evaluation.

3 . Analysis of All Samples

The Internal Reference Standard is now used by the producer to analyze all

the cylinders in the lot. This is generally accomplished by measuring the

ratio of the signal for the sample to that for the internal standard. Table 4

shows the signal ratios, denoted as R, and the corresponding calculated con- centrations, denoted as C, using the value 44.6139 ppm for the internal

standard. Thus, each concentration is obtained by the equation:

C = R (44.6139)

where

signal for sample

signal for internal standard

The last column of Table 4 is used for two purposes: a) to obtain an additional estimate of the standard deviation among replicates, and b) to test whether . a significant systematic shift has occurred between the two sets ("first

analysis" and "second analysis"). The estimate of the standard deviation,

converted to a single measurement basis is 0.021 and is consistent with that

obtained previously (s = 0.018) [see section 2, preceding]. As to a possible

shift, there is no evidence for such an occurrence. A test of significance can be carried out as follows:

-0.0048

t = = -.50

0.030/710

14
This value is not significant, when compared with the critical value of

Student's t, for 10-1 = 9 degrees of freedom.

The standard deviations, 0.021 and 0.022, for the two sets of values in

Table 4 are mutually consistent. Moreover, since they are of the same order of

magnitude as the measurement error (as derived from replicate measurements on the same sample) , it may be concluded that no measurable heterogeneity exists

between the cylinders of this lot.

Table 4

Sample

First Analysis

R C

Second Analysis

R C

Calculated

Concentration

Difference

Between

Duplicates

5 1.000177 44.622 0.999601 44.596 0.026

6 1.000592 44.592

11 1.000592 44 . 640

13 1.000102 44.618 1.000406 44.632 -0.014

15 1.000392 44.631

21 0.999482 44.591

23 1.000904 44.654 0.999807 44.605 0.049

29 1.000438 44.633

30 1.000368 44.630

31 0.999906 44.610 1.000353 44.630 -0.020

33 0.999350 44.585 1.000307 44.628 -0.043

34 0.999520 44.592 1.000050 44.616 -0.024

35 1.000480 44.635

39 0.999563 44.594 1.000210 44.623 -0.029

45 - 0.999514 44.592 1.000050 44.616 -0.024

46 0.999415 44.588

48 0.999168 44.577

49 0.999800 44.605

50 0.999881 44.609 0.999304 44.583 0.026

51 0.999060 44.572

52 0.999452 44.589 0.999321 44.584 0.005

Average 44.6065 44.6099 -0.0048

Std. dev. 0.021 0.022 0.030*

*Since the numbers in this column are differences of two measurements, the

standard deviation found for this column is J2 times that for single mea- surements . Consequently, the standard deviation for single measurements

derived from the differences is (0.030)/./2 = 0. 021. This values does not

include possible variability between samples.

15
The best average value for the concentration of the lot is

:

(44.6065 x 10) + (44.6099 x 21)

10 + 21

44.6088

The standard error of this overall average is:

0.022

= 0.0040

731

This standard error does not include calibration error, or errors in the

value of the SRM and in the value of internal reference standard.

4 . Auditor's Calibration Data

Table 5 shows the auditor's calibration results.

Table 5

SRM No. Concentration* Signal (counts)

1677 9.79 ± .09 19,688

1678 45.3 ± .5 91,642

1679 97.1 ± .8 197,990

1680 476. ± .4 979,130

*The ± values are the total uncertainties given in the SRM certificates.

The sensitivities (signal/concentration) are successively:

2011, 2023, 2039, and 2057 counts/ppm.

These values indicate a trend due, either to the presence of a blank or to

curvature, or to both.

A regression analysis shows a slight amount of curvature, but otherwise

the calibration data appear satisfactory. Since the samples are of the order

of magnitude of SRM 1678, the latter will be used for conversion of signal to

concentration, through the equation:

signal = 2023 (concentration).

16
5 . Auditor's Sample Measurements

The auditor made 10 replicate analysis on each of two cylinders, no. 48

and no. 29. The results are shown in Table 6.

Table 6

Signal

Cylinder 48

Concentration Signal

Cylinder 29

Concentration

89866 44.422 89781 44.380

90044 44.510 90121 44.548

89495 44.239 89874 44.426

89570 44.276 90050 44.270

90024 44.500 89558 44.270

89997 44.487 89874 44.426

89568 44.275 90226 44.600

89708' 44.344 89888 44.433

89742 44.361 89987 44.482

89852 44.415 89655 44.318

Average 44.3829 44.4396

Std. dev. 0.099 0.101

6. Evaluation of Uncertainties and IntercomDari son of Results

a. Internal Comoarison of Auditor's Results

The standard deviation of a single measurement made by the auditor is

0.10. The results for cylinders 48 and 29 may be compared by Student's t-test:

44.3829 - 44.4396

t = -1.27

1 0.100 — +

10

1

10

17
The t-value is not significant. There is, therefore, no evidence of

heterogeneity between the two cylinders.

b . Total Uncertainty of Producers Values

The value of each sample obtained by the producer is obtained by a procedure represented by the following diagram:

Calibration Internal Reference

SRM Factor Standard Sample

1 1 1 I I I Calibration A B

The total error in the sample value is composed of four parts:

1) the uncertainty in the SRM value

2) the uncertainty in the calibration experiment

3) the uncertainty due to comparison A

4) the uncertainty due to comparison B

We use the rule (derived from the law of propagation of errors) that the

square of the relative error of the final value is equal to the sum of the

squares of the relative errors of the components.

More specifically, if cL is the final concentration value obtained for a particular sample (denoted by the subscript i) in the lot, we have (from step

B):

= R, Cg. f (l)

where Rx is the ratio of signals for sample i. to the reference sample, and CRef

is the concentration attached to the reference sample. But is obtained in

step A by averaging ten values obtained each as

Signal

k

( 2 )

where k is the calibration value derived from the calibration experiment. In our case, k = 1.205. The average of the ten measurements may be described by

S

^ef = -

k

where S is the average of ten replicate signal values. Combining (1) and (3)

gives

18
s (4)

A = Ri

k

The value of k is obtained experimentally from a single signal value divided by

the concentration of the SRM. Thus we may write:

So

k = — (5)

where CD is the concentration attached to the SRM by the SRM certificate and S 0 is the signal corresponding to it.

Combining (4) and (5)

we obtain finally:

_

c0 Cj = ^ S - (6)

s o

The law of propagation of errors gives

( ° rc i 2 • "R,

' 2

’ a s

' 2 2 ' a CD

'

>

l|

*H <o

. Ri .

+

. S .

+

. SD .

+

. C0 .

The first term of the right side represents step B; the second term, step

A; and the third term the uncertainty of the calibration experiment itself.

The last term represents the uncertainty of the SRM used for calibration.

We now estimate these four components.

"Ri

1) (step B) is obtained from the last column of Table 4:

Ri

Ra 0.021 = = 4.71 x 10' A (8)

Ra 44.61

CTs 2) — (step A) is obtained from the calculations derived from Table 3:

S

og- 0.0057 — = = 1.28 x 10- A (9)

S 44.62

19
a S0 3) ——- (calibration experiment) has not been measured, but we can assume the same precision as in step A for a single measurement:

*S0 0.018

= = 4.03 x 10' A (10)

S 0 44.62

ff c0 4) ——- (uncertainty of SRM) is derived from the uncertainty stated in the SRM

°o

certificate. This stated uncertainty is equal to two standard deviations.

Thus

a C0

1 0.5 = = 55.68 x 10~ A (11)

C0 2 44 .

Adding the squares we have

r rr 2 A

I'

4 ' 71

)

2 + (1.28) 2 + (4.03) 2 + (55 . 68)‘ x 10" 8

= 3140 x 10'

Hence

c-l/c-i = ±56 x 10- A

Since all c L values are approximately the same and equal to

c ± = 44.62 ppm, we obtain

a c t = 56 x 10_A x 44.62 = ±0.25 ppm (12)

We see that the predominant component of uncertainty, in this case, is that of

the SRM.

c . Comparison of Producer's and Auditor's Values

By a calculation similar to that above, we obtain, for the auditor's value

for a particular cylinder, say c^

° r*c i 2 2

1

f > °ct

2

1 <

-

* c i J . ct .

T

. ct ,

-r

. c;

20
Step A is not present for the auditor's data. The symbol ct represents a count, and ct an average of ten counts; C* represents the value given by the

certificate for the SRM used in the calculation of the calibration factor. We

have

( a c* } i 2

r o.io/yio

' 2

> o o

; 2

' 0.25

'

J L 44.4* J T

C 44.4 J k 45.3 ,

= (7 . 12x10" *

2 + (22.52x10'*

2 + (55 . 19x10' *2

= 3604x10' 8 = (60x10'*

2

.

Since c* =44.4 ppm for both samples analyzed by the auditor, we have:

a c* = 60x10' *x44. 4 = ±0.27 ppm

We now obtain the following summary results (Table 7):

Table 7*

Sample Sample

48 29

Producer 44.58 + .25 44.63 + .25

Auditor 44.38 + .27 44.44 + .27

*The ± values represent standard errors in this table.

It is apparent that the result obtained by the auditor for each sample is not

significantly different from that of the producer for the same sample. Thus,

the auditor's values in this case, substantiate those provided by the producer.

In general, it may be assumed that there is no significant difference

between the concentration claimed by the producer and that found by the auditor

if the following expression is satisfied:

* +

a T

ci

21
APPENDIX B

Report Forms

The following forms, A and B, are suggested report forms which will be

made available to both the producer and the auditor.

22
-FORM A. Report of Analysis of Certified Reference Materials

(Producer)

Preoared bv: (Company name & address)

(Individual preparing report

& date)

Descriotion:

Composition:

Corresponding SRM No.

Date Blended

Date Transferred

Analysis

SRM's Used as Calibrants:

No. Sample No. Cylinder No. Concentration Purchase Date

1 .

2 .

3 .

4 .

5 .

23
FORM A (Continued)

Brief Description of Methods :

Brief Description of Calibration Procedure :

Calibration Data :

SRM No , Signal or Sensitivity (Identify)

1 .

2 .

3 .

4 .

5 .

Results

Concentration

Analysis of Internal Standard (if used)

Sample No. Cylinder No.

Date of Analysis

1 .

2 .

3 .

4 .

5 .

6 .

7 .

8 .

9 .

10.

Average s.d.

24
FORM A (Continued)

Analysis of CRM's

Concentration on Date Shown

Sample No. Cylinder No.

Average Std. Dev.

25
FORM B. Report of Analysis of Certified Reference Materials

(Auditor)

Prepared bv :

Description :

Analysis :

SRM's Used as Calibrants:

No . Sample No. Cylinder No. Concentration Purchase Date

1 .

2 .

3 .

4 .

5 .

Brief Description of Method :

Brief Description of Calibration Procedure :

Calibration Data:

SRM No. Signal or Sensitivity (Identify)
FORM B (Continued)

Sample No.

Cylinder No.

Analysis of CRM

Sample No.

Concentration Cylinder No. Concentration

Average

Std. Dev.

Average

Std. Dev.

27
APPENDIX C

Carbon Monoxide

a . The Available SRM's are as Follows

CO in N2 SRM No. Nominal Concentration

1677 10 ppm

2635 25 ppm

1678 50 ppm

1679 100 ppm

2636 250 ppm

1680 500 ppm

1681 1000 ppm

2637 2500 ppm

2638 5000 ppm

2639 1 %

2640 2 %

2641 4 %

2642 8 %

CO in Air

SRM No. Nominal Concentration

2612 10 ppm

2613 18 ppm

2616 42 ppm

The concentrations shown are in parts per million (ppm) by mole and in

mole percent. The actual concentration of a particular SRM may differ by more than several percent relative from the nominal value and it is therefore

essential that the exact concentration of the SRM to be duplicated be known

before attempting the preparation of the CRM.

28
The maximum permissible levels of impurities in the CRM are as follows:

Water Vapor

Me thane

Methane

Carbon Dioxide

Carbon Dioxide

10 ppm

1 ppm

5 ppm

5 ppm

400 ppm

(CO in N2 ) (CO in air)

(CO in N2 ) (CO in air)

b. Cylinders

CRM's must be packaged in new aluminum cylinders or in used aluminum

cylinders that have been used exclusively for mixtures containing carbon

monoxide in nitrogen or air. The history of any cylinder to be reused must be

known and confirmation must be obtained by analysis that the contents have not

been diluted or contaminated. Cylinders to be reused must contain sufficient

gas from the last filling to allow an analysis. The concentration must lie

within ±1 percent relative of the original analysis. The cylinder must be

evacuated before refilling and the concentration of the CRM to be added must

lie within 2 orders of magnitude (±100% relative) of the concentration of the

CRM previously contained.

c . Preparation of Analysis

Prepare a lot of at least 10 cylinders all at identical concentration.

The concentration of carbon monoxide must lie within ±1 percent relative of the

certified concentration of the specific SRM cylinder with which the lot is to

be compared. Analyze a representative number of cylinders by direct comparison

to the SRM or by comparing cylinders in the lot to a single sample selected at

random from the lot (internal standard from the lot). Analyze each sample at

least two times. Calculate the average of all results for all samples and

calculate the standard deviation of the average.

The lot should be put aside to "incubate" for at least one month after

which time all samples in the lot are analyzed. The results for the first and

second analyses should be statistically identical. If the two analyses are the

same, then it may be assumed that the lot is homogeneous and stable. If the

analyses were performed by direct comparison to the SRM, then the analytical

value for the sample will be the certified value. If the analyses were performed by comparison to an internal standard for the lot, then it will be

necessary to determine the concentration of the internal standard. This is

done by repeated comparison of the internal standard to the SRM. The con- centration measured for the internal standard is then multiplied by the

measured ratio of each sample.

The uncertainty of the CRM is composed of the uncertainty of the SRM and

any random errors introduced by the analysis of the CRM. The error of the SRM,

Esrm , is given on the certificate for the particular SRM cylinder and is

defined as the estimated upper limit of the total uncertainty. The estimated

upper limit of the uncertainty for the CRM should be calculated as follows:

29
E2 (Total Error) = 2 esrm

2

s t j

+ a

2 + b2 +

where E2 is the uncertainty of the CRM; a is the imprecision of intercomparison

of the internal standard with the lot; b is the imprecision of intercomparison

of the internal standard and the SRM; and c is the error of the calibration

experiment if applicable (see appendix A).

The following table lists the methods of analysis that are applicable to

the analysis of CRM's of carbon monoxide in nitrogen or air. The method chosen

is not necessarily limited to those shown providing that the method chosen is

of adequate sensitivity, precision and linearity. The precision and

sensitivity are evaluated during the analysis described earlier and are reflected directly in the imprecision of intercomparison. The linearity must

be defined in the concentration range between the CRM and the SRM.

Table 1. Methods for the Intercomparison of CRM's and SRM's of Carbon

Monoxide in Air or Nitrogen

Method Concentration of CO

Gas Chromatography - TC Detector 8% - 0.5%

Gas Chromatography - Ultrasonic Detector 1% - 0.05%

Gas Chromatography Followed by Catalytic

Reduction to Methane with Flame

Ionization Detection 1% - 10 ppm

Non-dispersive Infrared 8% - 10 ppm

d. Period of Certification

Experience at NIST indicates that carbon monoxide mixture contained in

aluminum cylinders are stable for periods of three years or more. If no change

in concentration of the CRM's is indicated by the results of the producer's

first and second analysis, then stability for a period of at least two years

from the date of the second analysis is assured with a high degree of

probability.

30
e . Recertification (see appendix G for further details)

CRM's of carbon monoxide in nitrogen or air that have exceeded the period

of certification may be recertified by the original supplier according to the

following provisions.

1. The pressure remaining in the cylinder must be greater than 500 psi

(3.4 MPa)

2. The ratio of the signal generated by the CRM when compared to the

original internal standard must be the same as when originally measured

within the limits of uncertainty generated by both sets of

measurements

3. The concentration of the internal standard, determined by comparison to

the SRM with which it was originally compared, must be the same within

the limits of precision of the comparison.

4. The concentration of the SRM must be known either by comparison with a new SRM or the original SRM must have been recently recertified by

NIST.

5. If either the original SRM or the internal standard no longer exists,

then no recertification can be made.

6. The uncertainty to be assigned to the recertified value must include

the added uncertainty, if any, of the recertification process.

31
APPENDIX D

Nitric Oxide in Nitrogen

a. The Available SRM's are as Follows

SRM Number

Nominal Concentration of NO

Dorn (molar)

2627 5

2678 10

2629 20

1683 50

1684 100

1685 250

1686 500

1687 1000

2630 1500

2631 3000

The concentrations shown are in parts per million (ppm) by mole. The

actual concentration of a particular SRM may differ by more than several

percent relative from the nominal value and it is therefore essential that the

exact concentration of the SRM to be duplicated be known before attempting the

preparation of the CRM.

The maximum permissible levels of impurities in the CRM are as follows:

Water vapor 5 ppm

Nitrogen dioxide <1% of the NO concentration

b . Cylinders

Nitric oxide mixtures will remain stable in aluminum cylinders which have

been treated to assure stability. The treatment must be such that the sample

remains stable with regard to adsorption, absorption, and desorption. Because

many methods are available to achieve stability, the choice of method is left

to the producer of the CRM, and confirmation that stability has been achieved

32
will be based on the observed homogeneity of the batch and on the stability

analysis

Aluminum cylinders that previously contained mixtures of nitric oxide in

nitrogen,' either CRM's or other mixtures, may be reused. It is recommended

that if such cylinders are reused, the concentration of the new mixtures be

within one order of magnitude of the previously contained mixture (within 0.1

to 10 times) . The history of any reused cylinder must be known, and cylinders

in which mixtures of nitric oxide were observed not to be stable must not be

used for CRM's. Cylinders to be reused must have a residual pressure greater

than one atmosphere of the mixture of nitric oxide in nitrogen before

preparation for use with a CRM's.

c . Preparation and Analysis*

Prepare a lot of at least 10 cylinders of identical concentration. The

concentration of nitric oxide must lie within ±1 percent relative of the

concentration of the specific SRM with which the lot is to be compared.

Analyze at least 10 samples from the batch if the batch contains more than 20

samples. All samples should be analyzed in batches of 20 or less. Analyze the

internal standard from the lot by comparison to the appropriate SRM's. Analyze

each sample at least two times. Calculate the average for the samples analyzed

and calculate the standard deviation of the average using,

2d2 s i =

4 Nx -1

where d is the difference between a sample and the mean, and Nx is the number

of measurements , or the number of samples times the number of measurements per

sample

The lot should be put aside to "incubate" for at least two months (60

days) after which time all samples in the lot are analyzed. The internal

standard should be reanalyzed at this time using the same SRM's for comparison

as were used in the first analysis. The results for the first and second

analyses should be statistically identical (see appendix A) . If the two

analyses are the same, then it may be assumed that the lot is homogeneous and

stable. If the analyses were performed by direct comparison to the SRM, then

the analytical value for the sample will be the certified value. If the

analyses were performed by comparison to an internal standard from the lot then

it will be necessary to determine the concentration of the internal standard.

This is done by repeated comparison of the internal standard to the SRM. The

concentration measured for the internal standard is then multiplied by the

measured ratio of each sample to the internal standard to obtain the

concentration of each sample.

*The reader should be familiar with the procedures of analysis given in the

body of this document, particularly Section 3, "Suggested Analytical

Procedure" and appendix A, "Statistical Analysis of Data".

33
The uncertainty of an individual CRM must include the error of the SRM and

the added uncertainty due to the imprecision of the intercomparison.

The error of the SRM, ESRM , is given on the certificate for the particular

SRM cylinder and is defined as the estimated upper limit of the total uncertainty. The estimated upper limit of the uncertainty for the CRM should be

calculated as follows:

E2 (Total Error) = 2

r f i 2

N . 2 ,

+ b

2 + c

2

where E2 is the total uncertainty of the CRM; a is the imprecision of the

intercomparison of the internal standard with the lot; b is the imprecision of

the intercomparison of the internal standard and the SRM; and c is the

calibration error.

The recommended method of analysis for nitric oxide in nitrogen is

chemiluminescence. The instrument used must have adequate sensitivity and

precision in the concentration range represented by the SRM which the CRM is

duplicating and as many other SRM's at other concentrations which are necessary

to define the response curve of the instrument. A zero gas may be used as one calibration point when the lower concentration SRM's (2627 and 2628) are being

prepared.

Some general considerations concerning calibration are given in the main

body of this document. These considerations apply to the calibration of most

gas analysis instruments and may be directly applicable to the calibration of

instruments for the analysis of nitric oxide. In any case, it will be the

responsibility of the producers of CRM's to assess and document the linearity,

stability, and precision of their instruments.

d. Special Considerations

Certain peculiar effects may be observed when analyzing batches of CRM's

consisting of nitric oxide in nitrogen by the procedure outlined in the main

body of this document. If a number of samples are analyzed by comparison to an internal standard from the batch, the ratio of each sample to the internal

standard may be consistently less than 1.00. This effect has been observed for

nitric oxide mixtures and for certain other reactive gases . It is probably

related to the following: greater amounts of sample are removed from the

cylinder chosen as the internal standard than from each other sample in the lot

because the internal standard is used at frequent intervals throughout the

analysis of the samples. The phenomenon probably results from slight adsorption on the surface of the valve exposed to the sample inside the cylinder. A

very slight depletion of the nitric oxide probably occurs in the gas phase in

the vicinity of the valve and, because of the small quantity of gas removed

during analysis, the observed concentration will be slightly lower than that of

the bulk of the mixture remaining in the cylinder.

34
If such a difference exists it may be obscured by the imprecision of the

results, in which case it is compensated for in the final assigned uncertainty.

However, if the degree of precision of the intercomparison of the samples with

the internal standard is high, the difference may be readily observable. In

this case, and if the difference is small (0.1 percent relative or less)

compared to an estimated total uncertainty of greater than 1 percent relative,

a slight correction should be applied. This can be done simply by multiplying

the final concentration of each sample by the reciprocal of the average of the

measured ratios for all samples. For instance, if the average of the measured

ratios is 0.9995, multiply the concentration of each sample by 1.0005.

If the precision of measurement of the ratios in high and the average of

all ratios is significantly less than 1.000, some further work will be necessary to fully evaluate the problem. A simple approach is to reanalyze a large

number of samples choosing a different sample from the batch as the internal

standard. If the large difference is no longer observed then the problem may

have been with the first internal standard. If, however, the difference is

still evident then it is likely that the valves or perhaps the cylinders are not compatible with the particular gas mixture and the mixture should be remade

using more appropriate containers.

e . Impurities

The impurity of greatest concern in nitric oxide mixtures is nitrogen

dioxide. Careful preparation of the mixture from analyzed reagents and the

exclusion of all oxygen from the system is necessary to maintain the nitrogen

dioxide at the low level required (<1 percent relative to the NO content of the

sample) . This quantity of nitrogen dioxide can be measured with most chemiluminescent analyzers designed to measure, NO, N02 , or N0X

. The calibration of

such instruments to measure 1 percent relative or less of N02 in a mixture of

NO and nitrogen may be somewhat difficult. In general, however, if the three

components, NO, N02 , and N0X are measured and if the sum of NO and N02 is equal

to the N0X then the instrument is probably functioning adequately. If this is

not the case, then the instrument should be serviced and reevaluated.

It is possible to measure nitrogen dioxide in the presence of nitric oxide

by chemical means, but the details of such a procedure are complicated and

lengthy and will not be included here.

The nitrogen dioxide should be measured during both the first and second

analyses to be certain that no positive changes are occurring. Decreases in

nitrogen dioxide with time are expected and do not adversely affect the

samples. Increase in nitrogen dioxide, however, can only occur if there is a substantial amount of oxygen in the sample, and such a case would be cause for

discarding the batch.

It is not necessary to measure the nitrogen dioxide in all samples. If

the concentrations of all samples in the batch lie within the required limits

and the nitrogen dioxide is found to be less than 1 percent relative to the

nitric oxide in ten percent or more of the samples, then it can be assumed that

the batch concentration of nitrogen dioxide is within limits.

35
If a careful measurement is made of the nitrogen dioxide and if it is

initially found to be at a low relative concentration, then the probability

that any significant amount of oxygen is in the samples is quite low. If the

nitrogen dioxide then remains low or disappears the assumption that no oxygen

is present is probably valid.

The water content of the diluent nitrogen should be measured before

preparing the mixture, and several samples should be analyzed for water content

during the initial analysis. If the concentration is the same in the diluent

and the samples, it may be assumed that the sample cylinders and the transfer

systems were adequately dried before use.

The methods used and the results of analysis for impurities should be

reported along with the other required information for each lot of CRM's.

f . Period of Certification

Experience at NIST indicates that nitric oxide mixtures contained in

aluminum cylinders generally remain stable for periods of time greater than two

years. If no significant changes in the concentration of nitric oxide occurs during the incubation period and if the batch is reasonably homogeneous to

begin with, there is a high probability that samples will remain stable for at

least two years.

g. Recertification (see appendix G for further details')

CRM's of nitric oxide in nitrogen that have exceeded the period of

certification may be recertified by the original supplier according to the

following provisions.

1. The pressure remaining in the cylinder must be greater than 500 psi

(3.4 MPa).

2. The ratio of the signal generated by the CRM when compared to the

original internal standard must be the same as when originally measured

within the limits of uncertainty generated by both sets of measure- ments .

3. The concentration of the internal standard, determined by comparison to

the SRM with which it was originally compared, must be the same within

the limits of precision of the comparison.

4. The concentration of the original SRM must be known either by

comparison with a new SRM, or the original SRM must have been recently

recertified by NIST.

5. If either the original SRM or the internal standard no longer exists,

then no recertification can be made.

6. The uncertainty to be assigned to the recertified value must include

the added uncertainty, if any, of the recertification process.

36
APPENDIX E

Sulfur Dioxide in Nitrogen

a. The Available SRM's are as Follows :

Nominal Concentration of S02

SRM Number ppm (molar)

1693 50

1694 100

1661 500

1662 1000

- 1663 1500

1664 2500

1696 3500

The concentrations shown are in parts per million (ppm) by mole. The

actual concentration of a particular SRM may differ by more than several

percent relative from the nominal value and it is therefore essential that the

exact concentration of the SRM to be duplicated be known before attempting the

preparation of the CRM.

The maximum permissible levels of impurities in the CRM are as follows:

Water vapor 5 ppm

Sulfur compounds other than S02 0.1 ppm

b. Cylinders

Sulfur dioxide mixtures will remain stable in aluminum cylinders that are dry and which have been treated to assure stability. The treatment must be

such that the mixture remain stable with regard to adsorption, absorption, and

desorption. Because many methods are available to achieve stability, the

choice of method is left to the producer of the CRM and confirmation that

stability has been achieved will be based on the observed homogeneity of the

batch and on the stability.

Aluminum cylinders which previously contained mixtures of sulfur dioxide

in nitrogen, either CRM's or other mixtures, may be reused. It is recommended

that if reused, the concentration of the new mixtures be within one order of

magnitude of the previously contained mixture (within 0.1 to 10 times.) The

37
history of any reused cylinder must be known and cylinders in which mixtures of

sulfur dioxide were previously observed not to be stable must not be used for

CRM's. Cylinders to be reused must have a residual pressure greater than one atmosphere of the mixture containing sulfur dioxide before preparation for use with a CRM.

c . Analysis -General

Several methods of analysis are applicable to the analysis of mixtures of

sulfur dioxide in nitrogen. These include gas chromatography non- dispersive

infrared, electrochemical titration, pulse, fluorescence, and flame photometry.

Most of these are relative methods requiring calibrating gases. Another

method, the "peroxide” method, is an absolute technique requiring no calibrating gases.

Some general considerations concerning calibration are given in the main

body of this document. These considerations apply to the calibration of most

gas analysis instruments and are directly applicable to the calibration of

instruments for the analysis of sulfur dioxide.

The alternative to the relative instrumental methods, the peroxide method,

is a wet chemical method. It involves collection of the sulfur dioxide from a mea-sured volume of the sample, in a solution of hydrogen peroxide. The sulfur

dioxide is oxidized to sulfuric acid which is subsequently titrated with standard sodium hydroxide solution. The method as described in the literature* is

not directly applicable to the analysis of SRM's or CRM's in the concentration

range from 50 to 3500 ppm. Consequently, we will describe the method in some detail as adapted for the analysis of SRM's. (See section G, below).

d. Preparation and Analysis of the CRM's

Prepare a lot of at least 10 cylinders of identical concentrations of

sulfur dioxide in nitrogen. The concentration must lie within ±1 percent

relative of the concentration of the SRM with which the lot is to be compared.

Analyze at least 10 samples from the lot if the lot contains more than 20

samples. If the lot contains 20 or fewer samples, it is advisable to analyze

all samples. The lot should then be set aside to "incubate" for at least two

months (60 days), after which all samples in the lot are analyzed.

If an instrumental method of analysis is used, it will be necessary to

analyze the internal standard for the batch using the appropriate SRM's. This

analysis should be performed at the time of the first analysis and again at the

time of the second analysis. The analysis must confirm that no change in

concentration of the internal standard has occurred during this time and the

difference between the two values should be no greater than would be predicted

from the precision of the method. In such cases, the concentration assigned to

the internal standard is the average of the two separate sets of measurements.

*See for instance the description in "The Analytical Chemistry of Poisons,

Hazards, and Solvents", by Morris B. Jacobs, Interscience, 1941, p. 250.

38
If the peroxide method is used for the analysis, the "true" value will

have been directly obtained for each CRM sample. However, it will be necessary

to confirm the analyses by also analyzing the appropriate SRM using the peroxide method. The value obtained by analysis using the peroxide method should

agree with the value given on the certificate for the SRM. If the difference

between these two values is small and is consistent with the expected errors of

the analysis, then a correction to the CRM values may be made to align the CRM

value with the certified value for the SRM. This assumes that the difference

between the two values results from an accumulation of small random errors. If

the difference is large, then the source of error must be identified and

corrected.

At this point, the producer of the CRM will have completed two analyses

and will have assigned a concentration to the CRM's. These data will allow

decisions to be made regarding stability and homogeneity, and will indicate

whether there is compliance with the concentration requirements for a CRM.

Details of the procedures of analysis are to be found in the main body of this

document, particularly in section 3, "Suggested Analytical Procedure" and in

appendix A, "Statistical Analysis of Data". In addition, the total error of

the CRM may be calculated from the various errors generated during the analysis

of the CRM. Details of the calculation of the error where instrumental methods

are used will be found in appendix A. In the special case of the peroxide

method, the error will be calculated as described in section G, paragraph 5 of

this Appendix.

e . Period of Certification

Experience at NIST indicates that sulfur dioxide mixtures contained in

aluminum cylinders generally remain stable for periods of greater than two

years. If no significant changes in the concentration of sulfur dioxide occurs during the incubation period and if the batch is reasonably homogeneous to

begin with, there is a high probability that samples will remain stable for at

least two- years

f . Recertification (see appendix G for further details)

CRM's of sulfur dioxide in nitrogen which have exceeded the period of

certification may be recertified by the original supplier according to the

following provisions.

1. The pressure remaining in the cylinder must be greater than 500 psi

(3.4 MPa).

2. The ratio of the signal generated by the CRM when compared to the

original internal standard must be the same as when originally measured

within the limits of uncertainty generated by both sets of

measurements

3. The concentration of the internal standard, determined by comparison to

the SRM with which it was originally compared, must be the same within

the limits of precision of the comparison.

39
4. The concentration of the SRM must be known either by comparison with a new SRM or the original SRM must have been recently recertified by

NIST.

5. If either the original SRM or the internal standard no longer exists,

then no recertification can be made.

6. The uncertainty to be assigned to the recertified value must include

the added uncertainty, if any, of the recertification process.

7. The concentration determined by the peroxide method must agree with the

original value to within the combined limits of uncertainty of the two

measurements. The validity of the peroxide method must be confirmed by

analysis of the SRM.

g. Analysis - Hydrogen Peroxide Method

The sample is passed through an absorption train as shown in the

accompanying figure.

The sample cylinder is equipped with a metering valve, and a three-way

valve allows the flow to be set at the appropriate rate using the rotameter

before beginning the analysis. The midget impingers contain a hydrogen

peroxide solution. The wet- test meter measures the total volume of gas passed

through the train.

1 . Reagents

a. Hydrogen Peroxide Solution

Prepare a 1.5 percent peroxide solution by diluting 50 mL of 30

percent H2 02 to 1 liter with distilled water. Using a pH meter,

adjust to a pH of 5.5 by adding dropwise 0.1N nitric acid. This

reagent is stable at room temperature for one month.

b. Standard Sulfuric Acid (0.06N)

Carefully add 5.0 mL of concentrated reagent grade sulfuric acid to 3 liters of distilled water. Standardize against approximately 0.310 g

portions of THAM (tris [hydroxymethyl]aminomethane ) . This solution may

be stored indefinitely.

c. Standard Sodium Hydroxide (0.05N)

Dissolve approximately 4.1 g of NaOH pellets in 2 liters of

freshly boiled distilled water. Pipette 50.0 mL portions of the

standardized sulfuric acid into a beaker. Add a stirring bar and,

with moderate stirring, titrate with the sodium hydroxide solution

to a pH of 7.0. Store this solution in a polyethylene bottle and

restandardize it bimonthly.

40
2

Apparatus

a. Absorbers

Two standard midget impingers are connected in series with short

lengths of Teflon tubing.

b. Burettes

Several calibrated burettes will be needed depending on the

concentration of the sulfuric dioxide and the volume of the sample.

Minimum graduation should be 0.2 percent of the volume titrated.

c. Wet-Test Meter

A 1 liter per revolution wet-test meter is used to measure the volume

of sample. The meter must be calibrated with a maximum inaccuracy of

0.25 percent relative. Details of a procedure by which this accuracy

may be accomplished will be found at the end of this appendix.

3 . Procedure

a. Fill each impinger base with 10 mL of hydrogen peroxide solution.

b. Adjust flow to desired rate using rotameter.

c. Rotate 3 -way valve to flush tubing and impinger head.

d. Attach second impinger followed by first impinger.

e. Stop flow when desired volume has passed through train.

f. Quantitatively transfer contents of both impingers to a 100 mL beaker

rinsing impinger tube and body with distilled water. Fill beaker to

50 mL level.

g. Titrate solution with standard base to pH 5.5.

h. Determine a reagent blank using 20 mL of peroxide solution.

4. Calculation

At the temperature, pressure, and concentration of the components of the

mixture the following may be assumed to be correct.

V M

S02 S02 V + V

= M + M

so2 n2 nso2 n2 where V is volume and M is moles.

41
The volumes of both species, S02 and N2 , must be expressed at the same conditions of temperature and pressure. The volume of nitrogen measured with

the wet test meter, for instance, is corrected to 25 °C and one atmosphere (760 mm or 101.3 KPa) . The volume of sulfur dioxide is expressed at the same conditions using the relationship:

1 milliequivalent NaOH = 12.235 /xL S02 Volume NaOH (mL) x NNa0H x 12,235 = /xL S02 and

/xL S02

/xL S02 +

106

cone . in ppm

The following table is included to give some guidance for the selection of

sample size, burette size and reagent concentration needed to analyze samples

of various concentration. Factors such as the concentration of the alkali and

the sample size may be adjusted to accommodate the skill of the operator or the

size of burettes available.

Table 1

Concentration

: Sulfur Dioxide

Volume

of Sample

mL of

0.05N NaOH

Burette Volume

(mL)

50 ppm 15 L 1 5

100 ppm 12 L 2 5

500 ppm 15 L 11 50

1000 ppm 6 L 10 50

1500 ppm 3 L 7 50

2500 ppm 3 L 12 50

3500 ppm 3 L 17 50

5 . Errors

The peroxide method is essentially an absolute method and the total error should be definable by the random errors of the various elements of the

procedure. These sources of random error are as follows:

42
1. Calibration of wet- test meter.

2. Calibration of burettes.

3. Imprecision of preparation of reagent solutions.

4. Imprecision of titration.

5. Analysis of reagent purity.

In addition to these sources of random error there may be biases , but

these biases most likely will be small relative to the random error.

In the absence of blunders, the major sources of error will be the

uncertainty of measurement of the sample volume with the wet- test meter and the

imprecision of the titration of the absorbed sulfur dioxide. Both of these

errors will be combined into the uncertainty of the measured value of the

concentration and will be represented by the uncertainty of the average of

replicate measurements of single samples. Reliable reagents carefully used to

prepare standard solutions and careful standardization will result in very

small errors in the chemistry preceding the titration of the absorbed sulfur

dioxide

The magnitude of the random error and the possible existence of biases in

the system are evaluated by analysis of an SRM of appropriate sulfur dioxide

concentration. The SRM must be analyzed in triplicate, and the average of the

results should agree within experimental limits with the value appearing on the

certificate for the SRM. The following relationship should be satisfied if the

analysis of the SRM by the procedure has been done correctly:

I A B| < 2 + b

2

where A is the certified value of the SRM with the associated uncertainty, a, (as given on the SRM certificate) and B is the average of the producer's

replicate analysis of the SRM with the associated standard deviation, b, of the

average of its determination.

If this relationship is not satisfied and the value of b is reasonably

small (<0.5 percent relative), then further work will be necessary to discover

the source of error. Consultation with the appropriate group at NIST is the

logical first step in resolving any conflict at this point.

If the NIST value and the producer's values agree, then it can be assumed

that no significant bias exists between the two values and the error of the

producer's value can be calculated from the observed random errors of the

measurement

Stepwise, these include the following:

43
a. The imprecision of standardization of the sulfuric acid based on titration of at least three weighed portions of THAM. This error,

represented by the relative standard deviation of the average

measured normality of the acid includes both the weighing error and the titration error.

b. The imprecision of standardization of the hydroxide solution

represented by the standard deviation of the average of replicate

titrations of the base with the acid.

c. The imprecision of measurement of the sulfur dioxide represented

by the standard deviation of the replicate measurements of the

sample. This uncertainty includes both the titration error and

the error associated with the repeatability of the volume

measurement

.

d. The error in the measured volume represented by the uncertainty in

calibration of the wet- test meter.

These errors may be summed as follows:

E = Jaz + b

2 + c2 + d2

In general, the first two factors, a and b, associated with standardization of

solutions' should not exceed 0.1 percent relative. The third factor, c, which

is the imprecision of measurement of the sulfur dioxide, will depend somewhat

on the concentration but should not exceed 0.25 percent relative. The errors

in calibration of the wet test meter should be less than 0.25 percent relative.

Thus, the total error should be in the vicinity of ±0.5 percent relative, or 1 percent at the 2o level.

h. Calibration of Wet Test Meter

Minimum uncertainty in a volume measurement using a wet- test meter is

obtained when the conditions of the calibration duplicate the conditions of use

as nearly as possible. The procedure described here is intended to satisfy

that requirement. The method is basically very simple - a weighed quantity of

gas is passed through the wet- test meter and the volume indicated by the meter

is related to the measured weight of gas. The gas is contained in a cylinder

which is weighed. Gas from the cylinder is then passed through the meter and

the cylinder is once again weighed. The weight loss of the cylinder represents

the mass of gas passed through the meter. This mass can be expressed as a volume if temperature and pressure are known. The only equipment needed other

than the wet- test meter is a high-capacity balance capable of weighing to

±0.1 g and a small cylinder equipped with a regulator. The weight of the

cylinder filled with gas and equipped with the regulator should not exceed the

capacity of the balance.

The cylinder is filled with air and nitrogen to a pressure that will allow

the release of at least 200 liters of gas at N.T.P. (normal temperature [25 °C]

and pressure [1 atm.]). The regulator is attached and the cylinder is

44
connected to the wet- test meter. The outlet control valve on the regulator is

adjusted for the flow rate to be used in the analysis, and the cylinder valve

is then shut off without changing the adjustment of either the regulator or the

outlet control valve. The cylinder is then weighed and reattached to the

system. The zero reading of the wet- test meter is recorded, as is the

temperature and barometric pressure. The cylinder valve is opened and about 50

L of gas is allowed to flow through the meter. The cylinder valve is closed

and the cylinder is reweighed. This sequence should be repeated at least three

times

Fifty liters of air or nitrogen will weigh somewhat over 50 g, and the

weighing error should not exceed about ±0.2 percent relative. The observed

weight loss of the cylinder represents the mass of gas passed through the

meter. This mass is expressed as a volume of gas, Vg

, at the measured

temperature and pressure using the ideal gas relationship,

Vs = £ RT

M

If the gas from the cylinder is dry and no saturation occurs except in the

wet test meter it will be necessary to reduce the volume indicated by the

meter, Vm , by a fraction representing the volume increase in the sample due to

uptake of water vapor, or more simply:

V + V„ = Vm 6 H2 0

m

The water vapor correction (V ) is V

H2 0

h2 o' , where P is the

saturation pressure of water at the temperature of the wet test meter and P is

the barometric pressure and, thus,

Vv m = Vv g

1 +

H2 0

A simpler approach is to saturate the flowing gas between the cylinder and

the wet- test meter. In fact, the apparatus used for collection of sulfur

dioxide may be used in the calibration of the wet- test meter. The system as shown contains a rotameter to facilitate setting the flow, and the impingers

can be used to saturate the gas, eliminating the need for water vapor

corrections. In this case, the relationship becomes:

V* =

If, however, the meter does not read correctly, a factor, a, should be

applied such that

a • Vm = Vg or

Vm = V. 1 m g

h2 o

45
Using this method, and using sufficiently large quantities of gas, it is

possible to calibrate a wet- test meter in place under identical conditions to

those which will be used in the analysis with an absolute uncertainty not

exceeding 0.25 percent relative.

46
SAMPLING

TRAIN

FOR

PEROXIDE

METHOD

47
APPENDIX F

Propane

a. Introduction

This, appendix describes the preparation of Certified Reference Materials

(CRM's) consisting of propane in nitrogen or propane in air. The reader should

be familiar with the general description of the preparation and analysis of

CRM's as described in the main body of this document.

b . Current SRM's

Following is a list of SRM's of propane in either nitrogen or air for

which CRM's may be made. The concentrations shown are in parts per million

(ppm) by mole and in mole percent. The actual concentration of a particular

SRM may differ by more than several percent relative from the nominal value,

and it is therefore essential that the exact concentration of the SEIM to be

duplicated be known before attempting the preparation of a CRM.

C3 H8 in Air C3 H8 in N2 SRM No. Nominal Concentration SRM No. Nominal Concentration

1665 3 ppm 2643 100 ppm

1666 10 ppm 2651 100 ppm C3 H8 ; 5% 02

1667 50 ppm 2652 100 ppm C 3 H8 ; 10% 02

1668 100 ppm 2644 250 ppm

1669 500 ppm 2645 500 ppm

2646 1000 ppm

2647 2500 ppm

2648 5000 ppm

2649 1%

2650 2%

c

Cvlinders

CRM's must be packaged in new aluminum cylinders equipped with valves of

appropriate material which conform to CGA recommendations for these particular

mixtures. Aluminum cylinders previously used for propane CRM's or which have

48
been used only for propane in air or in nitrogen mixtures may be used for

CRM's. The history of a reused cylinder must be known, and, in the case of a CRM cylinder, an analysis must be performed of the gas mixture remaining in the

cylinder to assure that the contents have not been diluted or contaminated.

The concentration of the remaining gas mixture must lie within ±1 percent

relative of the concentration of the original CRM, as previously certified.

The cylinder must be evacuated before refilling and the concentration of the

new CRM gas mixture must lie within one order of magnitude (within 0.1 to 10

times) of the concentration of the previous CRM or other propane mixtures

previously contained.

d. Preparation and Analysis

Prepare a lot of at least 10 cylinders all at identical concentration.

The concentration of propane must lie within ±1 percent relative of the

concentration of the specific SRM with which the lot is to be compared.

Analyze at least 10 samples from the lot, if the lot contains more than 20

samples. If the lot contains fewer than 20 samples, it is recommended that all

samples be analyzed. Analyze the samples by direct comparison to the SRM or by

comparing samples in the lot to a single sample selected at random from the lot

(designated as the "internal standard" of the lot). Analyze each sample at

least two times. Analyze for impurities as required in Section E, below.

After this first analysis, the lot should be put aside to "incubate" for

at least two months after which time all samples in the lot are analyzed and at

which time the impurities are redetermined. If the two sets of analyses are

the same, then it may be assumed that the lot is homogeneous and stable. If

the analyses were performed by direct comparison to the SRM, then the analytical value for the sample will be the certified value. If the analyses were

performed by comparison to an internal standard for the lot, then it will be

necessary to determine the concentration of the internal standard. This is

done by repeated comparison of the internal standard to the SRM. The con- centration of the internal standard must be determined at the time of the first

and second analysis by comparing it to the appropriate SRM's. The measured

concentration of the internal standard should be the same for both analyses

within the limits of error of the analyses . The concentration measured for the

internal standard is multiplied by the measured ratio of each sample to the

internal standard to obtain the concentration of each sample.

The uncertainty of the CRM includes the error of the SRM and the

uncertainty due to the imprecision of the intercomparison. The error of the

SRM, Esrm , is given on the certificate for the particular cylinder, and is

defined as the estimated upper limit of the total uncertainty. The estimated

upper limit of the uncertainty for the CRM should be calculated as follows:

E2 (Total Error) = 2 -srm + a

2 + b

2 + c:

49
where E2 is the estimated upper limit of the uncertainty of the CRM; a is the

imprecision of the intercomparison of the internal standard with the lot; b is

the imprecision of intercomparison of the internal standard and the SRM; and c is the error of the calibration.

e . Analytical Method

Analysis of these CRM propane mixtures may be done by gas chromatography

or any other method specific for propane determination. The method chosen

should have adequate sensitivity and precision, and the nature of the instrument response must be known for the concentration region bracketing the SRM and

the CRM. The precision and sensitivity are evaluated during analysis, and are reflected directly in the "imprecision of intercomparison. " The instrument

response characteristic is evaluated over the small concentration range between

the SRM and CRM by observing the response over a wider range using SRM's as calibration standards

i

f . Impurities

The major impurities in SRM's of propane in air or propane in nitrogen are other organic compounds. The propane content of an SRM is certified, while the

concentrations of other hydrocarbons are given for information only. If a method of analysis is used that is specific for propane (gas chromatography,

for example) , then the presence of other hydrocarbons will not affect the

measurement of propane. However, if a non-specific method (such as using a total hydrocarbon analyzer) is used, then it is essential that the type and

concentration of each other organic compound be known. Furthermore, the source

of the other organics must be known to assure future assessments of the

stability of the samples.

There are three major sources of organic contamination of SRM's or CRM's.

These include trace hydrocarbons, usually methane, present in the diluent air or nitrogen, other aliphatic hydrocarbons that are present in the reagent

propane, and solvents, usually chlorinated compounds, which are used to clean

cylinders and valves and which are not always completely removed before

assembly of the cylinder. The first two sources generally do not create

analytical problems, provided that the total concentration of the hydrocarbon

is low and further provided that the specific hydrocarbons are identified. The

last source may cause serious problems if the concentration is high and if the

concentration increases with time as the material desorbs inside the container.

The aliphatic hydrocarbons in the CRM's can be measured by gas

chromatography, and their concentration and identity should be consistent with

the impurities in the reagent propane and in the diluent nitrogen or air (which

should have been analyzed before the preparation of the CRM's). The presence

of other organic material may be determined by exhaustive analyses utilizing

any number of trace analytical techniques, but if reasonable precautions were used in excluding organic impurities from the preparatory process, a simple

test will suffice to assure that the only impurities are those originating in

the reagents . This involves comparing the hydrocarbons measured with a total

50
hydrocarbon analyzer, with the sum of the aliphatic hydrocarbons individually

measured by gas chromatography. If the two values are the same or similar,

there is a high degree of assurance that the other organics in the sample are aliphatic hydrocarbons originating from the reagent gases. If the value

obtained by total hydrocarbon analysis is significantly higher than the sum of

the compounds measured chromatographically , then a serious problem exists, and

either the batch of CRM's must be discarded or the source of the other organics

must be identified. Further, it must be determined that these organic

impurities will not create future analytical problems.

In general, the following requirements must be met in regard to

impurities

1. The hydrocarbon impurities* must be less than one percent of the

propane

2. Organic impurities other than aliphatic hydrocarbons must be less than

0 . 25 percent of the propane

3. Neither the aliphatic hydrocarbons nor the other organics should

increase between the first and second analysis.

Compliance with these requirements can be confirmed by use of the

following procedure. This procedure is the minimum effort that should be

expended to characterize the impurities, and it is not intended to discourage

more strenuous efforts to identify the exact nature of the impurities

1. Select at least five samples from the batch for analysis of total

other hydrocarbon [TOH] and total organics [TO] . This analysis must

be performed within 10 days after preparation of the batch, and again

sixty days after the first analysis.

2. Analyze the selected samples for total organics using a flame

ionization total hydrocarbon analyzer. Calibrate the analyzer with

SRM's of propane in air or propane in nitrogen, whichever are appropriate. (The total concentration of hydrocarbon in the SRM,

[TH] srm , is the sum of the propane and of the reported other hydrocarbon, expressed as propane, whereas the total organics [TO] is the sum of the propane, the total other hydrocarbon [TOH], and any other

organic compounds , such as chlorinated hydrocarbons .

3. Calculate the total organics [TO] in the CRM using the following

relationship

*The concentration of each impurity is expressed as the equivalent

concentration of propane. When measured with a total hydrocarbon analyzer

calibrated with propane, the measured concentration of the impurities is

equivalent to propane. If the individual compounds are measured by

chromatography, the concentration of each is multiplied by

^ where n is

the number of carbon atoms in the particular molecule.

51
XCRM

[TOJcrm = [TH] SRM 7

^-SRM

where i is the signal generated by the CRM or the SRM. The total

other hydrocarbon (TOH) in the CRM is simply the total hydrocarbon

expressed as propane minus the propane. ([TH] = [TOH] + Propane).

4. Analyze the CRM's for aliphatic hydrocarbons using gas chromatography.

The analysis must be such that hydrocarbons from through C 7 can be

quantitatively determined. Several different analyses may be required

to accomplish this.

5. Calculate the total hydrocarbon [TH] in the CRM by summing the

individual hydrocarbons after expressing each as the equivalent

propane concentration. This is done as follows:

[TH] crm = 1/3 [CHJ + 2/3 [C2 H6 ] + .... ^

[Cn H2n + 2 ]

6. Compare the value for total organics measured with the hydrocarbon

analyzer to the total hydrocarbon measured by chromatography after

first subtracting the propane concentration from each. The concentration of other organics is at an acceptable level if the other organics

([TO] - [C3 H8 ]) do not exceed the total other hydrocarbons ([TH] - [C3 H8 ]) by more than 25 percent or

([TO] - [

3 H8 ] ) < 1.25 ([TH] - [3 H8 ] )

Furthermore, the total organics must not exceed 1 percent of the

propane , or

[TO] < 1.01 [C3 H8 ]

Finally, the value for total organics must not differ between the

first and second analysis in excess of what would be calculated from

the uncertainty of the two analyses

In addition to the limit for the concentration of organic impurities , the

water vapor content must not exceed 5 ppm by mole.

g. Reporting Results

A complete report concerning the concentration of impurities will be

required in addition to the information concerning the concentration of propane

and the stability of the propane concentration. The report on impurities

should summarize the results obtained in steps 2 through 6 of the previous

section on impurities.

52
h. Period of Certification

Experience at NIST indicates that propane mixtures contained in aluminum

cylinders are stable for periods of more than two years. If no change in

concentration of the CRM's is indicated by the results of the producer's first

and second analysis, then stability for a period of two years from the date of

the second analysis is assured with a high degree of probability.

i

Recertification (see appendix G for further details)

CRM's of propane in air or in nitrogen that have exceeded the period of

certification may be recertified by the original supplier according to the

following provisions.

1. The pressure remaining in the cylinder must be greater than 500 psi

(3.4 MPa).

2. The ratio of the signal generated by the CRM when compared to the

original internal standard must be the same as when originally

measured within the limits of uncertainty generated by both sets of

measurements

3. The concentration of the internal standard, determined by comparison

the SRM with which it was originally compared, must be the same within

the limits of precision of the comparison.

4. The concentration of the SRM must be known either by comparison with a new SRM or the original SRM must have been recently recertified by

NIST.

5. If either the original SRM or the internal standard no longer exists,

then no recertification can be made.

6. The uncertainty to be assigned to the recertified value must include

the added uncertainty, if any, of the recertification process.

53
APPENDIX G

Extension of Certification (Recertification)

and Date-of- Sale Certification

a. Introduction

In Appendices C through F and Appendices H and I of this document , the

period of certification for a Certified Reference Material (CRM) has been

limited to two years from the date of the second analysis of its CRM lot by its

producer. This limitation is necessary in the absence of confirmatory

measurements proving long-term stability (>2 years). Short-term stability (up

to 2 years) of CRM's must be established before they can be certified, but

long-term stability can only be proven by periodic reanalyses after

certification. Guidelines for establishing short-term stability are given in

the main body of this document (see section 3.3.1). Changes which might not

affect the certified concentration over a two-year period may become evident

after more than two years

.

The purpose of this appendix is to provide CRM producers with procedures

for extending the certification of CRM samples that either are unsold or are still in use at the end of the original period of certification.

Two approaches may be taken to achieve this: (1) the first approach is to

extend the certification of all unsold (or sold samples, if returned to the CRM

producer for recertification) in a batch for a specified period following

reanalysis of these samples (see section b, below); and (2) the second approach

is to certify individual samples in a lot for a specified period, beginning at

the time of sale of each sample to a purchaser (see "date-of-sale" certification; section c, below). The procedures to be followed to demonstrate longterm stability are slightly different in each of these two approaches. Simply

stated, date-of-sale certification (section c) requires that the CRM producer

retain the samples for a longer period of time and perform a third analysis,

six months after preparation, before submitting data for approval of the lot as CRM's, and, subsequently, perform additional reanalyses at 18 and 30 months on samples of the lot remaining unsold.

This appendix also contains a procedure for converting existing lots of

CRM's (that had originally been limited to two years from the date of the

second analysis of the lot by the CRM producer) to date-of-sale certification

(see section d, below).

b . Procedures to Extend the Certification of a CRM

The certification period for a CRM may be extended if it can be

demonstrated that the concentration, on reanalysis, has not changed from that

of its initial certification. The period of recertification will begin at the

time of completion of the reanalysis and will continue for an additional period

of time not to exceed two years. A CRM may be recertified more than one time,

as long as the concentration, on reanalysis, has not changed from that of its

54
initial certification. This section is written to describe the first

recertification of a CRM, which represents its third analysis (the first two

analyses being performed during the original certification of the CRM). Hence,

in the discussion given below, for subsequent recertifications of the same CRM

sample, the new recertification analysis is compared to the most recent

previous recertification analysis. The recertified CRM may be one or more of

the samples remaining unsold in a particular lot, or may be a sample that has

been returned by a purchaser for recertification.

The analyses performed during recertification must be sufficient to show

that the concentration of the sample has not changed within the limits of the

combined error of the original two analyses and the recertification analysis.

For CRM samples that are recertified more than once, the analyses performed

during any subsequent recertification must be sufficient to show that the

concentration of the sample has not changed within the limits of the combined

error of the original two analyses and all subsequent recertification analyses.

Any recertification analysis is performed in an identical manner to the

second analysis of the original certification (see section 3.3 in the main body

of this document), except that a smaller number of samples may be involved.

The statistical test to demonstrate that the concentration has not changed

between the second and the third analyses is the same as that which was used to

show that no change had occurred between the first and the second analyses

The third analysis must be performed with reference standards that are traceable to the same source as was used for the first and second analyses.

These standards must include one of the following combinations:

1. The original SRM and the original internal standard of the lot;

2. A new SRM and the original internal standard of the lot; or

3. The original SRM and a new internal standard of the lot.

Any SRM used in the recertification of a CRM must either be within the original

period of its certification by NIST or must have been recertified by NIST and

still be within the period of its recertification.

The linearity and precision attained with the instrument used for the

third analysis must be similar to that attained with the instrument used in the

first and second analyses. The linearity in the concentration region between

the CRM and the SRM must be confirmed by the method outlined in section 2.3 of

the main body of this document and as illustrated in section II of appendix A.

The imprecision of the measurement method is evaluated by ten (10) replicate

measurements of the internal standard of the lot (e.g., see section II in

appendix A) . The value of the imprecision thus obtained should be the same as or smaller than that obtained during the first and second analyses

The analyst should be aware of the effect of precision on the decision

concerning the stability of the samples being analyzed. Ultimately, a decision

must be made as to whether an observed difference between the second and third

analyses is real, or, if no difference is observed, whether the third analysis

55
was sufficiently precise to reveal a difference. Thus, if the imprecision of

the third- analysis is the same as or smaller than that of the first or second

analyses, it will be possible to recognize small real differences in

concentration that have occurred during the interval between the second and

third analyses. However, if the imprecision of the third analysis is large

(i.e., two or more times that of the second analysis), then larger differences

in sample concentration values would need to be observed to conclude that a group of samples was unstable (i.e., it would not be possible to state with

confidence that samples in a lot are stable and, hence, acceptable for

recertification, if the imprecision of the third analysis is large)

The analyst should not proceed with the analysis of the samples in the lot

if the imprecision of the analysis (evaluated by replicate measurements of the

internal standard, as stated above) or if the concentration of the internal

standard is questionable. The concentration of the internal standard should be

the same as measured during the second analysis. This can be determined by

observation if agreement is close and the imprecision is small, or, more rigorously, by application of a simple "t" test of the form:

t = (C2- C3 )/S

p (l/n2 + l/n3 )°

5

where (C2 -C3 ) is the difference between the average concentration of the

internal standard for the second and third analyses, the pooled standard

deviation^ S

p is the average of the standard deviations for the second and

third analyses of the internal standard, and the value for n (the number of

measurements) is ten (10) for both of these analyses. If the value of t lies

in the interval from +2.2 to -2.2., there is no significant difference between

the concentrations measured during the second and third analyses. Practically

speaking, if the difference between the two averages is equal to or less than

the larger of the two standard deviations, then it can be confidently assumed

that no change has occurred in the concentration of the internal standard.

Once it has been shown that the concentration of the internal standard has

not changed and that the imprecision and linearity are within acceptable

limits, then analysis of the samples to be recertified should be performed.

This analysis is intended to reveal whether or not a change in the concentration of any sample has occurred during the time internal following the second

analysis. Because this time interval is much greater than that between the

first and second analysis, it will be easier to recognize any samples in which

slow changes in concentration have been occurring. Individual samples that

have changed significantly should be evident by visual inspection of the

results. An entire batch that has changed in concentration will be evidenced

by a significant change in the average concentration of all samples, or by the

sign of the difference between the second and third analyzed concentration

values being the same for all samples. However, in some cases, the change may

be small and may not be easily recognized. In these cases, the paired results

of the second and third analyses are treated in the same manner as the paired

results of the first and second analyses (see appendix A, section II, part 3).

There are three situations which might result from a comparison of the

second and third analyses. These are:

56
1 . An obvious and unquestionable change in the concentration of some or all samples in the batch has occurred.

2. No significant difference is found between the second and third

analyses

3. A small but real change in concentration is revealed on application of

statistical tests for differences.

In the first situation, an obvious problem exists which requires further

action, probably of a drastic nature. In the second case, all is well and the

certification period may be safely extended. In the third case, we are faced

with deciding whether or not a difference is significant. That is, will a statistically significant difference translate into an analytically significant

difference, considering the total uncertainty of the certified concentration

and the length of the period of certification. For example, suppose that the

second and third analyses have been executed properly and that the imprecision

of the measurement of the difference between the two sets is less than 0.1

percent relative. In principle, it should be possible to detect real differences of 0.1 percent relative between the second and third analyses. Because

of the nature of the changes which take place in compressed gases contained in

cylinders, a change of 0.1 percent relative over one year can be extrapolated

to a change of no more than 0.3 percent relative over three years. Therefore,

a 0.1 percent relative change will not compromise the analytical utility of the

sample, and may be safely ignored. Changes greater than 0.1 percent relative

per year, however, may or may not be acceptable with respect to extending the

certification period. A final decision may have to be based on factors in

addition to the results of the third analysis. Factors which might be con- sidered are: (i) the results of the first and second analyses as compared to

the thirds (ii) the relative contributions of random errors and systematic

errors to the total error of the CRM; and/or (iii) the sign and nature of the

observed change.

Evaluation of the Data

The producer must submit to the NIST the results of the analysis of the

internal standard, the imprecision and linearity data, and the individual

results obtained for each sample for which extension of the period of cer- tification is sought. NIST will examine these data and will decide whether or not the results justify an extension of the period of certification. The

producer will be notified of NIST's decision in writing. If the decision is

negative, the producer will be supplied with the reason and a summary of all

statistical treatments of the data. In either regard, EPA will be notified of

the decision in writing and will be informed of any further action to be taken

either by the producer or by NIST.

57
Consequence of the Third Analysis

If the evaluation of the data of the third analysis concludes that the

batch is stable or that observed changes will not become analytically significant during the period of the recertification, then all samples analyzed as part of the third analysis may be certified for two years or more. If changes

have occurred that can be extrapolated to become significant changes by the end

of the requested period of recertification, the samples cannot be recertified

and will remain in certification only through the original period of certification. If the analysis reveals changes that are substantial and are considered

significant even during the original period of certification, then all samples

still within the certification must be recalled from the purchasers. However,

before a "recall" decision is made, NIST and the CRM producer will make every

effort to determine whether or not the change is real or if it is the result of an analytical error committed at the time of the second or third analyses

Since a recall could occur, a CRM producer should maintain records of the names and addresses of all purchasers of CRM's.

c . Analytical Procedures for Date-of-Sale Certification

This section describes procedures that allow a CRM producer to certify an individual CRM for a two-year period starting from the date of its sale to a customer. These procedures differ slightly from those for regular CRM cer- tification. The principal difference is that all samples in the lot must be

analyzed three times rather than twice before approval as a date -of- sale

certified CRM. An initial analysis of the lot should be performed as soon as practical after its preparation, followed by reanalyses at 3 and 6 months after

its preparation. After the third analysis, the CRM producer should submit a comprehensive report of the three analyses to NIST for review and approval.

The CRM producer should at this time also send two samples, randomly selected

by EPA from the lot, for an EPA Performance Audit. If the review of the

producer's and auditor's analytical results by NIST indicates that the batch

has satisfied the requirements of a CRM, NIST will notify EPA and the producer

by letter that the lot may be certified for sale. If reservations concerning

the stability or homogeneity of the lot arise as a result of the review of the

analytical results by NIST, an additional analysis may be required or,

alternately, the lot may be rejected.

The period of certification for each CRM sample in an approved lot will be

two years from the date of sale to a customer for those sales that occur within

two years after preparation of the lot. Individual CRM's may continue to be

sold after this two-year period with a two-year certification from date of sale

if additional analyses are performed at 18 and at 30 months. For these

analyses, only 20 percent of the lot (or at least 4 cylinders) need to be

reanalyzed. The results of the reanalysis should be submitted to NIST for

review.

During each analysis of the lot, ten (10) replicate measurements must be

performed on an internal standard from the lot. Each analysis must be performed with reference standards, all of which are traceable to the same source. This traceability can be established with:

58
1. The original SRM and the original internal standard of the lot;

2. A new SRM and the original internal standard of the lot; or

3. The original SRM and a new internal standard of the lot.

Any SRM used as a reference standard must be within the period of original

certification or within the period of some subsequent recertification by NIST.

It should again be emphasized that the analyst should never proceed with the

analysis of the lot if the imprecision of the analysis or the concentration of

the internal standard is questionable.

The CRM producer's report to NIST should contain information concerning

the imprecision and linearity of the analytical method, the stability of the

internal standard, and the stability and homogeneity of the lot. The

statistical tests to determine the stability and homogeneity of a lot before

its certification are described in appendix A of this document. After

certification, the stability of the reanalyzed cylinders should be determined

as described in section B of this appendix (appendix G)

The CRM producer shall provide a "Certificate of Analysis" and a copy of

the certification authorization letter from NIST to EPA, to any purchaser of a CRM that is certified from date of sale. The certificate shall include, as a minimum, the sample number, the cylinder number, the certified concentration of

the CRM, the uncertainty assigned to the certified concentration, the date of

sale, and the period of certification. The certificate also must identify the

SRM used by the producer to establish the traceability of the CRM.

d. Converting from Fixed Period Certification to Date-of-Sale Certification

It should be apparent from the preceding narratives in this appendix that

there is little difference between the analytical requirements for a fixed

period certification (or extension of a certification through recertification)

and a date-of-sale certification. In fact, there is no reason why a lot

originally certified for a fixed period cannot be converted to date-of-sale

certification if the additional necessary analyses are performed and the

resulting data are made available for NIST review. These data must support the

conclusion that all samples in the lot are stable and can be expected to remain

stable for at least 4 years if no changes are observed during the first six

months. If the initial six-month observation period was extended to two years,

for instance, and no changes were observed, then the assumption that the lot

was stable and could subsequently be certified from date of sale would be

equally valid.

Thus, unsold samples remaining from a lot originally certified for a twoyear certification could be certified for subsequent date-of-sale certification

if all samples are analyzed and found to be stable (as specified in section C,

preceding) . The results of this analysis would be treated in the same manner

as the third analysis of the date-of-sale certification described in section C

of this appendix. Samples from the lots old before this analysis could not be

59
retroactively recertified unless they are reanalyzed at the same time as the

remainder of the lot.

A CRM producer who wishes to convert from fixed period certification to

date-of-sale certification should proceed as follows:

1. Analyze all remaining samples in the lot, being certain that either the

original analytical standards (SRM or internal standard) or acceptable

replacements are available.

2. Forward all data to NIST, together with a letter requesting conversion

of the remaining samples in the lot to date-of-sale certification.

NIST will examine the data for the reanalysis as well as for any previous

analyses, and will advise both the CRM producer and EPA as to whether or not

the request should be granted. If the data indicate that the request is

justified, NIST will inform the CRM producer and EPA by letter of its approval.

If deemed necessary, NIST may require either an additional analysis by the

producer, or that another pair of samples be submitted for an EPA Performance

Audit

.

60
APPENDIX H

Oxygen in Nitrogen

a. Introduction

This appendix describes the preparation of Certified Reference Materials

(CRM's) consisting of oxygen in nitrogen. The reader should be familiar with

the general description of the preparation and analysis of CRM's given in the

document, and the material in Appendix G for extending the certification of

CRM's and for date-of-sale certification of CRM's.

b. Current SRM's

Following is a list of SRM's of oxygen in nitrogen for which CRM's may be

made. The concentrations shown are in mole percent. The actual concentration

of a particular SRM may differ by more than several percent relative from the

nominal value and it is therefore essential that the exact concentration of the

SRM to be duplicated be known before attempting the preparation of a CRM.

SRM No. Nominal Concentration

2651 5.0 mole percent*

2652 10.0 mole percent*

2657 2 mole percent

2658 10 mole percent

2659 21 mole percent

c

Cylinders

The CRM's must be packaged in new aluminum cylinders equipped with valves

of appropriate material that conform to CGA recommendations for these particular mixtures; alternatively, aluminum cylinders previously used for oxygen

in nitrogen CRM's or which have been used only for oxygen in nitrogen mixtures

may be used for these CRM's, provided that the history of a reused cylinder is

well known, and, in case of a CRM cylinder, an analysis must be performed of

the remaining gases to assure that the contents have not been diluted or con- taminated. Note: The concentration of the remaining gas mixture must lie

within plus or minus one percent relative of the concentration of the original

CRM, as certified. In the case of reused cylinders, the cylinder must be

evacuated before refilling, and the concentration of the new CRM gas mixture

must lie within one order of magnitude (within 0.1 to 10 times) of the

concentration of the previous CRM or other oxygen mixtures previously

contained.

*SRM Nos. 2651 and 2652 also contain 0.01 mole percent propane.

61
d. Preparation and Analysis

Prepare a lot of at least 10 cylinders all at identical concentration.

The concentration of oxygen must lie within ±1 percent relative of the con- centration of the specific SRM with which the lot is to be compared. Analyze

at least 10 samples from the lot, if the lot contains more than 20 samples. If

the lot containing fewer than 20 samples, it is recommended that all samples be

analyzed. Analyze the samples by direct comparison to the SRM or by comparing

samples in the lot to a single sample selected at random from the lot (internal

standard from the lot). Analyze each sample at least two times. Analyze for

impurities as required in Section F.

After the first analysis, the lot should be put aside to "incubate" for at

least one month after which time all samples in the lot are analyzed and at

which time the impurities are redetermined. If the two sets of analyses are the same, then it may be assumed that the lot is homogeneous and stable. If

the analyses were performed by direct comparison to the SRM, then the analytical value for the sample will be the certified value. If the analyses were performed by comparison to an internal standard for the lot, then it will be

necessary to determine the concentration of the internal standard. This is

done by repeated comparison of the internal standard to the SRM. The con- centration of the internal standard must be determined at the time of the first

and second analysis by comparing it to the appropriate SRMs . The measured

concentration of the internal standard should be the same for both analyses

within the limits of error of the analyses. The concentration measured for the

internal standard is multiplied by measured ratio of each sample to the

internal standard to obtain the concentration of each sample.

The uncertainty of the CRM includes the error of the SRM and the

uncertainty due to the imprecision of the intercomparison. The error of the

SRM, Esrm , is given on the certificate for the particular cylinder and is

defined as the estimated upper limit of the total uncertainty. The estimated

upper limit of the uncertainty for the CRM should be calculated as follows:

E2 (Total Error) = 2

r v^SRM

N . 2 ,

+ a

2 + b2 + c2

where E2 is the estimated upper limit of the uncertainty of the CRM; a is the

imprecision of the intercomparison of the internal standard with the lot; b is

the imprecision of the intercomparison of the internal standard and the SRM;

and c is the error of the calibration.

e . Analytical Method

Analysis of these CRM oxygen mixtures may be done by gas chromatography,

paramagnetic analyzers, or any other method specified for oxygen determination.

The method chosen should have adequate sensitivity and precision, and the

nature of the instrument response must be known for the region between the SRM

62
and the CRM. The precision and sensitivity are evaluated during analysis and

are reflected directly in the "imprecision of intercomparion. " The instrument

response characteristic is evaluated over the small range between the SRM and

CRM by observing the response over a wide range using SRMs as calibration

standards

f . Impurities

The major impurity in SRMs of oxygen in nitrogen is argon. The oxygen

content of an SRM is certified while the concentration of argon is given for

information only. If a method of analysis is used that is specific for oxygen

(paramagnetic analysis for example) then the presence of argon will not affect

the measurement of oxygen. However, if a non-specific method is used, then it

is essential that the concentration of argon be known.

There are several sources of argon contamination in SRMs or CRM's. First,

contamination may be present in oxygen or the diluent nitrogen. In addition,

atmospheric leakage through cylinder valves or transfer system connections and

valves can contribute to argon contamination. The atmospheric concentration of

argon is approximately 0.93 mole percent. The argon in the oxygen, nitrogen,

and CRM's can be measured by gas chromatography. The argon concentration in

the CRM should be consistent with the impurities in the reagent oxygen and the

diluent nitrogen that were analyzed prior to the compounding of the CRM's. If

the concentration of argon in the CRM is consistent with the pure gases, it may

be assumed that the sample cylinders and the transfer systems were adequate.

The maximum concentration of impurities shall not exceed the following:

Hydrocarbons as methane 3 ppm

Water vapor 5 ppm

Total other impurities except argon 15 ppm

Argon in 2% 02 /N2 30 ppm

Argon in 5% 02 /N2 40 ppm

Argon in 10% 02 /N2 50 ppm

Argon in 21% 02 /N2 100 ppm

g. Period of Certification

Experience at NIST indicates that oxygen mixtures contained in aluminum

cylinders are stable for periods of more than two years. If no change in

concentration of the CRM's is indicated by the results of the producer's first

and second analysis, then stability for a period of two years from the date of

the second analysis is assured with a high degree of probability.

63
h. Recertification (see Appendix G for further details)

CRM's of oxygen in nitrogen that have exceeded the period of certification

may be recertified by the original supplier according to the following

provisions

.

1. The pressure remaining in the cylinder must be greater than 500 psi

(3.4 MPa).

2. The ratio of the signal generated by the CRM when compared to the

original internal standard must be the same as when originally measured

within the limits of uncertainty generated by both sets of

measurements

.

3. The concentration of the internal standard, determined by comparison to

the SRM with which it was originally compared, must be the same within

the limits of precision of the comparison.

4. The concentration of the original SRM must be known either by

comparison with a new SRM, or the original SRM must have been recently

recertified by NIST.

5. If either the original SRM or the internal standard no longer exists,

then recertification can only be made using a new SRM.

6. The uncertainty to be assigned to the recertified value must include

the added uncertainty, if any, of the recertification process.

64
APPENDIX I

Carbon Dioxide in Nitrogen

a. Introduction

This appendix describes the preparation of Certified Reference Materials

(CRM's) consisting of carbon dioxide in nitrogen. The reader should be

familiar with the general description of the preparation and analysis of CRM's

given in the main text of this document and the material in Appendix G which

provides procedures for extending the certification of CRM's and for date- of

sale certification of CRM's.

b . Current SRM's

Following is a list of SRM's of carbon dioxide in nitrogen for which CRM's

may be made. The concentrations shown are in mole percent. The actual con- centration of a particular SRM may differ by more than several percent relative

from the nominal value and it is therefore essential that the exact concentration of the SRM to be duplicated be known before attempting the preparation of

a CRM. The eight SRM's covering 2619a through 2626a listed below were

originally developed for use in calibrating combustion efficiency measurements;

the remaining originally developed for use in calibrating mobile source

emission measurements.

SRM Number Nominal Concentration of C02

1674b

1675b

2619a

2620a

2621a

2622a

2623a

2624a

2625a

2626a

2632

2633

2634

7 .0 mole

14 .0 mole

0 .5 mole

1 .0 mole

1 .5 mole

2 .0 mole

2 .5 mole

3 .0 mole

3 .5 mole

4 .0 mole

300 ppm

400 ppm

800 ppm

percent

percent

percent

percent

percent

percent

percent

percent

percent

percent

The preparer of carbon dioxide in nitrogen CRM's should note that the C02 concentrations in the eight combustion efficiency SRM's (2619a through 2626a) are certified to within 0.1 percent relative at the 95 percent confidence

level; the C02 concentrations in the remaining five SRM's are certified to

within ±1 percent relative at the 95 percent confidence level.

65
c . Cylinders

The CRM's must be packaged in new aluminum cylinders equipped with valves

of appropriate material that conform to CGA recommendations for these

particular mixtures; alternatively, aluminum cylinders previously used for

carbon dioxide in nitrogen CRM's or that have been used only for carbon dioxide

in nitrogen mixtures may be used for these CRM's, provided that the history of

a reused cylinder is well known, and, in case of a CRM cylinder, an analysis

must be performed of the remaining gases to assure that the contents have not

been diluted or contaminated. Note: The concentration of the remaining gas

mixture must lie within plus or minus one percent relative of the concentration

of the original CRM, as certified. In the case of reused cylinders, the

cylinder must be evacuated before refilling, and the concentration of the new CRM gas mixture must lie within one order of magnitude (within 0.1 to 10 times)

of the concentration of the previous CRM or other carbon dioxide in nitrogen

mixtures previously contained.

d. Preparation and Analysis

Prepare a lot of at least 10 cylinders all at identical concentration.

The concentration of carbon dioxide must lie within ±1 percent relative of the

concentration of the specific SRM with which the lot is to be compared.

Analyze at least 10 samples from the lot, if the lot contains more than 20

samples. If the lot contains fewer than 20 samples, it is recommended that all

samples be analyzed. Analyze the samples by direct comparison to the SRM or by

comparing samples in the lot to a single sample selected at random from the lot

(internal standard from the lot). Analyze each sample at least two times.

Analyze for impurities as required in Section F.

After the first analysis, the lot should be put aside to "incubate" for at

least one month after which time all samples in the lot are analyzed and at

which time the impurities are redetermined. If the two sets of analyses are

the same, then it may be assumed that the lot is homogeneous and stable. If

the analysis was performed by direct comparison to the SRM, then the analytical

value for the sample will be the certified value. If the analyses were

performed by comparison to an internal standard for the lot, then it will be

necessary to determine the concentration of the internal standard. This is

done by repeated comparison of the internal standard to the SRM. The con- centration of the internal standard must be determined at the time of the first

and second analysis by comparing it to the appropriate SRM's. The measured

concentration of the internal standard should be the same for both analyses

within the limits of error of the analyses. The concentration measured for the

internal standard is multiplied by measured ratio of each sample to the

internal standard to obtain the concentration of each sample.

The uncertainty of the CRM includes the error of the SRM and the

uncertainty due to the imprecision of the intercomparison. The error of the

SRM, Esrm , is given on the certificate for the particular cylinder and is

defined as the estimated upper limit of the total uncertainty. The estimated

upper limit of the uncertainty for the CRM should be calculated as follows:

66
E2 (Total Error) = 2

2 + a

2 + b

2 + c2 ^SRM

4 2

where E2 is the estimated upper limit of the uncertainty of the CRM; a is the

imprecision of the intercomparison of the internal standard with the lot; b is

the imprecision of the intercomparison of the internal standard with the SRM;

and c is the error of the calibration.

e

Analytical Method

Analysis of these CRM carbon dioxide mixtures may be done by gas

chromatography, paramagnetic analyzers, or any other methods specified for

carbon dioxide determination. Analysis of SRM's 2632-2634 also employed a non-dispersive infrared technique. The method chosen should have adequate

sensitivity and precision, and the nature of the instrument response must be

known for. the region between the SRM and the CRM. The precision and sen- sitivity are evaluated during analysis and are reflected directly in the

"imprecision of intercomparison." The instrument response characteristic is

evaluated over the small range between the SRM and CRM by observing the

response over a wide range using SRM's as calibration standards.

f

Impurities

The maximum concentration of impurities shall not exceed the following:

Carbon monoxide 0.1 ppm

Nitrous oxide 0.5 ppm

Hydrocarbons as methane 1 ppm

Water vapor 5 ppm

Total other impurities 10 ppm

g.

Period of Certification

Experience at NIST indicates that carbon dioxide in nitrogen mixtures

contained in aluminum cylinders are stable for periods of at least two years

If no change in concentration of the CRM's is indicated by the results of the

producer's first and second analysis, then stability for a period of two years

from the date of the second analysis is assured with a high degree of probability. Any samples not sold within two years of the second analysis should be

recertified (see section h. , below) and the period of certification should be

appropriately extended.

67
h. Recertification (see Appendix G for further details)

CRM's of carbon dioxide in nitrogen that have exceeded the period of

certification may be recertified by the original supplier according to the

following provisions.

1. The pressure remaining in the cylinder must be greater than 500 psi

(3.4 MPa)

2. The ratio of the signal generated by the CRM when compared to the

original internal standard must be the same as when originally measured

within the limits of uncertainty generated by both sets of measurements .

3. The concentration of the internal standard, determined by comparison to

the SRM with which it was originally compared, must be the same within

the limits of precision of the comparison.

4. The concentration of the original SRM must be known either by

comparison with a new SRM, or the original SRM must have been recently

recertified by NIST.

5. If either the original SRM or the internal standard no longer exists,

then recertification can only be made using a new SRM.

6. The uncertainty to be assigned to the recertified value must include

the added uncertainty, if any, of the recertification process.

68
NIST-1 14A U.S. DEPARTMENT OF COMMERCE

(REV. 3-89) NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

1 . PUBLICATION OR REPORT NUMBER

EPA-600/7-81-010

BIBLIOGRAPHIC DATA SHEET

2. PERFORMING ORGANIZATION REPORT NUMBER

NBSIR 81-2227

3. PUBLICATION DATE MARCH 1990

4. TITLE AND SUBTITLE A Procedure for Establishing Traceability of Gas Mixtures to Certain National Institute

of Standards and Technology Standard Reference Materials. (Revised November 1989)

5. AUTHOR(S)

Ernest Hughes and John Mandel, NIST

6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

7. CONTRACT/GRANT NUMBER

ha i lunML ino iiiuic wr o i MnuMnuo mhu i cunnuLuu t GAITHERSBURG, MD 20899 8. TYPE OF REPORT AND PERIOD COVERED

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP) Jointly sponsored by:

National Institute of Standards and Technology, Gaithersburg,

U.S. Environmental Protection Agency, AREAL, RTP, NC

MD 20899

10. SUPPLEMENTARY NOTES

DOCUMENT DESCRIBES A COMPUTER PROGRAM; SF-1B5, FIPS SOFTWARE SUMMARY, IS ATTACHED.

11. ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBLIOGRAPHY OR

LITERATURE SURVEY, MENTION IT HERE.)

This procedure includes the specifications and requirements that must be followed by gas

during the preparation of compressed cylinder gas Certified Reference Materials (CRM). A

CRM is a certified gas standard prepared at a concentration that does not exceed ± 1 percent

of currently available NIST Standard Reference Material (SRM) cylinder gases. The procedure

includes specifications and requirements for: (1) preparation of compressed gas samples in

cylinders prepared in lots of ten or more of identical concentration with the average concentration for the lot within ± 1.0 percent relative to the concentration of a specific SRM;

(2) tests to verify compressed gas sample stability and within lot homogeneity; (3) simultaneous submission by the gas manufacturer of analysis results to NIST and cylinder gas

numbers to USEPA (without analysis results); (4) random selection by USEPA of two cylinders

per lot for a USEPA performance audit analysis; (5) submission by USEPA of audit results

to NIST; and (6) an evaluation by NIST to determine whether the lot satisfies the criteria

for a NIST-traceable CRM. Individual CRM procedures are described for CO in N2 or air

(Appendix C) , NO in N2 (Appendix D) , SO2 in N2 (Appendix E) , propane in N2 or air (Appendix F)

O2 in N2 (Appendix H) , and CO2 in N2 (Appendix I) . Procedures for CRM recertifications and

for date-of-sale certification are given in Appendix G.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITALIZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

Traceability, Certified Gas Standards, National Standards, Nitric Oxide, Sulfur Dioxide,

Propane, Carbon Dioxide, Carbon Monoxide, Oxygen, Quality Assurance, Air Pollution,

Environmental Measurements, Data Quality

13. AVAILABILITY 14. NUMBER OF PRINTED PAGES

X UNLIMITED 75

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS). ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE, WASHINGTON, DC 20402.

IS. PRICE A04

_X- ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD, VA 22161. ELECTRONIC FORM


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