, NBS

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NArL INST OF STANDARDS & TECH R.I.C

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nrinS°M«c^,l^.".^^''<* reference material QC100 .U5S6 V148;1975 C.I NBS-PUB-C 1975

NBS MONOGRAPH

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

The Role of

Standard Reference Materials

in Measurement Systems
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901.

The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade,

and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry,

and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Nuclear

Sciences - — Applied Radiation - — Quantum Electronics ^ — Electromagnetics ' — Time

and Frequency " — Laboratory Astrophysics — Cryogenics

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; provides

advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard

Reference Materials and the following divisions:

Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and

Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice arid services to Government agencies upon request. The Institute consists of a Center for Building Technology and the following divisions and offices: Engineering and Product Standards — Weights and Measures — Invention and Innovation — Product Evaluation Technology — Electronic Technology — Technical Analysis — Measurement Engineering — Structures, Materials, and Life Safety* — Building

Environment * — Technical Evaluation and Application * — Fire Technology.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research

and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing

equipment, techniques, and computer languages. The Institute consists of the following

divisions : * Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal

Government; promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National

Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The Office consists of the following

organizational units:

Office of Standard Reference Data — Office of Information Activities — Office of Technical

Publications — Library — Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg,

Washington, D.C. 20234.

2 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

* Part of the Center for Building Technology.

Maryland, unless otherwise noted; mailing address
I 1977

\ CO. '

•S ^ THE ROLE OF STANDARD REFERENCE

^"^^ MATERIALS IN MEASUREMENT SYSTEMS

\^

J. P. Cali, T. W. Mears, R. E. Michaelis, W. P. Reed,

R. W. Seward, C. L. Stanley, and H. T. Yolken

Office of Standard Reference Materials

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

and

H. H. Ku

Applied Mathematics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

Ij^C, NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued January 1975
Library of Congress Catalog Card Number: 74-600196

National Bureau of Standards Monograph 148

Nat. Bur. Stand. (U.S.), Monogr. 148, 54 pages (Jan. 1975)

CODEN: NBSMA6

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1975

For sale by the Superintendent of Uocuments, U.S. Government Printing Office, Washington, D.C. 20402

(Order by SD Catalog No. C13.44:148). Price $1.15
Preface

This publication has been prepared as an aid to all users of Standard Reference Materials,

and should be especially helpful to users in countries that are developing national measurement

systems. Standard Reference Materials (SRM's) represent one aspect of the total measurement system needed to produce adequate measurements for science, technology, and industry. This publication attempts to describe the role SRM's play in the total measurement system, but is not

intended to be an exhaustive description of the NBS-SRM program.

The information contained in this publication must be viewed within the context of the in- dividual nation's measurement needs and priorities, which of course vary from country to country.

Further, this information should be useful not only to new industries, but also to mature industries

seeking to improve their quahty control procedures.

This pubhcation covers the role of SRM's in a measurement system, both general and specific uses of SRM's, and selected fields in which SRM's have made significant contributions.

The desirabihty of a general publication on the use of Standard Reference Materials has long

been recognized. However, the initial impetus to undertake such a task came from the U.S. Agency

for International Development (U.S.A.I.D.) as part of their program to provide technical assistance

to developing countries. U.S.A.I.D. also furnished partial financial support for this endeavor.

For further information on Standard Reference Materials, write to the Office of Standard

Reference Materials, National Bureau of Standards, Washington, D.C. 20234.

iii

Contents

Page Preface iii

I. Introduction 1 A. The Measurement Process 1 B. Compatibility in Measurement 2

C. Meaningful Measurement 2

D. A Systems Approach to Meaningful Measurement 3

E. Economic and Social Costs of Making "Bad" Measurements 7

References 8

II. Considerations for the Use of SRM's 9

A. Proper SRM Selection 9

B. Potential Pitfalls in Standards Usage 10

C. Special Handling and Storage 11

D. Sources of Information on NBS-SRM's 11

References 12

III. Certified Values and Their Meaning— Uncertainties 13

A. Material Variability 13

B. Measurement Errors 14

C. Instrument and Operator Errors 16

D. Differences Among Measurement Methods 16

E. Uncertainties of Certified Values 17

F. Statements of Uncertainty of SRM Certificates 17

References 18

IV. Specific Areas of Use for SRM's 19

A. Metals Industries 19

References 22

B. Rubber Industry 22

References 23

C. Nuclear Measurement System 23

References 24

D. Inorganic Standards 24

Sources of Information 25

E. The Clinical Measurement System 26

References 31

Appendices 34

V

Standard Reference Materials: The Role of SRM's in Measurement Systems

J. P. Cali, T. W. Mears, R. E. Michaelis, W. P. Reed,

R. W. Seward, C. L. Stanley, and H. T. Yolken

Office of Standard Reference Materials

Institute for Materials Research, National Bureau of Standards,

Washington, D.C. 20234

H. H. Ku

Applied Mathematics Division

Institute for Basic Standards, National Bureau of Standards,

Washington, D.C. 20234

This publication is a guide to the use of Standard Reference Materials (SRM's) and should be

useful to all users of SRM's particularly those in countries developing national measurement systems.

It is not intended to be an exhaustive description of the NBS-SRM program, but rather a review of the role SRM's play in the measurement system, how SRM's are certified, and what the certification means. To illustrate the use of SRM's, several selected industries are described in which SRM's have

made significant contributions.

Key words: Certification; meaningful measurement; measurement; measurement system; precision;

reference method; specificity; SRM; standard; Standard Reference Material; systematic error.

I. INTRODUCTION

Standard Reference Materials represent one facet

of the national measurement system in the United

States. Before describing the role they play, the

meaning of measurement must first be discussed.

Most measurements are made to communicate

information on properties of material things in a purposeful way to accomplish useful goals. Communications can be effective or ineffective. In

parallel, measurements can be meaningful or not

meaningful. Meaningful measurements allow us

to make decisions on a solid, objective basis. For

example, a transaction involving 10 kilograms of sucrose of 99 percent purity is likely to cause little controversy between buyer and seller, because

analytical instruments and scales are available to determine 99 percent purity and 10 kilograms to within some specified degree of tolerance. On

the other hand, measurements of fragrance and

taste on a scale of desirability are stiU highly

subjective, and decisions based on results of

"experts" are frequently subject to controversy.

The principle difference in the two situations is that the first involves numbers associated with

distinct, well-defined properties, while the second

does not. To see what constitutes meaningful

measurements, and how they may be propagated

throughout the industrial and technological communities, the measurement process must be

examined.

A. The Measurement Process

The Measurement in science and technology is that process whereby a numerical value is associated with a distinct, specific, and unique property

of a material. The magnitude of the number is related to the amount or degree of that property in a particular material or similar class of materials.

The word "material" is taken in its broadest sense

to include all those things considered to constitute

the physical objects of the observable universe.

In 1939, Shewhart [1] ^ pointed out two aspects

of the measurement process that he described as quantitative and qualitative. The former aspect concerns numbers associated with a scale, pointer

reading, counter, or the like. In much of this publication, this quantitative consideration wiU be asso- ciated with a well-characterized material, called a Standard Reference Material (SRM). In these

materials, one or more properties will have a number

of numbers assigned in a manner analogous to the

numbers associated with a meter stick, although in

many instances the SRM will have one, unique

value, rather than a series of incremental values.

Thus, while a meter stick may have numerous divisions of the meter along its length, a copper alloy

SRM will have just one number associated with its copper content. In any case, the SRM represents

the quantitative aspect of measurement, especially

useful where composition is the property under

' Figures in brackets indicate the literature references at the end of each chapter.

1
measurement. The qualitative aspects of measure- ment are included in what is often called the procedure, or the method. Included in this factor are such things as apparatus, reagents, indeed all those

things that are used or can affect the course of the

measurement. Obviously, the experimenter or measurer, the sequence of operations, control of

the ambient conditions, etc., must be stated in the

written method used by the operator to make the

actual measurements.

Experience over many centuries has taught man

that, if he can agree on one universal set of coherent

scales, he can more effectively communicate with

his fellow man across time and geographical boundaries. In priciple, there is no logical reason why

many different sets could not be utihzed, as historically they have, but the economic, political, and

social benefits of one universal set are so apparent,

that most of the world's nations have now agreed to use the set of measurement scales called the Inter- national System of Units (Systeme International

d'Unites) and abbreviated as SI. This rational, self- consistent system of units of measurement includes

the base units (mass, length, time, electric current,

thermodynamic temperature, luminous intensity,

and the mole), and the derived units (area, density,

energy, etc.), together with rules for their use [2, 3].

Having defined the units, access to the units is provided through highly refined measurement processes. In some cases one can reconstruct the unit,

in others one relies on artifacts such as sets of

weights, gauge blocks, and the like whose magnitudes in terms of the unit have been carefully

established — especially when the magnitudes met

in local measurement practice are far removed from

(i.e., are large multiples or small fractions of) the

base unit. The uncertainty in the use of such refer- ence standards is a function of both the method and

the process precision. The uncertainty of the

assigned value of the reference standards becomes

a systematic error of the process in which the arti- fact is used.

B. Compatibility in Measurement

If measurements between nations, between

industries , between buyer and seller, indeed between

any two or more parties are to be useful for some re- alizable purpose, the measurements must be compatible. Assume two different' laboratories measure

the same specific property on samples taken from the

same lot of a stable material. If the two independently

determined values agree, tlie two measurements,

and hence, the two laboratories are said to be

compatible. The critical question is, "Agree within what limits?" In practical measurement situations, these limits should be defined in terms of

the useful end requirements. Having estabhshed

the limits, one is concerned with verifying that

tEe results of the measurement are compatible

with the limits.

Dr. Robert D. Huntoon, former Director of the

NBS Institute for Basic Standards, discussed fully

his concept on the importance of compatibility

at the 6th Materials Research Symposium, Standard Reference Materials and Meaningful

Measurements [4].

C. Meaningful Measurement

By definition, a measurement system produces

the numerical value of the amount of a well-defined

property of a material.

The technique by which such a numerical

property of a material is obtained is called a meas- urement method. The use, or the reahzations, of

the method over time yields values produced by the

measurement process.

The practical measurement processes of in- dustry are varied and complex. In some instances,

such as the measurement of the disintegration of

nuclear particles using a radiation detector, the

process is essentially a counting operation and

therefore conceptually simple. However, in the

majority of situations, the process is far more

complex and consists of a sequence of operations,

each of which may be a process of some complexity.

At the end of this sequence of operations, a numerical result emerges together with an estimate of

uncertainty. In most cases this result can be ex- pressed in units according to an accepted system

of base and derived units — for science and technology, that system is the SI. While every one

could start with a fresh realization of the base

units, the accumulative systematic error as one moves through the process can be large. Such an

approach, in addition to being costly may not

produce results that are within the desired limits.

It is here that the role of SRM's emerges. The SRM

serves a role similar to that of a set of weights or gauge blocks. With carefully prepared SRM's, the

uncertainty of the result is mostly a function ot the uncertainty of the SRM and the measurement

process.

If a measurement process is to be meaningful,

then the numerical values obtained should be

specific, precise, and free of systematic error (or

bias) within the agreed on or practical limits

required for the end use. When these goals have

been achieved, then the measurement results may

be said to be accurate. Thus, by this definition, a meaningful measurement is termed an accurate

measurement. A fuU and complete discussion of

all concepts of accuracy is not appropriate for this

publication as it is a subject that measurement

scientists are not completely agreed upon in all of its ramifications. (For detailed discussions on

the term "accuracy," see references [5, 6].)

1 . Specificity.

During the measurement process, only the

property under test must be measured, and not some

2
combinations of properties that may give the false

impression of singularity. Non-specificity can be

considered a special case of systematic error, and

could be included in that discussion. However,

especially in the measurement of chemical composition, its insidiousness as a special source of error

is so striking (when found) that special emphasis is warranted.

2. Precision.

A high degree of precision in a measurement

process is demonstrated when essentially the same

numberical value is repeatedly obtained. In some

measurement circles, the measure of precision

within the same laboratory is called repeatability;

between different laboratories, reproducibility.

(Alternatively, the terms, "intra-" and "interlaboratory" precision, respectively, are often used.)

The interplay and complications of varying degrees

of inaccuracy with varying degrees of imprecision

in a measurement process are discussed in depth by

Eisenhart [6]. In practice, as opposed to theoretical

considerations, a high degree of accuracy is usually

positively correlated with a high degree of precision.

But, also in practice, highly precise systems are sometimes found to be highly inaccurate. This is a

real danger and must be carefully considered. More

detail on this is presented in Chapter III (B).

3. Systematic Error.

The third requirement for a measurement to be

meaningful is that it be free of systematic error. When systematic errors are present then the

numerical result differs from the "true value." From

a practical point of view for the large majority of

measurements made in industry and technology,

the "true value" can be considered in an operational

sense. This requires that a careful assessment be

made of the systematic errors in each step of the

measurement process. When the systematic errors have been identified and eliminated, the resulting

numerical value can be equated to the "true value."

Furthermore, the value obtained by this process

should be essentially the same as that obtained by

any other acceptable process used to measure the

same property of the same material.

4. Other Desirable Characteristics.

There are, of course, other desirable attributes of

measurement — sensitivity of detection, a large

dynamic range, ease of operation, speed, low cost,

and several others. These, however, are pragmatic

considerations by and large, while specificity, precision, and freedom from systematic error are absolute essentials to the attainment of meaningful

measurement. The use of SRM's in measurement

will be shown to have some impact in attaining

many of these other desirable attributes.

D. A Systems Approach to Meaningful

Measurement

There are several ways in which a meaningful

measurement system can be built, maintained, or expanded. Many of these modes are now in operation in various sciences, industries, and technologies. Principal among these are: calibration services,

especially for instruments calibrated at a central,

competent source and returned to the user; publication of standard reference data, which if critically

evaluated and given together with the detailed

measurement procedure, allows others to use the

data directly or to reproduce the original measurements; the provision of measurement signals

(time interval, frequency, etc.) via a central source

to users; transfer through methodology dependent

upon locally produced materials of realizable purity

and stability (e.g., specification of the purity of

platinum used to realize the candela); manufactured devices and/or materials made available to produce compatibility in a narrow field on a relative

rather than absolute (or accuracy) basis.

In this discussion emphasis will be placed on a meaningful measurement system based on SRM's

and reference methodology. Together, these provide a mechanism whereby compatibility can be

transferred with speed and modest cost into

practical measurement fields. There are five major

components of this system that will be described,

some in more detail than others. The relationships

of these components are shown in figure 1.

MEANINGFUL MEASUREMENT SYSTEM

UNITS

of MEASUREMENT

Ti]

STANDARD

REFERENCE

MATERIALS

ASSURE

QUALITY

MFCS of WORKING STANDARDS

REFERENCE

METHODS

1

ASSESS ACCURACY

FIELD METHODS 14 QUALITY CONTROL

LONG TERM

H

QUALITY CONTROL

ASSURANCE

THIS SYSTEM V CAN PRODUCE

MEASUREMENT VALUES

THAT ARE PRECISE, SPECIFIC, AND

FREE OF SYSTEMATIC ERRORS.

Figure 1.

3
1. Component I— A Rational, Self-consistent System of Units of Measurement.

As stated earlier, international agreement on a

large scale has been reached to make this system

the SI. In scientific measurement areas (metrology,

physics, chemistry, etc.) the SI is now used on a wide scale. Full implementation has not yet occurred,

primarily in areas of industrial technology. The

most obvious example is that of the U.S. where

many engineering and technical measurements

are still made and reported in non-SI units. Hopefully, this situation will be resolved within the next

10 years or so, since a recommendation has been

made to the U.S. Congress that the U.S. "go

metric" (i.e., adopt the SI) [7]. For well over 90

percent of the SRM's issued in the U.S. through

NBS, the properties are given in SI units, although

the corresponding non-SI units may also be re- ported. For some engineering oriented SRM's,

arbitrary, non-coherent units are used where the

SRM is made part of a test recipe (e.g., the "flame

spread index" of the Surface Flammability SRM).

For purposes of this publication, component 1 is the SI, even though occasionally certain exceptions

must be taken.

2. Component 2 — The Materials to Realize in

Practice the SI Units and Their Derivatives.

To reali/^ t determine) the SI unit candela,

platinum of a specified and known purity is necessary because the candela is defined in terms of the

radiation of a black body at the freezing temperature

of platinum. In carrying out this determination a

rigorously written procedure will also be specified

and the eventual accuracy will depend on both the

purity of the platinum and the adequacy of the

method. Such methodology is called a reference

method (see. Component 3, next). Similarily, if the

mass of copper in an industrial copper alloy is to be

determined with known accuracy, so that compatibility throughout the copper industry is to be propagated, then a copper alloy of known copper content

and a reference method of analysis to specify the

actual steps in its determination must be available.

In much of the world, these well-characterized

materials are called Standard Reference Materials

(SRM's) and are prepared, measured, and certified,

in most instances, by national standards laboratories. At NBS, the formal definition of an SRM,

which includes these primary uses, is given in

figure 2. Examples of these uses, for specific areas

of technology, are included in later chapters of this

publication.

The key characteristic of an SRM is that the properties of interest be measured and certified on the

basis of accuracy. At NBS there are three routes that are used to accomplish this goal.

A Formal Definition of NBS Standard Reference Materials (SRM's)

SRM's are well-characterized and certified materials, produced in quantity:

(1) To help develop reference methods of analysis or test; i.e., methods proven to be accurate .

and/or

(2) To calibrate a measurement system in order to:

(a) Facilitate the exchange of goods

(b) Institute quality control

(c) Determine performance characteristics

(d) Characterize at scientific frontiers

and/or

(3) To assure the long-term adequacy and integrity of the quality control process.

thus

Ensuring the Compatability and Meaningfulness of Measurement in the

Nation

for

. . . Science and Technology

. . . Production and Distribution of Goods and Services

. . . Government

Figure 2.
(a) Measurement of the property using a previously validated reference method. By definition, a reference method is a method demonstrated to be

accurate. When such a method exits, then any

unknown material (falhng within the scope and limitations of the method) may have its property

measured accurately. To minimize the possibihty

of personal bias, or other unforeseen difficulties,

two or more analysts, working independently, but

using the same methodology, must perform the

work. Table 1 gives an illustration of this route of

measurement.

Table 1. SRM 610— Trace elements in glass

(Lead at 500 ppm — nominal)

Rod No. Analyst 1 Analyst 2

2 426.5 13 426.2

18 425.6 425.9 48 426.1 426.0 56 426.9 425.0 66 426.0 425.4 78 426.2 425.6 106 425.7

Average 426.15 425.58

cr , ±0.41 ±0.40

95% L.E... ±.98 ±1.11

(b) Where previously established reference

methods do not exist, then two or more independent,

rehable measurement methods are used. A reHable

method is one of high precision, but one whose

systematic biases have not been fully discovered

and evaluated. In this route, the estimated systematic biases of each method are evaluated by the

analyst and must be small relative to the overall

uncertainty of the final certified value. Table 2

illustrates this route.

(c) The third route is a variation of the second.

Where a previous issue of an SRM is available to be used to assure intralaboratory quality control,

then many laboratories can be formed in an ad hoc

network to perform the work. Each laboratory uses the method felt to be most rehable (and

accurate) under that laboratory's operating conditions, but must run the prior SRM in parallel with

the unknown as a check. The results will be used

only when the certified value of the prior SRM is obtained. Obviously, only laboratories of the

highest technical competence and thoroughly

knowledgeable and familiar with the particular

material under study should participate in such

work. At NBS, this route is most often used to measure and certify renewal SRM's. Renewal

SRM's are replacements for SRM's the supply

of which has been exhausted; they are materials

closely similar to the original SRM's. Table 3

illustrates this route.

Table 3. SRM 337 -Basic Open-Hearth Steel

(Carbon by interlaborat<jry comparison)

Analyst Method/variation Percent

(Note: Combustion step common to all) carbon

1 Gravimetric — Ig sample 1.08 2 Gravimetric — 3g sample 1.06

2 Volumetric— Ig sample 1.06 3 Gravimetric — factor weight (2.73 g) sample 1.06 3 Gasometric — 1 g sample 1.07 4 Gravimetric — half-factor weight (1.36 g) 1.06 sample

5 Gravimetric — half-factor weight (1.36 g) 1.08 sample

5 Thermal conductivity — 0.7 g sample 1.08 6 Gravimetric — 0.7 g sample 1.07 Mean 1.07 4 Different methods

6 Different sample weights

AU Labs used SRM 16d as control

Table 2. SRM 1577 -Bovine Liver

(Cadmium by 2 or more independent methods)

Concentration (/.ig/g)

ID-MS

0.32

.29

.26

.27

.27

.27 .28

.26

.28

.04

r.ic aosorp.

0.29

.24

.26

.26

.27

.24

.27

.30

.26

.27

.04

Range (all results) 0.24 - 0.32 Recommended value 0.27 ± 0.04

Polarography

0.26

.16*

.28

~.28"

.27

.01

* Outlier, discarded for sounH reasons.

Finally, in the definition of an SRM, the phrase

"produced in quantity" has important implications

for anyone considering SRM's as • n integral part

of their quality-control process. At NBS, a six to ten year supply of an SRM is usually produced at the time of certification. As the supply of an SRM

gets low, a renewal SRM is planned. This assures

all I 3 of an adequate, continuous, and rehable source of supply, so that the quahty control of their

measurement procedures may h'^ maintained and

improved on a continuing basis over the life of their

operations.

3. Component 3 — Reference Methods of Measurement Used With or Base on SRM's.

A reference method is defined as "a method of

proven and demonstrated accuracy." These have

\ jen called variously: umpire methods, referee

methods, standard methods, and so ^-^rth. In any

5
case, the operational definition just given is the crux

of the matter, although international agreement

on a descriptor would help to avoid future misunderstanding. Absolute accuracy, implying

methods w^ith no systematic biases, is an unattainable goal, not achievable by mere mortals. It is important to realize that the cost of obtaining greater

accuracy increases exponentially. Therefore, only

that degree of accuracy required should be sought,

making allowance for further advances in the cur- rent state-of-the-art. A good guideline is to strive for a reference method whose accuracy is three times

better than that currently required by the end use. The development of reference methods is a timeconsuming, expensive, and complex process, in- volving these steps (although permutations are

possible).

(1) A group of experts surveys the Literature to choose a candidate method — one expected to have small systematic biases. They also decide

what the accuracy goal should be for the

reference method, considering the required

end use.

(2) A central laboratory is chosen to: coordinate

the work; develop the statistical design; prepare and distribute samples that have been

previously measured by the central laboratory

using an independent method of known ac- curacy, but one not usually available to the

field in question; and distribute the SRM —

a

necessary precondition is the availability of

the appropriate SRM.

(3) The group of experts, in conjunction with the

central laboratory, writes the first version of a detailed procedure (protocol), and helps select a group of measurement laboratories (usually

6 to 10 laboratories) wilhng to cooperate in

performing the work called for in the protocol.

(4) The central laboratory distributes the protocol,

sample, SRM, and instructions to the co- operating laboratories. The cooperating

laboratories perform the work according to a schedule. The analytical data plus other pertinent information are returned to the central

laboratory.

(5) The group of experts and qualified personnel

from the central laboratory, including statisticians, analyze the data, identify sources of error and then revise the protocol to eliminate

them.

(6) Steps 4 and 5 are repeated as often as necessary until the accuracy goal is achieved.

(7) The protocol is written in final form and published in a journal, a collection of reference

methods, or another appropriate pubHcation.

A recently developed reference method in clinical

chemistry, which illustrates the conditions just set forth, is that for the determination of calcium in

serum [8]. The number of reference methods, world-wide,

is discouragingly small, and in view of the needs.

an interim solution to this question of accuracy may

have to suffice for now. Some scientists have proposed accuracy by edict, a scheme whereby experts

declare a particular method to be the accurate

method against which all other alternative methods

will be assessed. If the method chosen by edict is carefully selected and some modest interlaboratory

testing done, then this interim solution may meet

present pressures. This approach should assure

compatibility at least. In the long run no substitute exists for the hard,

scientific work that establishes in the laboratory the

accuracy of the analytical method.

4. Component 4 — Establishment of Compatibility

into a Wider Area of Technology via the SRM and

Reference Method.

Components 1,2, and 3 are sufficient in themselves

to bring about accurate measurements in a few wellqualified laboratories. The real problem is, however,

to improve the quality of and make compatible the

measurement in the average laboratory on a routine

basis. There are two aspects to this problem, one

involving the field (routine) methods per se, the

other concerned with commercially produced (or

in-house) working standards.

Assessment offield methods. As reference methods

and SRM's become available, responsible groups

should begin the assessment of the various field

methods currently in use. When the test materials to be used in the assessment process are characterized on an absolute (accuracy) basis via the reference

method and SRM, the inaccuracies of the tested field methods will become readily apparent. Alternatively,

in some cases, an SRM can be used alone for this

purpose. As the testing data accumulate and become widely disseminated, a selection process will

occur, and highly inaccurate methods will tend to

fall into disuse and eventually disappear. For those

field methods having desirable characteristics

(speed, low-cost, etc.) it should be possible to correct

or eliminate any systematic biases found, thereby

placing them, in turn, on an accurate basis.

Many reference methods will not be suitable

(because of complexity, cost, or lack of speed)

for use in daily routine practice. Furthermore, not

every laboratory will have the facilities or instruments

required by the reference method, while other

laboratories may prefer to determine some constituents through use of simpler methods. Indeed, it is not necessary nor even desirable to do away with

present field methods, as long as they are tested

against the reference method.

Upgrading the quality ofworking standards. Given

SRM's and reference methods, the manufacturers

of working, or secondary standard materials, in- cluding reagents, and instruments will be able to test those products for accuracy. In the U.S., some

manufacturers already are using SRM's, where

available, to test the quality of their reagents and

secondary standards. Without reference methodol6
ogy, this testing must necessarily be on a relative

rather than an absolute basis.

The implementation of these assessment activities is more complex and difficult, than those of

Components 1, 2, and 3. Modes of implementation,

including legal requirements, differ from country

to country. In the U.S., standardization in most

fields of technology is strictly a voluntary process,

as opposed to the practice in many nations where

standardization procedures are usually legally

imposed. Agreement on the quality of products

moving in international trade is possible and current

trends indicate that efforts in this diiection are increasing.

5. Component 5—Assuring the Long-Term Integrity

of the Measurement Process.

Measurement systems are notorious in one

respect. Unless carefully monitored, they tend to get

out of control. Loss of precision is usually the first indication that the measurement process is not in a state of quality control. In most measurement

laboratories, this question is one of almost daily concern and one that has been extensively studied

and addressed. Although each individual laboratory

must ultimately be responsible for assuring its own quality control, professional societies and

governmental agencies can, and often do, provide

a mechanism that helps to assure, to a degree,

long-term quality control.

If SRM's and reference methods are available, the

mechanism for assuring the long-term integrity of

the measurement process in a large number of

measurement laboratories is quite straightforward:

(a) The sponsoring or testing agency prepares a

series of test samples incorporated in a suitable

matrix that cover the range of values likely to be

encountered in real life. (b) The properties are determined by the sponsors'

laboratory (or laboratories) using the reference

method to obtain values of known accuracy.

(c) The test samples, as unknowns, are distributed

with suitable instructions and reporting forms to the

laboratories under test who perform the work as instructed. In true bhnd studies, these samples

will not be differentiable from daily, routine samples.

(d) Results are returned to the sponsoring agency

and statistically analyzed. In a well-designed and

controlled program, each laboratory should receive

back the following information for each property

tested: its day-to-day precision within the laboratory;

the accuracy of the method used; its rank compared

to other laboratories using the same methodology;

the accuracy of its method compared to alternative

methods; a statement of acceptability of the results

(if norms for that technology have been established).

E. Economic and Social Costs of Making

"Bad" Measurements

In today's highly technological society, the costs

of making "bad" measurements can be monumental.

On the other hand, the benefits that can accrue from

"good" measurements, both economically and

socially, can be equally large. A measurement system that is non-compatible is obviously a wasteful

system. In such systems, the transfer of useful

measurement data across different technological

or geographic boundaries becomes difficult or impossible, and certainly wasteful. In measurement

systems not in a constant state of quality control,

the expenditure of a significant portion of the available measurement time in redoing measurements

obtained during the out-of-control period is not un- common. The examination of two large U.S. in- dustries illustrates the economic side of measurements. The first is that of the U.S. steel industry,

whose measurement system is long established and

well under control. The second is the U.S. "health"

industry, especially clinical chemistry, which has

areas in which lack of agreement between laboratories has received publicity.

1. The Steel Measurement System in the U.S.

In the U.S. today the quality and composition of

well over 90 percent of all the steel produced is maintained or measured through a measurement

system based on the concepts outlined. Over 300

SRM's are made available by NBS to cover essentially aU of the major steels produced. Reference

methods developed over many years are maintained

and published primarily through the American

Society for Testing and Materials (ASTM), a standards body strongly supported by the steel industry.

The manufacturers of the working standards check

and maintain their quality against the primary

NBS-SRM's. Long-term quality control is a constant activity of the various steel producers'

laboratories working in collaboration with the

various technical committees of the ASTM. Through

implementation and maintenance of their measure- ment system, steel producers and users routinely

produce results with an accuracy of 1 to 2 percent,

making more than one-half billion (5 X 10^) analyses

every year, with turn-around times of 3-5 minutes

from the time the sample is taken to the time the

analytical results arf^ available.

The cost per analysis of steel by spectrographic

means is between one and two dollars. If the present

measurement system used in the steel industry

eliminated only 1 percent of the "bad" analyses

(and the figure over many years has been much

higher than this), then the current, direct saving

today is at least $5,000,000 per year.

2. The Clinical Chemistry Measurement System

in the U.S.

The situation in clinical chemical measurements

is vastly different. Because the work load in clinical

7
chemical laboratories increased so dramatically over the past few years, hand methods of analysis were replaced by automated methods of unknown

accuracy, and in many instances, of unknown

specificity. Furthermore, no reference methods as defined herein existed in cUnical chemistry whereby

the new automated methods could be assessed.

Imposed on the system also were shortages of

trained personnel, a rapid proliferation of instruments and new methodologies, the need for very

fast response times, etc. In short, meaningful

measurement throughout large segments of the system did not exist. It should be added, however,

that scientists and physicians in this area are fully aware of these problems and are instituting on a national basis those steps necessary to bring about

meaningful measurement.

Meanwhile, the economic costs of the current

situation are very high. In the U.S. in 1973, according to one source [9], approximately 3 X 10^ chnical

tests will be performed by the nation's 12,500

chnical laboratories. The director of a large hospital

chnical laboratory estimates that from 10 to 25

percent of all measurements made must be repeated

because of error, or are sufficiently unreliable as to provide no useful information to the physician, and

are therefore worthless.

The cost per analysis in chnical chemistry is one

to two dollars. If the situation is no worse than the 10

percent mentioned above, then the current (1973)

cost of "bad" measurements will be at a minimum,

$3 X 10*. The potential savings in this field alone

would pay several times over the total per year

investment of aU the world's standards laboratories

for the production, measurement, and certification

ofSRM's.

3. Social Costs.

Social costs of "bad" measurements are not so

readily assessed. Such costs do exist, however, as one small example wiU show. In 1971, the editors of

the San Francisco Examiner were interested in

the degree of contamination of San Francisco Bay.

Samples of mud were sent as unknowns to three

local measurement laboratories. The results for

four contaminants are shown in table 4. These data

are meaningless in terms discussed in this pubhcation. From such discordant data, only fruitless controversies will arise, which will tend to obscure the

real problems — in this case, the level of pollution

in San Francisco Bay.

Table 4. Results reported in San Francisco Examiner

September 19, 1971 {in figlg). DDT DDE Mercury Lead

Lab 1 0.80 2.80 0.80 3.30 Lab 2 .68 L20 .10 21 Lab 3 .14 0.47 .10 50

References

[1] Shewhart. W. A., Statistical Method from the Viewpoint of Quality Control, Dept. of Agriculture, (Washington, D.C.

1939).

[2] Metric Practice Guide, ASTM, E-380-70 (1970).

[3] ISO Recommendation R 1000, reprinted by ANSI (New York,

New York, 1%9).

[4] Huntoon, R. D., Standard Reference Materials and Meaningful Measurements — An Overview (Proceedings, Sixth

Materials Research Symposium, Gaithersburg, Md.,

Oct. 29, 1973). In Press.

[5] Ku, H. H. , editor. Precision measurement and calibration, Vol. 1, Statistical concepts and procedures, NBS Spec.

Publ. 300-1 (1969).

[6] Eisenhart, C., Realistic Evaluation of the Precision and Ac- curacy of Instrument Calibration Systems, J. Res. Nat. Bur. Stand. (U.S.) 67C (Eng. & Instr.) No. -2, 161-187

(Apr.-June 1%3). Also available as Paper 1.2 in Reference 5.

[7] De Simone. D. V., A Metric America, Nat. Bur. Stand. (U.S.), Spec. Publ. 345 (1971).

[8] Cali, J. P., Mandel, J., Moore, L., and Young, D. S., A referee method for the determination of calcium in serum, Nat.

Bur. Stand. (U.S.), Spec. Publ. 260-36, 136 pages (May

1P72).

[9] Untitled news article. Lab World, 1246 (Nov. 1972).

8
II. CONSIDERATIONS FOR THE

USE OF SRM's

The introductory chapter provided the rationale

for using Standard Reference Materials and out- lined their relationship to meaningful measurements. As such the preceding chapter presented an overview of the NBS-SRM program and the

general concepts of SRM usage. In this chapter,

other considerations that must be faced by SRM

users are described.

SRM's are designed to provide analysts with the

means to check or calibrate within their laboratory

their entire measurement system by providing them

with well-characterized materials — SRM's --and

the analytical results of the characterization

procedures — Certificates. As such, SRM's eliminate

the need to ship instruments to a central calibration facility or to have inspection teams come to the laboratory.

The first problem facing the user or potential user is that of availability. Is the material he needs

available as a well-characterized standard? What

are the sources of availability? Next, the user must

decide what grade of SRM is needed and what form

is needed (solid, powders, liquid, gaseous).

An of these questions must be answered if the user is to make a rational decision. An overriding

question that all users must answer is that of cost. Not only the cost of the actual SRM's or the cost

of analysis, but especially the cost that would result

by not using SRM's should be considered. The cost

aspect is specific to each user and different in each

case, and can only be discussed in general terms.

The purpose here is to point out that cost is often a prime consideration in the use of SRM's.

This chapter is not designed to answer these

questions, but rather to provide users with information to help them answer their own questions. Therefore, this chapter will cover the process of selecting

appropriate SRM's and problems encountered in

their use; and sources of information on NBS-SRM's.

A. Proper SRM Selection

Basically, SRM's have three possible uses. They may be used as "control" materials analyzed

simultaneously with "unknown" materials; as

"calibration" materials to calibrate instrumentation; or, as "known" materials in the development

of new techniques or instrumentation. In each

instance, the SRM provides the results that the

analyst should obtain. Hence, the analyst is provided

with the means for a critical examination of his

entire measurement system.

The NBS-SRM program began with cast iron-chip

standards provided by the American Foundrymen's

Association. These metal standards together with

those subsequently prepared by NBS were produced

to meet the needs of the expanding American iron

and steel industry in the early 1900's. These early

standards, applicable to the analytical methods then

employed, are typical of "control" standards. As

"controls," the SRM's are analyzed simultaneously

with the "unknowns" regardless of the specific test method used. When the analytical results obtained

for the SRM agree with the certified values within

stated limits, then the method of analysis is in control and the results obtained for the "unknown"

are reliable. The methods used, such as gravimetry,

titrimetry, and volumetry, are chemical methods and

thus have a high degree of specificity and use relatively simple detection systems.

When an SRM is used as a "control," the method

of measurement may be biased because of the

matrix of the material; consequently, to avoid possible matrix effects, both the SRM and the unknown

must have similar compositions. The degree of

correspondence required is related to the selectivity

of the method — the greater the selectivity, the less

need for a one-to-one correspondence. In addition,

three other factors should be considered: homogeneity of the unknown sample, sample size (of

both SRM and unknown), and the acceptable degree

of agreement (or disagreement) between the

analyst's results and those certified for the SRM.

In any analytical work, sample size is a function of

both material homogeneity and the analytical

method. For NBS-SRM's, sample size is not normally

specified because the material is homogeneous for

the sample sizes normally required by the particular

method for which the SRM was intended. For those

SRM's where homogeneity may be a problem, the

minimum sample size to be used is stated on the

Certificate.

The types of methods using SRM's as "control"

standards usually involve mass or volume measurements. As technology advanced, new demands were

placed on the analyst giving rise to instrumental

techniques. The absolute measurement of mass

and volume were replaced by relative measurements, e.g., light intensity, plate density, or current.

In many cases, the selectivity of the chemistry employed was replaced by the selectivity of the instrument in measuring a certain physical property, such

as thermal conductivity. This resulted in a demand

for new SRM's to cahbrate the instruments. Be- cause instrumental techniques are relative, i.e.,

they do not measure a chemical property directly,

matrix effects must be closely examined and ac- counted for. The first major use of "calibration" SRM's was

in the preparation of emission spectroscopic curves

in the early 1940's. These curves were strictly

empirical in nature and had to be established by

relating instrumental response to a series of

SRM's. Examination showed that the response or shape of the analytical curves was affected by a variety of matrix effects, that included matrix composition and metallurgical condition of the specimen.

The impetus for SRM's to generate such curves came from the steel industry as did the original

demand for "control" SRM's. The need for "calibration" SRM's was two-fold, first to standardize

9
analytical results from laboratory to laboratory and

from run to run, and secondly to reduce the time

required to analyze samples accurately so that

analyses could be made quickly (3-5 minutes)

during production of large batches of material.

The problems that were encountered in the steel

industry then are being faced today by many other

segments of technology that are now using instrumental methodology, that is, the use of rapid, relative methods, to produce accurate analyses of a

large number of samples in a relatively short period

of time. Consequently, the use of SRM's for calibration work is of particular interest to a large and growing segment of both the scientific and technical

communities because of the widespread use of

instrumentation.

A variety of ways exists for using calibration

SRM's to generate calibration curves. Usually one

or more of the following problems must be

considered:

(a) Matrix— the sample and the SRM's used to make the calibration curve should give the same

instrumental response with respect to interferences

from the matrix. The user must know what the possible interferences are and to what extent they

might interfere.

(b) Calibration curves — when possible, the calibration curves should be prepared using two or more SRM's so that the value of the unknown is closely bracketed and may be interpolated from

the curve. Unless the curve is supported by a proven theory, one point calibrations may be most

unreliable. Extrapolation from such calibration curves may result in misleading conclusions if the

analytical situation is not clearly understood.

(c) Physical condition— physical condition of

SRM's and unknowns must be similar. For ex- ample, in x-ray fluorescence spectroscopy both

SRM's and unknowns should have the same surface

finish to obtain an equivalent instrumental response.

NOTE: Certified SRM's do not, by themselves,

guarantee meaningful measurements, i.e., they do

not obviate the prerequisite for trained, experienced personnel.

A third use of SRM's is for the development of

new analytical methods. Perhaps a more general

term would be scientific use. This use is characterized not so much by the need for precise measure- ment of a specific property, but more by the need

to provide a common matrix material containing

known concentrations of various elements for developing new measurement techniques. Such

SRM's permit researchers in different locations

to cooperate in developing new methods. For

example, the development of the reference method

for calcium in serum [1] was greatly simplified

because a clinical SRM for calcium existed —

Calcium Carbonate, SRM 915. Currently NBS

issues SRM's for this purpose in several areas,

e.g., trace elements in orchard leaves and trace

elements in glass. Both of these SRM's contain a

variety of trace elements — one is a botanical matrix

similar to many biochemical and environmental

samples, and the other is a silicate matrix similar

to many rock-type samples of interest to geologists

and others interested in minerals and ores. In

addition, a special effort has been made to provide

a few high-purity materials for use as source material for analytical method development, and in

some cases, characterization of physical properties.

The elements certified in this type of SRM are measured by several different techniques and

possible interferences are identified. Such certifi- cation makes these SRM's invaluable for comparison of various measurement techniques.

The development of any SRM is generally undertaken to answer a specific measurement need.

Once the SRM is prepared and knowledge obtained

of its composition, homogeneity, physical condition,

and other factors, it has often been found useful

for a variety of other measurement needs. AU of

these needs, however, wiU encompass either control,

calibration, or method development.

B. Potential Pitfalls in Standards Usage

Regardless of the end use, several other factors

must be considered in the selection of the appropriate SRM. Two of these are the grade of the

standard required and any special handling or storage requirements the user must foUow to maintain the integrity of the standard. Standards are

often discussed in general terms without a clear

definition of what is meant by the word "standard."

Various international and national scientific and

technical groups have established official definitions

for various grades of standards that pertain to their

field. There is little agreement on these definitions

among the various groups. At the National Bureau

of Standards, when an adjective is used to describe

the grade of an SRM, the adjective generally correlates with the definition given by the scientific

group for which that SRM is most applicable. For

inorganic chemicals, the International Union of Pure

and Applied Chemistry (lUPAC) definitions re- garding "primary," "working," and "secondary"

standards are used. Thus, the lUPAC definition

of a primary standard "a commercially available

substance of purity 100 ± 0.02 percent," is used for

NBS Primary Chemical Standards. A more detailed discussion of the classification of purity of

chemicals is given in chapter IV, E, Reagent Chemicals. In the following discussion the use of the

words "primary" and "secondary" does not necessarily conform to any specific definition of grades,

but is used merely to distinguish between two

"levels" or "grades" of standards.

Primary standards serve a specific need, but are not generally used on a daily routine basis. Rather

they are used to ascertain the quality of secondary

standards that would be used daily. Therefore the

grade of the standard material to be used must be

decided by the user based on the intended use.

10
Secondary standards come into existence in a

variety of ways. Commercial firms, study groups,

individual laboratories, or groups of laboratories

may prepare and issue these standards. Ideally

these standards should be related to and measured

against the primary standards and therefore be

more useful in the measurement system. Of course,

each time accuracy is transferred along the meas- urement infrastructure an additional uncertainty

is added. Therefore, the more measurement levels

between the primary and the secondary standard,

the greater the uncertainty will be. However, within

a well controlled measurement system, using a reference method and a well characterized primary

standard, the degree of degradation should not be

great.

The use of primary standards issued by a rec- ognized standards body and the use of secondary

standards prepared by commercial firms or groups

of laboratories strengthens the whole measure- ment system. Experience at NBS has shown that no one organization can possible supply all of the

standard reference materials needed by one nation. Therefore, a national measurement system

should include a recognized standards laboratory

that produces primary standards, and another level

of standards production that prepares secondary

standards directly related to the primary standards.

Scientifically, the preparation of secondary

standards provides a second group of scientists

who compare the primary to the secondary standard and thus provides an important check on the

quality of the primary standard. This independent

checking of the primary standard provides an extra

base of scientific skills and helps strengthen the

measurement infrastructure.

The availability of lower-cost secondary standards

promotes wider use of standards. Such availability

reduces the demands upon the national standards

laboratory to be the major supplier of all standards.

This in turn permits the national standards laboratory to devote more of its resources to the development of other needed primary standards.

C. Special Handling and Storage

Any storage condition that would alter the certified value of a material must be avoided. Despite

the obviousness of this statement, careful examination of storage conditions is mandatory to avoid

many potential problems.

With inorganic materials, e.g., steel chips, little warning is necessary. Furthermore, the usual

requirements for a standard is that it be stable,

nonhygroscopic, and nonefflorescent in most

atmospheres. These requirements tend to make

standards easy to store. However, in most laboratories and industries, certain specific cases arise

where standards must be used that do not have all of these properties. In such cases, storage and

shipment involve real and serious problems. NBS

has issued many SRM's that pose storage problems,

but which are adequate for the intended purpose

if properly handled. Some of these SRM's include

coal, which contains volatiles; cement, which

tends to hydrate; and SO2 permeation tubes, which

are affected by a high humidity. Careful laboratory

storage of these SRM's in desiccators, sealed vials,

amber bottles, and similar containers is usually

called for. These are typical laboratory requirements, and should present no added problem for the

user. Storage in inventory and the packaging for

shipment do however present difficult problems for a standards producer that must be solved to make

a standard reliable and generally available. Here

the nature of a material must be considered at the

very start of an effort to produce a standard.

With many new packaging materials available, a variety of ways exists to solve these problems and

most materials can be suitably packaged for both

storage and shipment.

Perhaps the major consideration for users in the

storage of standards is that of maintaining the

material's integrity in the period from certification

to use. At NBS all SRM's are stored in sealed containers. Once the seal is broken by the user and

the container is opened, the original contents may

easily be contaminated. Thus in a laboratory situation, some policy concerning the use of standards

must exist to prevent contamination. The integrity

of a laboratory's measurement process rests on this

consideration as much as with the analyst's technique and the rehabiHty of his method.

D, Sources of Information on NBS SRM's

These sources of information include: SRM

Certificates; SRM Catalogs and Catalog Supplements; a series of NBS Special Publications (the

"260 Series"), specifically devoted to SRM's; brochures on selected groups of SRM's; and individual

announcements of new SRM's. In addition, papers on SRM's and the SRM program are presented in

technical publications and scientific meetings in

the United States and other countries by NBS staff

members.

The Certificate issued with an SRM is the single,

most important source of information for a specific

SRM. It is an integral part of the SRM as it provides

the certified data, accuracy hmits, instructions for

the correct use of the SRM, and other pertinent

information. With very few exceptions, each SRM

is issued with some form of certification. The few

exceptions are so noted in the Catalog with the reason for the exception.

The form of certification varies with the type of

SRM. For SRM's certified for chemical composition, a "Certificate of Analysis" is issued. For en- gineering type standards and SRM's certified for a physical property, a "Certificate" is issued.

Occasionally, such terms as "Certificate of Calibration" and "Certificate of Viscosity Values" have

been used. In this publication no distinction is

11
made between different titles and for the most part

the term "Certificate" is used.

For many years, NBS has issued some Certificates with the word "Provisional" printed on them.

Although what was to be inferred from this word

had never been formally explained, its intended

use was to indicate that additional information

would be made available at a later (usually unspecified) date.

The type of information to be made available

would include such possibilities as:

(1) Some certified values were to be remeasured

and recertified at higher levels of accuracy

than was originally done.

(2) Some values given "For Information Only"

were to be certified at a later date.

The use of the word "Provisional" has caused

confusion among some SRM users and will no

longer be used on future Certificates. Any technical

limitations or restrictions that are necessary are stated on the Certificate.

The most valuable source of information for the

selection of SRM's is the Standard Reference

Materials Catalog issued by NBS. This Catalog

Hsts and describes all of the SRM's available from

NBS as well as many of the materials currently in

the process of being certified. Supplements to the

Catalog are issued periodically describing SRM's

issued since the Catalog was printed, listing those no longer available, and giving current prices of

SRM's.

Another valuable source of information is the

series of NBS Special Publications devoted to Standard Reference Materials. Called the "260

Series," these publications are numbered as: 260-1, 260-2, etc., and all have as a main title,

"Standard Reference Materials." The "260 Series"

is dedicated to the dissemination of information on all phases of SRM's as related to meaningful

measurement including the preparation, measurement, certification, correct use of NBS-SRM's, and

the development of reference methods of analysis.

In general, much more detail will be found in these

publications than is generally allowed or desirable

in scientific journal articles. These publications

enable the user to assess the validity and accuracy

of the measurement processes employed, to judge

the statistical analysis, and to learn the details of

techniques and methods utilized for work entailing

the greatest care and accuracy. An additional purpose for these publications is to supplement the

information given on the Certificate so that new

applications may be developed in diverse fields not

foreseen when the SRM was originally issued.

Appendix A is a complete listing of the "260

Series."

A group of brochures that list SRM's prepared

for specific fields is yet another effort to assist users in selecting SRM's. Some of these list additional SRM's that have a general applicability.

The availability of each new SRM is announced by

NBS through several mechanisms, primarily by

direct mailing to users and potential users of SRM's.

A source of information that should be of particular interest to all SRM users is the "Proceedings

of the 6th Materials Research Symposium, Standard Reference Materials and Meaningful Measurements " [2]. This symposium reviewed the SRM

programs of several Governments and national and

international organizations and the SRM needs and

measurement problems in many industries and

scientific disciplines.

References

[1] Standard Reference Materials. A Referee Method for the Determination of Calcium in Serum, Nat. Bur. Stand.

(U.S.). Spec. Publ. 260-36, 136 pages (May 1972).

[2] Proceedings 6th Materials Research Symposium, Standard

Reference Materials and Meaningful Measurements,

Oct. 29-Nov. 2, 1973 (in press).

12
III. CERTIFIED VALUES AND

THEIR MEANING

-UNCERTAINTIESIn the development of each SRM, assurances must be obtained that the material used is uniform

and stable, that test methods yield repeatable and

consistent results, and that the conditions under

which the material is to be used are carefully

described. Eventually, these qualitative state- ments will have to be translated into quantitative

terms, using data generated from the tests, and

condensed into a Certificate that will be understandable and useful to the user. This condensation of information is no easy task!

Ideally, the experimental conditions could be

described in detail and all of the numerical values

of individual determinations could be presented

so that the user could judge how best to use these

results. This has been done in a few cases and published in the "260 Series."

Generally, the cost and work involved in a detailed

presentation is not justified in relation to the

number of times it will be profitably used. In most

cases, therefore, the data are processed and con- densed into the form presented on the Certificate.

The numerical values are normally expressed in

two parts: the certified value of the property and

the uncertainty of this value.

The uncertainty of the certified value denotes

how well this value is known. A number of different

expressions have been used for the statement of

uncertainty, depending on how the SRM was

developed, which group of scientists was involved,

and the use for which it was intended. A number of

commonly used expressions are described in this

section.

In many cases, the statements of uncertainty

are based, to a certain extent, on the subjective

judgement of the scientists involved, rather than on a strict interpretation of the data. Acquisition

of data is expensive, sometimes prohibitively so, and these factors must be weighed against the

intended use of the SRM.

The uncertainty of the certified value of an SRM

is made up of at least two components: inhomogeneity of the material and measurement errors. Regardless of the form of an SRM, none are issued

with a guarantee that the property of interest is identical for each item in a lot. The homogeneity

of the material is usually checked by a rapid method

of analysis on selected portions of the bulk material,

or on a randomly chosen number of items. If the

results show that the variability of sampled material is within the specified limits, the material is accepted.

A. Material Variability

The selection of samples and the analysis of

data are usually performed in consultation with a

statistician. The emphasis here, depending on the

form of material, is to detect trends or patterns;

e.g., from one end to the other of a steel rod] from

the center to the edge of a plate; from the top

to the bottom portion of material in a drum; or to check on the variability of the material among

ampoules and bottles. A proper, statistically designed experiment helps to assure that conclusions

are vaHd, and to reduce the number of samples used

to reach such conclusions.

The outcomes possible from homogeneity testing

are:

(1) Homogeneity is not a problem, or material

variabihty is negligible in relation to either measure- ment errors or to the use of the SRM (pp. 17-21,

NBS 260-1);

(2) Material variability is a major factor in the

total uncertainty, in which case the material is rejected, reworked, or each specimen is individually measured and certified; or

(3) Material variability is of the same magnitude

as the measurement error, and must be included as

a component of the uncertainty.

Of the three cases, the last is the one most

frequently encountered. Two sub-classes are apparent: one where a trend is detected and one where

no trend is detected.

Where a trend has been detected, e.g., along a

steel rod to be cut into pieces, the unusable portion

is discarded and, hopefully, the trend in the re- maining portion is linear. In such a case, a fine can

be fitted to the values measured along the rod. The

maximum departure from the average of points on

the fitted line is taken as a measure of inhomogeneity, assuming measurement error is small in

comparison to the trend.

Where no trend is detected, but the results of

measurements showed variability that is not

negligible, a statistical concept called "statistical

tolerance interval" could be used. To illustrate this

concept, suppose a solution is prepared and packaged into 1000 ampoules, of which 30 are analyzed.

For this example, the tolerance hmit concept states

essentially that based on the measured values of

the 30 ampoules almost all of the 1000 ampoules

will not differ from the average of the 30 ampoules

by more than the given Umits. In statistical terms,

it would read: "The tolerance interval (average ±A)

is constructed such that it will coyer at least 95 percent of the population with probability 0.99.

This statement does not guarantee that the tol- erance interval will include all of the ampoules.

It says 99 percent of the time the tolerance interval

will include at least 95 percent of the ampoules.

The "99 percent of the time" refers to the way this

tolerance interval is constructed, i.e., if 30 ampoules

were selected from the population repeadedly, and

the same experiment were performed over and over

again, 99 percent of the tolerance intervals so con-

^The statement is true only for a population of infinite size; however, the correction

for populations of finite size is negligible where finite size is large.

13
structed would cover at least the proportion (95%)

of the total population as specified.

How is this interval constructed? First, the aver- age and standard deviation from the 30 ampoules

are computed:

Average:

Standard deviation: S =

(1)

1

n _ "11/2

1=1 -I (2)

where Xi,X-2, . . . Xi ._. .X,, are the measured values,

with n — 30. Here X is an estimate of the mean,

m, of the 1000 ampoules, and S is an estimate of

the measure of the dispersion, a, among these

ampoules. These two values, X and S, contain practically aU the information available on the 1000

bottles and can be used to calculate the tolerance

interval "X ± A."

The value of A is computed as a multiple of S,

that is, A = KS. The value of K depends on three

parameters:

1. n: The number of samples measured (30),

2. P: The proportion of the total to be covered

(95%), and

3. y: The probability level specified (0.99).

A table of "factors for two-sided tolerance limits

for normal distributions" [1] gives the value for

K as 2.841 when n = 30, y = 0.99, and P = 0.95.

The term "two-sided" means that we are interested in both over and under limits from the

average. The term "normal distribution" refers to the distribution of all the values of interest and is a symetrical, bell-shaped distribution usually en- countered in precision measurement work.

Figure 1 is a histogram of the ratios of the emission rate of '^^Cs in SRM 4233, '^^Cs Burn-Up

Standard, to a radium standard, and frequency

curve of a normal distribution can be fitted to these

data.

There were 98 ampoules of '^''Cs involved; each

ampoule was measured in April, September, and

November, 1972. By averaging the three measurements, the measurement error was considerably

smaller than the difference of weights of active

solutions among these ampoules, and the plot

showed essentially the inhomogeneity of the amount

of solution in the ampoules.

Values of 30 ampoules were selected from the

98 values by using a random number table, and

X and S are computed by expressions (1) and (2), giving:

Z = 0.11675, 5 = 0.000801

30

24

hJ

-IO

a.

18

S '2

UJ

IDz

3 6 Z

'^^Cs SRM NQ4233

^WEIGHED SOURCES

Oinoinomoino'noi') 1x7 aaO O — — — ~ — tiH— Cs

Qo g) CO op RRS20

Figure 1. Histogram of the frequency (number of ampoules)

versus the ratio of the activity of Cs—137 standards to a radium

reference standard, RRS20.

The tolerance interval for at least 95 percent

coverage with probability level 0.99 is Z ± KS, or;

0.81168 -2.841 X 0.00080 = 0.80941

to 0.81168 + 2.841 X 0.00080 = 0.81395

This interval covers the values of 96 ampoules,

i.e., all except the single ampoule on the extreme

left and the one on the extreme right.

If 30 values were selected repeatedly, the

values of X and S would not be the same, but the

intervals from X — KS to X + KS will have the

property of covering at least 95 percent of all values

99 percent of the time.

Tolerance limits and interval are useful concepts

when a finite number of samples are taken from a larger stock of material. As the number of samples,

n increases, the value o{ K decreases and eventually

settles down to 2 (actually 1.96) when X=m,

and S = a. Over the years, however, chemists have

resisted the use of this term mainly because the

concept is somewhat complicated. The more

conservative ones use a factor of 3. and sometimes

the range between maximum and minimum is used.

B. Measurement Errors

In discussing measurement errors, the terms

"precision," "systematic error or bias," and "accuracy" are usually used. The meanings of these

terms are not rigidly fixed, but depend to a large

extent on the interpretation and use of the data.

If two equally trained operators, A and B, make

four analyses on a uniform material each day for

4 days on one instrument, and 4 days again on a

similar instrument, the results, 16 sets of 4 measurements, may look like those in figure 2. What can be seen from this plot?

14
(1) The spreads among each set of four values

are comparable, perhaps slightly smaller for

Instrument II than Instrument I.

(2) There appears to be more variability between

daily results than within sets of daily re- sults, particularly for Instrument I.

(3) Operator B gives lower results than Operator A.

(4) Instrument I gives lower results than In- strument II. Figure 2 is constructed for the purpose of demonstration, and actual measurements could be better or worse than shown. However, this plot does show

some four types of factors that contributed to the

total variability of these measurements: (1) factors

acting within days, (2) factors acting between days,

(3) factors due to instrument systems, and (4)

factors due to operators.

1 1 1 o o 1 o 1 n n r\

o OOo 0 , O 0 O 1

1 1 1 1 ° 1 °

.1 o

0

0 o

V ^ 0 1 1 ° 1 1 0 O 0 0 • •

1 1 o ' • • 1 •

1 ° 1 1 * 1 **

•

• •

•

• • •

1 1 " 1 1 1 1 1 1

• 1**1* 1 1 * *

• • •

•

w

• ^

1

1 OPERATOR «

' OPERATOR E

0•

DAY t DAY 2 1 DAY 3 1 DAY 4

INSTRUMENT 1

DAY S 1 DAY 6 1 DAY 7

INSTRUMENT 2

DAY 8

Figure 2. An example of results of measurements by two

operators using two instruments on eight different days.

Appropriate techniques are available for the

separate estimation of the effect of these four fac- tors and standard deviations could be computed

corresponding to each of them. However, the limited

number of operators and instruments prevent the

computation of standard deviations as reliably for

factors (3) and (4), as for factors (1) and (2). The

time and work involved certainly impose limits on any efforts to do so. The exclusion of factors relating to instruments

and operators is one of the main causes for the un- reasonable differences usually encountered in in- terlaboratory, or round-robin types of tests [4]. Because instruments vary from time to time and

operators change, the result from a laboratory at a given time represents only one of the many results

that could be obtained, and the variability caused

by these two sources must be considered as part of

the precision of the laboratory. The standard deviation computed without regard to these effects would

underestimate the precision.

If by the proper use of standards and reference

methods [2], these two sources of errors were eliminated, the standard deviation computed from the

16 averages of sets of 4 measurements would be

the proper measure of precision. Presumably the

grand average of the 16 values would be reported.

The average of many values is more stable than

individual measurements, and the relationship between the standard deviation of individual measure- ments and the average of n such measurements can

be expressed as:

a^{X„) = , n

in other words, the standard deviation of the aver- age is smaller than standard deviation of individual measurements by a factor of One

Vn

important provision must hold for this relationship

to be true, i.e., that the n measurements are independent of each other. "Independence" can be

defined in a probability sense, but for this paper,

measurements may be called independent if they

show no trend or pattern. This is certainly not true

in figure 2, and to say that the standard deviation

of the average of all 64 values is Vs of the standard

deviation of an individual measurement would

grossly overestimate the precision. Moreover, this

relationship is exact only if the true value of standard deviation, symbol cr, is known. As the computed standard deviation, symbol S, is itself an

estimate of cr from the set of measured values, the

relationship is only approximately true. The use of the standard deviation computed from

daily averages rather than individual values is preferred because a component of variability between days, or over time, is usually present in

precision measurement.

The basic information available on the measure- ment errors are:

(1) The number of independent determinations

or the number from which an average was

computed and reported;

(2) An estimate of the standard deviation. S.

From (1) and (2) several useful, derived statistics can be computed:

(3) Standard deviation of the mean of n measurements, S{X„) = — This is sometimes called

Vn

standard error of the mean to differentiate from the

standard deviation of individual determinations.

Note: as n becomes large, the value of S(Xn) becomes very smadl, showing that the average of

large numbers of measurements approaches a constant value "m" which is usually the objective of

the measurement procedure.

15
(4) Confidence interval for the mean. Each time

n measurements are made, a value of the average

of the measurements is reported. These averages

will differ from time to time within certain limits.

One interval of the type X ±b can be constructed

such that the interval between X — b \.o X-\- 8 will

be fairly certain to include the value of m desired.

The interval is computed by:

where "f" is a tabular value of Student's statistic

depending on two parameters {n — \ ) and (1 — a).

The probability level, (1— a), is similar to "y"

used in tolerance limits, and is usually set at 0.95

or 0.99 and commonly referred to as 95 percent

or 99 percent confidence level. The statement may

read: "The interval Z - 5 to Z + 6 is the 95 percent confidence interval for the mean based on n

determinations," or "The half-width of the 95

percent confidence interval for the mean is 8,

based on n determinations" [1]. The confidence interval for the mean is concerned

only with the probability of including the mean

and not of including a specified proportion of population as was the case of the statistical tolerance

interval. With n large, the confidence interval tends

toward zero, whereas the tolerance interval approaches some constant width, as previously

explained.

(5) 2-sigma (or 2s), 3-sigma (or 3s) limits. These

limits are similar to the tolerance interval in the sense that the interval between these limits will cover a large percentage of the individual measure- ments that were or might be made. The expected

coverage, however, depends critically on the number of measurements used to estimate cr. Hence,

if a measurement is made by the user of an SRM

having the same precision as that obtained by

NBS, his measurements should fall within these

limits when a is well established. Otherwise

there is evidence of systematic difference. (For a discussion of two-sigma and three-sigma limits see ASTM Standard E-177-71 ) [3].

C. Instrument and Operator Errors

Instrument and operator types of errors have

not yet been treated. An ideal situation would be

to eUminate them from the measurement process,

or to use more instruments and more operators

and then estimate standard deviations associated

with these sources. When neither of the above is feasible or practical, the least that can be done is to use two instruments. If the confidence intervals

for the results of the two instruments do not overlap, then there is good evidence of instrument

difference. The difference between these two

instruments is probably the lower bound of errors caused by the instruments, because adding a

third instrument could only make the total range

of difference larger, and not smaller.

Using his experience and judgement, an analyst

may arrive at reasonable bounds for these types of

errors. If the bound is not computed from measure- ment data, then its validity can not be supported

by statistical analysis. In such cases, these bounds

are "guesstimates" and the only recourse is to

treat them as limits to systematic errors. The detection of differences and the separation

of the total variability into its identifiable components can be facilitated through careful planning

and statistical design of the experiment.

D. Differences Among Measurement

Methods

Each measurement method purports to measure

the desired property of a material, but seldom

does a method measure the property directly. In

most cases the method actually measures some

other property that is related to the property by

theory, practice, or tradition, and then converted

to the value of the desired property through these

relationships. Discrepancies among results of different measurement methods are legend, even for

measurements leading to the determination of

fundamental physical constants [5]! In the preparation of an SRM, usually two or more measurement methods are employed for each

property measured. If these methods are well

established by virtue of past experience, the re- sults yielded by these methods usually agree to within the uncertainty assigned to each method.

In a few cases these differences are so large that

the results cannot be reconciled, and these results are then reported separately for each individual

method, but not certified. An example of this type

of reporting is SRM 1091, Stainless Steel. The nitrogen content was measured by vacuum fusion and

pressure bomb-distillation, and gave results of

861 and 945 ppm, with standard deviations of 3 and

20 ppm, respectively. A report of the average of the

two methods would be highly misleading.

Clearly one or both methods have a systematic

error that is large compared to the variability of material or the measurement uncertainty. By definition

the nitrogen content in the sample of steel is a constant, and the difference between the value

obtained (including its own measurement uncertainty) and the true nitrogen content is a measure

of the inaccuracy of the method.

Accuracy in its absolute sense is a luxury that

we can hardly afford. In practice, inaccuracy of

some standards is defined by using a reference

method so that at least the same benchmark will

be used by every one in the field. Primary Standard

Reference Materials are intended for such purposes. The importance of reference methods to supplement the use of these standards is also being

emphasized [2].

16
E. Uncertainties of Certified Values

The term "uncertainty," while indicating the

likely inaccuracy of a reported value, does not

seem to give such a rigid connotation as that of

"accuracy." The uncertainty of an SRM value is usually made up of three components, some supported by data and some not:

(1) A tolerance limit giving bounds to material

inhomogeneity based on data and statistical

computations.

(2) A confidence limit for the mean giving bounds

to measurement error based on data and

statistical computations.

(3) A combination (addition of absolute values or the square root of the sum of the squares) of

estimated bounds to "known" sources of possible systematic error based on experience

and judgement (in other words, there is no

data, or not enough data, to make a statistical

calculation).

The word "known" is quoted above to contrast

with systematic errors that are "unknown" or un- suspected. These unsuspected errors could occur

in a number of ways — a component in the physical

system, a minor flaw in the theoretical consideration,

or the rounding error in a computation. As more

uniform material becomes available, and more

precise measurement methods are developed, these

types of errors will be detected by design or by

chance and hopefully will be eliminated. Improved

accuracy in the measurement of a property is basically an expensive iterative process and un- warranted demand for accuracy could mean the

waste of resources.

F. Statements of Uncertainty on SRM

Certificates

A variety of statements of uncertainty can be

found in past and current Certificates issued for

SRM's. Some of these statements are well formulated and supported by data, others are not; some

of these statements contain a wealth of information

that is useful to exacting users, but overwhelming

to others; some statements are oversimplified with

a resulting loss of information. Because the originator of an SRM has to keep all classes of users in

mind, the use of a single form of statement is not

usually possible. The intention is that all these

statements are unambiguous, meaningful, and

contain all the information that is relevant for

potential users. In practice, however, it is difficult to impose these requirements on all of the more than

850 items now listed in the SRM Catalog.

Some of the commonly used statements are

listed below:

Example 1: Uncertainty expressed in significant

digits.

AISI 4340 Steel (SRM 1261)

Element Percent, by weight

Carbon 0.38 2 Manganese 0.66

According to the explanation given in the text: "The value listed is not expected to deviate from

the true value by more than ± 1 in the last significant figure reported; for a subscript figure, the

deviation is not expected to be more than ± 5."

Thus, the carbon percent by weight is between

0.377 and 0.387; and that for manganese is between

0.65 and 0.67. These uncertainties include material

inhomogeneity, measurement errors, and possible

bias between laboratories, because these values

are ". . . the present best estimate of the true value

based on the results of the cooperative analytical

program."

When twenty to thirty elements are to be certified

for one sample, this method gives a concise and

convenient summary of the results. As these limits are expressed in units of 5 and 10, some information

is unavoidably lost for some of the elements. However, it is important to use all of the digits given

including the subscripts.-^

Example 2: Standard deviation of a single

determination.

Fe-3Si AUoy Microprobe Standard (SRM 483)

Silicon 3.22 ± 0.02 weight percent

The value "± 0.02" is explained as "Standard

deviation of a single determination based on results

obtained on SRM's 483 and 125b, but also including

similar method error estimations from previous

determination" [6]. A tolerance-interval type of uncertainty statement,

e.g., 3s =0.06, might have been more meaningful,

because it would include both method imprecision

and material inhomogeneity.

Example 3: 95 percent confidence limits for the

mean.

Rubidium chloride (SRM 984)

Absolute abundance ratio . . . 2.593 ± 0.002

"The indicated uncertainties are overall limits of error based on 95 percent confidence limits for the

mean and allowances for the effects of known

sources of possible systematic error."

Because the isotopic ratio is a constant and is not subject to errors of material inhomogeneity, the

17
95 percent confidence limits for the mean refer

to measurement error only. This is computed from.

Example 6: Average, standard deviation, and

number of determinations.

t —p, as previously described.

The effects on known sources of possible sys- tematic error are discussed in detail in. Absolute

isotopic abundance ratio and atomic weight of

terrestial rubidium [7].

Example 4: 95 percent confidence limits for a

single determination.

Rubidium chloride (SRM 984)

RbCl, rubidium assay, weight percent . . . 99.90 ± 0.02

"The indicated tolerance is at least as large as the 95 percent confidence level for a single

determination."

The 95 percent confidence limits for a single

determination are calculated as "tS," using the

same expression as that in (3) but without the

denominator, V^. The expression "^S" is without

statistical interpretation and a more meaningful

expression is desirable.

Example 5: 2-sigma or 3-sigma limits.

Glass Filters for Spectrophotometry (SRM 930a)

Absorbance . . . 0.500 ± 0.0022

"This uncertainty is the sum of the random error

of ± 0.1 percent {2SD limit) and of estimated biases

which are ± 0.4 percent."

Each glass filter was individually calibrated, and

the standard deviation refers to measurement

error, including the cleanliness of the surface. As

these glass filters wiU be used time after time, a multiple of the standard deviation is a proper

measure of variability. Note: The estimated sys- tematic error is much larger than the measurement

error, as usually is the case in measurements of

high precision.

Oxygen in Ferrous Metals (ppm by weight)

SRM Material Vacuum Neutron Inert Gas

No. Fusion Activation Fusion

X s n X s n

1090 Ingot Iron 484 14 216 492 28 6 497 13 12 1091 Stainless Steel 131 8 286 132 7 6 129 8 11 (AISI 431)

1092 Vacuum Melted 28 2 105 28 4 5 29 5 20

Steel

X = mean oxygen value; s = standard deviation of a single

determination; n = number of independent determinations.

(Note: The standard deviation includes error due both to the imprecision of the analytical method and to possible hetero- geneity of the material analyzed.)

One criticism against this mode of presentation

is that the user will have to compute the uncertainty

based on his own understanding of the relationships.

References

[1] Natrella, M. G., Experimental Statistics, NBS Handbook

Nat. Bur. Stand. (U.S.), HandB. 91, (Aug. 1966).

[2] Cali, J. P., Mandel, J., and Moore, L. J., A Referee Method

for the Determination of Calcium in Serum, Nat. Bur. Stand. (U.S.), Spec. Publ. 260-36. 136 pages (May 1972).

[3] Standard Recommended Practice for Use of the Term

Precision and Accuracy as Applied to Measurement of a Property of a Material, ASTM designation E177-71,

American Society for Testing and Materials, 1916 Race

Street, Philadelphia* Pa. 19103.

[4] Ku, H. H., Editor, Precision Measurement and CalibrationStatistical Concepts and Procedures, Nat. Bur. Stand.

(U.S.), Spec. Publ. 300, Vol. 1, 346 pages (Feb. 1967).

[5] Eisenhart, C. E., Contribution to Panel Discussion on Adjustment of the Fundamental Constants, Nat. Bur. Stand.

(U.S.), Spec. Publ. 343, (1970).

[6] Yakowitz, H., Fiori, C. E., and Michaelis, R. E., Standard

Reference Materials: Homogeniety Characterization of Fe-3Si Alloy, Nat. Bur. Stand. (U.S.), Spec. Publ. 260-22,

30 pages (Feb. 1971).

[7] Catanzaro. E. J., Murphy, T. J., Gamer, E. L., and Shields,

W. R., Absolute Isotopic Abundance Ratio and Atomic

Weight of Terrestial Rubidium, J. Res. Nat. Bur. Stand.

(U.S.), 73A (Phys. and Chem.), No. 5, 511-516 (Sept.-Oct.

1969).

18
IV. SPECIFIC AREAS OF USE

FOR SRM's

In today's technologically advanced world,

standardization is a must. The application of

meaningful measurement is being applied to an

ever growing list of properties of materials. Thus,

the documented need for additional Standard

Reference Materials has grown to unprecedented

proportions.

Science and technology are continuously invading

and exploring new and more severe environments

such as the reaches of space, the depths of the

ocean, and travel at supersonic speeds. The re- quirements for SRM's therefore have become

correspondingly more exacting and complex. For

example, chemical composition SRM's certified

for a half-dozen elements with an uncertainty of a

fraction of a percent were adequate for the steel

industry in the I920's. Some 50 years later, certifi- cation of up to 40 elements (some close to the onepart-per-million level) is necessary for certain

SRM's designed for composition control in evaluating

alloys having special metallurgical properties such

as strength, ductility, and corrosion resistance.

Significantly, recent specifications covering the

chemical composition of materials have been set with reduced allowable tolerances, and this trend

seems likely to continue. These specifications are

generally designed to ensure that the material

will fulfill all of the performance requirements of

the material's end-use. An example is the hightemperature alloys used in turbine blades, particularly those in jet engines. Not only is close control

necessary for major and minor constituent elements,

but also for certain tramp elements such as lead and

bismuth that have proved harmful to the performance of these materials. Recently specifications have been written limiting the content of lead

to a maximum of 5 parts-per-million and bismuth

to a maximum of a half-part-per-million. For applications where material failure could be catastrophic,

the close scrutiny of almost every element in the

periodic table wiU soon be necessary. Similar examples can be found for almost all segments of science

and industry. The following sections describe the

availability of SRM's, sources of methodology, and

standardization activities in five selected industries.

A. Metals Industries

Well over half of the currently available SRM's

are metals widely used in industrial applications.

Although a number have been certified for their

physical properties, the great majority of SRM's

are certified for chemical composition. The chemical

composition SRM's generally are provided in two

main forms— chips and solids. The chip SRM's are used in procedures that involve dissolution of the

sample before chemical analysis. These SRM's are used as controls or in the validation or development of chemical methods. The solid metal SRM's

are used primarily for the calibration and analytical

control using optical emission and x-ray spectroscopic methods of analysis.

1 . Chip Form Metal SRM's.

These SRM's consist generally of alloys whose

nominal compositions have been selected to provide a wide range of analytical values for the

various elements that are of vital concern to the

chemist and metallurgist. The chips are usually

sized between 16- and 40-mesh sieves and are prepared from selected portions of commercial

ingots produced to NBS specifications. The Certificates provided with each SRM give the chemical

composition determined at NBS and most Certificates also include values obtained by other

laboratories that cooperated in the analytical

program for certification. NBS issues a Catalog of

SRM's that provides the nominal chemical composition for each of these SRM's to serve as a guide to the user in the selection of the appropriate standards to meet his needs. The official document, however, is the Certificate. For illustrative purposes,

the Certificate of Analysis for Standard Reference

Material 8j, Bessemer Steel (Simulated) 0.1 percent

carbon is given as Appendix B. This is the official

document that accompanies SRM 8j. Some features

of this Certificate are important to note. The first standard was prepared in 1909 and it was called

Standard Sample No. 8. Subsequent renewals of

this material were numbered 8a, 8b, 8c. . . . 8j.

Thus, 8j is the eleventh lot of this type of material.

While each of the lots may differ somewhat in

detailed analyses, all retain the relatively high level

of phosphorus and sulfur and the low alloy metal

content characteristic of this type of steel. While the Bessemer steel making process is little used today (hence "Simulated" appears in the

descriptive name), the demand by analysts for this

composition provided sufficient justification to warrant its renewal in 1972. NBS and three in- dustrial laboratories cooperated in the analytical

program for certification of this standard. Two parts

of the Certificate are of most concern to the analyst:

first, the values reported by the laboratories and the

average or mean of those values, and secondly, a

brief synopsis of the methods employed for the determination of the various elements in the cooperating laboratories. This enables the analyst to see immediately the variability among laboratories.

Thus the analyst can objectively view the results

in the light of the methodology used to obtain them.

With respect to methodology, emphasis is made

that relative methods are normally not used in the

actual certification of a standard. Only reference

or umpire methods of demonstrated accuracy are used in the actual analyses. Although many laboratories have developed their own reference or umpire

methods of analysis, both NBS and cooperating'

laboratories rely heavily on Standard Methods

developed in interlaboratory (round-robin) compari19
son programs and subsequently published by the

American Society for Testing and Materials

(ASTM).

When an SRM similar in composition to the un- knowns to be analyzed is run as a "control" along

with the unknowns, analytical results obtained for

the SRM that are divergent from the certified value

usually indicate possible methodology problems.

The analyst should not apply correction factors to the "unknowns" based on divergent results of the

SRM's. Rather, modification and closer control of

his method should be undertaken until the results on the SRM control are substantially in agreement

with the NBS certified values. Then and only then can the analyst have reasonable assurance that the

numbers reported for the "unknowns" are reliable.

The chip form SRM's have been lathe cut with a serrated tool. The lathe cuttings are machine

ground, sieved (usually through 16- to 40-mesh

sieves) and then thoroughly blended. This provides

highly homogeneous lots of material of uniformly

sized particles. For many materials, particularly

cast irons, the finer sized particles may differ appreciably in composition from larger size particles;

consequently, the "fines" (smaller than 40 mesh)

and the very coarse (larger than 16 mesh) are discarded. However, in analyzing "unknowns" representative samples must be taken that include the

proper proportion of both coarse and fine particles.

Sampling and sample preparation techniques are described in detail in the Annual Book of ASTM

Standards (currently 1973 edition) [1].

2. Solid Form Metal SRM's.

A wide variety of SRM's have been prepared in

solid form for calibrating optical emission and

x-ray spectroscopic methods of analysis. These

methods are extensively used in the metals in- dustry by producers for the production quality

control of about 90 percent of their output, and by

consumers for acceptance testing of more than 75

percent of their purchases. Basically, spectroscopic

methods of analysis are comparative type methods

and require SRM's matching the test samples

closely in composition, form, and physical condition. To utilize the prime advantages of high speed

and high accuracy in the analysis of metals and

alloys by these methods, SRM's must be in forms

suitable for use without further processing. When

the first spectroscopic standards were issued in

1944, the materials were prepared in the form designed for emission spectrographic analysis only.

As direct-reading emission spectrometers came into

being and with the advent of x-ray fluorescence

analysis in the early I950's, a disk form equally

suitable to either method evolved. Since the late

1950's, all of the solid form steel standards made

available from NBS have been prepared as disks,

usually 32 mm (1 V4 in) in diameter and 19 mm

(^/4 in) thick. In recent years, material from the

same melt has been prepared in several forms to serve the needs of most analytical methods.

Spectrometric methods of analysis require a graduated set of SRM's for establishing analytical curves. Previously, most spectroscopists maintained

that it was necessary to have a minimum of three

standards for each different alloy to be analyzed,

and many spectroscopists wanted the standards

available in both chiU-cast and wrought metallurgical conditions. With the many thousands of

different kinds of alloys being produced, standardizing agencies such as NBS soon realized that they

could hope to fulfill only the basic needs of in- dustry for these standards. They also recognized

that the producer was in a position to prepare his

own standards (usually chiU-cast materials), but

the consumer normally did not have the capability

to produce the standards he needed for acceptance

testing (usually wrought products). Therefore,

NBS concentrated largely on the preparation of

wrought-type SRM's.

The solid form metal SRM's available from NBS

include various iron and steel compositions and

nonferrous base alloys. Despite the variety of

compositions of these SRM's and their individual

development, the history of the low-alloy steels

and associated iron SRM's present an excellent

example of the development of solid metal SRM's.

The low alloy steel SRM's are divided into three

groups: those prepared in the mid-l940's, those

in the mid-1950's (called the "1100" series), and

those introduced in 1970 (called the "1200" series).

The original low alloy steels were prepared for

spectroscopic methods of analysis from the cores

that remained after the ingots were lathe cut to produce the chip form SRM's. Those materials

that were found to be satisfactory after homogeneity

testing were fabricated into the final shapes and

sizes. Although entirely satisfactory for conventional

spectroscopic methods, these older SRM's generally

do not meet the stringent homogeneity requirement

needed by the newer microchemical methods.

Consequently many of these will be discontinued

when the present supply is exhausted.

The "1100" series was prepared in the mid-1950's

and consists of eight SRM's "tailored" to provide

a useful concentration range for some 25 elements.

The planning, preparation, homogeneity testing,

and certification of these SRM's were performed

through a major cooperative program between in- dustry and NBS. This series was planned to meet

the newer, critical requirements for calibration in

the iron and steel industry. Material for these SRM's

was prepared by the most modern melting, casting,

and fabricating techniques then available to provide

large quantities of the material of the highest

homogeneity. The materials were extensively

characterized including investigations by means of

electronprobe microanalyzers and quantitative

metaUographic techniques [2, 3].

First issued in 1957, the "1100" series gained

widespread acceptance. The demand for these

SRM's proved so great that the supplies of some of

20
the disk type SRM's were exhausted before their

planned replacements were available.

Consequently, a set of five SRM's -the "1200"

series — was planned and prepared to replace the

"1100" series.

The "1200" series consists of 4 low-alloy steels

and an electrolytic iron containing a graded series

of 40 elements. Material from the same melt is available in 4 forms: disks, SRM's 1261-1265,

31 mm in diameter and 19 mm thick for optical emission, and x-ray spectrometric methods; chips,

SRM's 361-365, for solution chemical methods;

rods, SRM's 661-665, 3.2 mm in diameter and 51

mm long for microchemical methods; and rods,

'SRM's 1095-1099, 6.4 mm in diameter and 102 mm

long for determining gases in metals by vacuum or

inert-gas fusion techniques or neutron activation

methods of analysis.

Extensive planning, preparation, homogeneity

testing, and certification were carried out for the

"1200" series involving a major cooperative program

between industry and NBS. Appendix C shows

the present status of the certification of the "1200"

series.

NOTE: The more recent Certificates of Analysis

for solid spectrochemical SRM's contain the following statement on precision and accuracy.

"The value listed for a certified element is the present best estimate of the "true" value based on the results of the analyti- cal program. The value listed is not expected to deviate from

the "true" value by more than ± 1 in the last significant

figure reported; for a subscript figure the deviation is not ex- pected to be more than ± 5. Based on the results of homogeneity testing, maximum variations within and among

samples are estimated to be less then the uncertainty figures

given above."

An example of the use of this statement is il- lustrated for SRM 1264. The carbon content in

1264 is certified at 0.87 o percent. This signifies

that the "true" carbon content is expected to be

in the range from 0.865 to 0.875 percent. The

silicon in 1264 is certified at 0.067 percent. This

signifies that the "true" silicon content of the

standard is expected to he within the range of

0.066 to 0.068 percent.

Unless the user has access to and utilizes the

proper methodology, SRM's alone are essentially

useless. The main reference for methodology is the

ASTM Book of Standards, Part 32. All proposed,

tentative, and standard methods of analysis for

both optical emission and x-ray fluorescence are

included. In addition, ASTM Committee E2 on Emission Spectroscopy publishes a compilation

that includes not only all of the methods and practices included in Part 32, but also a variety of

suggested methods that are based solely on the

authors' experiences. This compilation has been

invaluable in establishing suitable methods for

analysis of a variety of products — not only for new

laboratories, but also for well-established laboratories that are confronted with new analytical

problems. This compilation is generally published

every four years.

Numerous other books and periodicals exist

that contain both chemical and spectrochemical

methods of analysis. Most of these are referenced

Hi Chemical Abstracts published by the American

Chemical Society.

Where differences arise and agreement must be

reached between producer and consumer, the use

of ASTM methods together with suitable SRM's is recommended and extremely useful. These ASTM

methods have been tested and proven in extensive

interlaboratory comparison programs. Often these

programs will specify the use of standards, such as SRM's, together with unknowns analyzed by ASTM

chemical procedures. The general agreement that

is reached on these unknowns by the volunteering

laboratories attest to both the precision and to the

accuracy of the methods under test. Neither the National Bureau of Standards nor any other national laboratory can hope to provide

all the SRM's needed. Consequently, users should

know the commercial sources of standards [4] or be prepared to make their own standards.

3. Gases in Metals.

A variety of SRM's. both ferrous and nonferrous,

are available for the determination of gases in

metals, particularly oxygen and nitrogen, but also

including hydrogen, for titanium-base alloys. These

SRM's are for application in the vacuum fusion

method for the determination of gases in metals.

The nitrogen content, however, can be determined

by chemical and Kjeldahl techniques, while more

recently neutron activation has been used rather

extensively in the determination of oxygen in metals.

In addition, inert gas fusion techniques may be

used to determine oxygen, nitrogen, and hydrogen

in metals.

4. High-Purity Materials.

Several high-purity metal SRM's have been made

to meet the needs of analysts determining impurity

elements in metals. They are intended to serve as benchmarks in testing methods and equipment.

They are also expected to be valuable in developing

new or improved methods or techniques that extend

the sensitivity of detection in the determination of

trace constituents in various materials. On the

Certificate of Analysis for high-purity SRM's the

state-of-the-art information on the chemical composition is supplied. The Certificates are revised

whenever significant, new information is generated.

SRM's certified for their physical properties play

a vital role in servicing the National Measurement

System [5]. They serve as the transfer mechanism

for physical property measurements between

national standardizing laboratories and industrial,

university, and government laboratories.

A number of metal SRM's certified for their physical properties have been made available and

considerable information regarding these may be

found in the SRM Catalog [6J. Included are SRM's

certified for physical properties such as thermal con21
ductivity, thermal expansion, magnetic susceptibility, thermal emittance, and specular reflectance.

Specific details regarding the certification

measurements and the use of these SRM's are

stated on the Certificate that accompanies each

SRM. Most Certificates provide literature refer- ences to additional information that may be of

value to the user. For several of these SRM's, an NBS Special Publication in the "260 Series" has

been written and is available.

References

[1] ASTM Designation E30-73: Standard Methods for Chemical

Analyses of Steel, Cast Iron, Open-Hearth Iron, and

Wrought Iron. 1973 Annual Book of ASTM Standards, Part

32, American Society for Testing and Materials, Phila- delphia, Pennsylvania (1973).

[2] Standard Reference Materials: MetaUographic Characterization of an NBS Spectrometric Low-Alloy Steel Standard,

Nat. Bur. Stand. (U.S.), Misc. Publ. 260-3 (October 1964).

[3] Standard Reference Materials: Homogeneity Characterization of NBS Spectrometric Standards II: Cartridge Brass and Low-AOoy Steel. Nat. bur. Stand. (U.S.), Misc. PubL

260-10 (Dec. 1965).

[4] Report on Available Standard Samples, and High-Purity

Materials for Spectrochemical Analysis 1963, ASTM Spec.

Tech. Publ. No. 58-E. American Society for Tesimg and

Materials, Philadelphia, Pennsylvania (1963).

[5] Huntoon, R. D., Concept of a National measurement system.

Science 158, No. 3797, pp 67-71 (October 6. 1907).

[6] Standard Reference Materials: 1973 Catalog, Nat. Bur. Stand. (U.S.), Spec. Publ. 260, 1973 ed. 96 pages (April

1973).

B. Rubber Industry

Crude rubber, be it natural or synthetic, is a highly variable material. There is considerable art in the manufacture ot uniform, salable consumer

products from these crude materials.

Prior to World War II, the rubber products used

in the United States were made from natural

rubber. As World War II approached it became

evident that the supply of natural rubber would be

endangered. Research and development was begun

on a number of synthetic materials to assure the

supply of this critical commodity. By 1943, a number

of synthetic rubber plants were in operation.

In order to guarantee the uniformity of products

from the various plants, the U.S. Government took

steps almost immediately to establish a standardization program. The history of these developments

and the problems related to the establishment of a rubber SRM program have been described by

Roth and Stiehler [1]. The principal source of Standard Reference Materials for the rubber industry is the National

Bureau of Standards. In the selection of rubbers

and rubber-compounding materials to be issued as SRM's, the National Bureau of Standards depends

on the advice and recommendations of the ASTM

Sub-Committee Dll.20 on Compounding Materials

and Procedures. The present NBS-SRM program

is based on the following five rubbers:

SRM Type

385 ilallilal I tJJJiJcl 386 Styrene-butadiene type 1500 rubber (SBR 1500)

388 Butyl rubber

389 Styrene-butadiene type 1503 rubber (SBR 1503) 391 Acrylonitrile-butadiene rubber

Each of these rubber SRM's are characterized

for homogeneity and comparability to the preceding lots. The specific rubber is blended with the

appropriate SRM rubber-compounding ingredient

and vulcanized according to the most current re- vision of ASTM Method D15 [2]. The vulcanized

rubbers are then measured for Mooney viscosity, stress at a given elongation and at failure, and

strain under a given load. All except the natural

rubber (SRM 385) are measured for electrical conductivity [3]. Each rubber SRM is issued with a

Certificate.

Rubber-compounding SRM's are not issued

with Certificates. Consequently each renewal of a rubber-compounding material must be as similar as possible to its predecessor. At present, the state- of-the-art in rubber chemistry is such that numerical

values cannot be assigned to the properties of

interest. Indeed, at this time, it is not even certain

what the specific properties are that affect the vari- ous properties of rubbers. Therefore, every effort

is made to supply renewals that are chemically

similar to their predecessor, but more importantly,

the properties they impart to the vulcanizate must

be similar to those imparted by their predecessors.

The rubber-compounding SRM's currently available are:

SRM Type

370 Zinc oxide 371 Sulfur 372 Stearic acid 373 Benzothiazyl disulfide (MBTS)

374 Tetramethylthiuram disulfide (TMTD)

376 Light magnesia

377 Phenyl-B-naphthylamine

378 Oil-furnace black (HAF)

379 Conducting black

380 Calcium carbonate

381 Calcium silicate 382 Gas-furnace black (SRF)

383 Mercaptobenzothiazole

384 A^-tertiary-Butyl-2-Benzothiazolesulfenamide

When these materials are renewed, they are evaluated in the same manner as the rubber SRM's.

This is done by making two recipes, one with the

intended renewal and the other with the material

to be replaced. The new lot is accepted as the re- newal only after a consistency is established between the various rubbers and rubber-compounding

materials that is independent of the renewal lot.

22
Huge amounts of carbon black are used in the

formulation of rubber; therefore, a secondary

standard supply known as "Industrial Reference

Blacks" has been established by ASTM Committee

D-11 (Rubber) and D-24 (Carbon Black). The

present material is "Industrial Reference Black

No. 4" and is available from any sales office of the

Geoffrey Cabot Corporation.

Because of the importance of the changes in

properties of rubbers upon immersion in oils, a

series of ASTM Reference Oils has been developed.

These are designated as "ASTM Reference Oils

No. 1, No. 2, and No. 3" and are described in

ASTM Method D47 [2]. Oil No. 1 is no longer

available, and a replacement is being sought. Oils

No. 2 and No. 3 are available from the Sun Oil Co.,

Philadelphia, Pa.

The rubber industry in the United States has been

most active in the International Standards Organization Committee ISO/TC 45 on Rubber. Consequently, many of the ISO methods strongly resemble

those of ASTM. The ASTM methods for both rubber

and carbon blacks are described in Part 28 of the

Annual Book of ASTM Standards [2]. These standard procedures are developed and are continually

revised by ASTM Committee D-11 on Rubber

and D-24 on Carbon Black.

References

[1] Roth, F. L., and Stiehler, R. D., Standard Materials for Rubber Compounding, Proc. Intern. Rubber Conf. No.

8-13, 1959, p 232, American Chemical Society, Washington, D.C., 1959.

[2] Annual Book of ASTM Standards, Part 28, Rubber, Carbon

Black, Gaskets, American Society for Testing and Ma

terials, Philadelphia, Pa., 1972.

[3] McKinney, J. E., and Roth, F. L., Carbon black differentiation by electrical resistance of vulcanizates, Ind. Eng. Chem.

44,159(1952).

C. The Nuclear Measurement System

In the area of nuclear measurement, as in most

other areas where the measurements to be made are chemical (analytical) in nature, the primary chemical SRM's issued by NBS are applicable. These

primary chemical SRM's conform to the lUPAC

definition of primary chemicals in that they are at least 99.98 percent pure substances. These NBSSRM's can be used to calibrate typical chemical

systems such as acid-base reactions and oxidationreduction reactions. They are listed in Appendix D.

In addition to the chemicals usually thought of as primary standards, NBS issues three other materials that have primary applicability in the area

of nuclear materials for the analysis of chemical

composition and also a variety of other SRM's

either directly or indirectly related to nuclear

measurement systems. These SRM's were developed over a long period of time in conjunction

with the U.S. Atomic Energy Commission (AEC)

and are listed in Appendix E.

The uranium oxide chemical assay Standard,

SRM 950a, was developed as an outgrowth of the

NBS position as a referee laboratory in the 1950-

1960 period. A complete description of the development and a discussion of the stoichiometry problems

and their resolution is included in Analysis of

Uranium Concentrate at the National Bureau of

Standards [1], and Analytical Mass Spectrometry

Section: Summary of activities, July 1969 to June

1970 [2]. The plutonium metal standard, SRM 949 (now

SRM 949d), has been issued by the National

Bureau of Standards since 1962. This high-purity

chemical SRM was prepared and issued in cooperation with the AEC and is intended as an assay

standard for the analysis of plutonium and plutonium

based fuels. This particular SRM is one of the

primary standards used by fuel processors, fabricators, users, and regulating agencies.

While the use of the plutonium metal SRM is quite broad, the packaging, preparation, and analytical details present problems of maintaining

adequate supplies of this SRM. To remedy these

problems, the preparation of a plutonium chemical

compound for assay purposes was undertaken in

1967. A variety of plutonium compounds was in- vestigated for stability and stoichiometry as possible "candidate SRM's." A report on the stoichiometry and stability of plutonium sulfate tetrahydrate

by C. Pietri was published in 1968 [3]. As a result

of this report and other work at NBS and the

AEC's New Brunswick Laboratories, a chemical

assay standard of plutonium sulfate tetrahydrate

was prepared and is now issued as SRM 944 by

NBS as an alternate SRM for the plutonium metal

SRM, 949d. While the former SRM does not dupHcate the matrix effect of the latter SRM, it is more

readily available, more convenient to use, and is suitable for most analytical work.

In addition to the uranium and plutonium

chemical compounds, NBS issues a series of

Uranium oxide isotopic SRM's and three plutonium

isotopic SRM's as described in Appendix E. These

SRM's are intended to be used as calibration standards in making mass spectrometric measurements.

The details of preparation and analysis of these

materials are found in NBS Special Publication

260—27, Uranium Isotopic Standard Reference

Materials [4] and in NBS Technical Note 546, p. 24 [2]. A variety of other isotopic SRM's available

from NBS relates to the measurement of the agedating isotopes of strontium and lead and are

listed in Appendix F. One of these, SRM 988, is a precisely measured solution of pure strontium-84

to be used as a spike solution to make isotope

dilution age-dating measurements. Another SRM—

987, Strontium Carbonate — is a contemporary rock

and chemical assay standard. In addition to the

strontium dating SRM's, three lead isotopic SRM's

are issued for the calibration of mass spectrometers making lead dating measurements.

23
Besides the NBS role in the standardization of

nuclear measurements, other laboratories, especially those within the AEC. issue standards

relating to this same activity. The New Brunswick

Laboratory for example issues several series of

standards: Uranium bearing ore standards; pitchblende; and monazite counting standards; UF4,

UO3, Th metal, Be metal, and UO2 issued for

chemical assay and/or impurities; and three graded

series, ThO^- UsOg, and BeO. certified for a variety

of trace elements.

The AEC's Oak Ridge National Laboratory

(ORNL) issues a series of stable isotopic materials

with nominal isotopic compositions that vary from

those usually found in nature. In the past these were listed in the ORNL isotopes catalog; however,

they are now listed in Radio Isotopes, Stable

Isotope Research Materials [5]. The number of nuclear materials issued by foreign

nations is somewhat limited. However, NBS distributes fissile nuclear material to the International

Atomic Energy Agency (IAEA) member countries,

through IAEA; and to countries that have entered

into an AEC agreement on the use of fissile material.

Measurement methodology for nuclear material

is available from several sources. As this particular

discipline is relatively new, most laboratories have

developed their own analytical handbooks to deal

with the analytical problems that they face. The

Los Alamos Scientific Laboratory (LASL), Argonne

National Laboratory (ANL), and others have written

their own procedures outlining in detail the analytical methods used. A valuable source of specific

information in the general analysis of these materials is Analysis of Essential Nuclear Reactor

Materials [6]. The basic reference methods have been or will

be incorporated into the Book of ASTM Standards.

These now appear in the 1973 Book of ASTM

Standards, Part 32; but after 1974. they wiU appear

in a new. separate volume. Part 45. In addition,

most methods approved by ASTM are incorporated

without change as standard methods by American

National Standards Institute (ANSI).

Although other sources of standard methods or reference methods are somewhat limited, most

specific methods and essentially all of the general

methodology have been published in scientific

journals. Periodic reviews of the appropriate

journals provide a convenient means of staying

abreast of current analytical procedures.

References

[1] Anaylsis of Uranium Concentrate at the National Bureau of Standards, Nat. Bur. Stand. (U.S.) Misc. Publ. 260-8, 96

pages (Dec. 1965).

[2] Analytical Mass Spectrometry Section: Summary of Activities July 1969 to June 1970, NBS Tech. Note 546, pp. 23-24 (1970).

[3] Pietri, C, The Stability of Plutonium Sulfate Tetrahydrate,

and Analytical Standard: A Ten-Year Evaluation, Talanta,

18, 849-852 (1971).

[4] Uranium Isotopic Standard Reference Materials, Nat. Bur. Stand. (U.S.), Spec. Publ. 260-27, 40 pages (Feb. 1971).

[5] Radio Isotopes, Stable Isotope Research Materials, available from the Isotope Development Center, Oak Ridge National

Laboratory, P.O. Box X, Oak Ridge, Tennessee.

[6] Rodden, C. J., Analysis of Essential Nuclear Reactor Materials, C. J. Rodden, ed., 1—157, U.S. Atomic Energy Commission, Superintendent of Documents, U.S. Government

Printing Office, Washington. D.C. 20402 (1964).

D. Inorganic Standards

The Standard Reference Materials described in

this section are economically important either in

themselves or in the preparation of some product.

For the most part, these are naturally occuring

materials; consequently, they contain a wide range

of minor and trace constituents. Depending upon

the end use of the material, these constituents may

enhance or detract from the economic value or technological use of a material. Therefore, producers must carefully select the material that has

the best balance between desirable and undesirable

constituents. To aid industry in this selection process, NBS has issued a variety of SRM's certified for

chemical composition, including such materials as ores, cements, fertilizers, minerals, refractories,

and glasses.

The standardization efforts of NBS for inorganic

materials have been directed toward the accurate

determination of those constituents that are essential to the users, but not necessarily toward the total

characterization of the material.

Two fluorspars, SRM's 79a and 180, illustrate

these efforts. Although both SRM's are certified

for calcium fluoride content only, they are certified

for different purposes. SRM 180, a higher purity

fluorspar, is issued primarily for use by geochemists,

while SRM 79a is issued as an assay standard for

the U.S. Bureau of Customs and fluorspar importers.

SRM 79a is certified at 97.39 wt % CaFa primarily

for use by the Bureau of Customs to determine the

duty to be assessed on imported fluorspar, which

is based on the CaFa content. For acid-grade

fluorspar (containing at least 97% CaF-z), the duty

charged per ton is less than the duty charged for

metallurgical-grade fluorspar (containing less than

97% CaFa). In 1972 the duty for fluorspar was $2.10

per long ton for the acid grade and $8.40 per long

ton for the metallurgical grade. In that year, the

total fluorspar consumption in the U.S. was 1,276,329

long tons, of which 1,181,530 long tons were imported and for which duty was paid. Thus, the eco- nomic importance of such an SRM is easily seen. While the fluorspar SRM's illustrate the importance of the major constituent of a material, many

glasses require minor constituent measurement and

certification. Glasses generally consist of a mixture

of the oxides of silicon, barium, calcium, sodium,

and potassium. The physical properties of the

finished glass depend on the quantities of these

24
oxides, the presence and quantities of other metal

oxides, and the preparation techniques used (heating rate, anneaHng procedures, etc.)- For example,

a low-expansion, heat-resistant glass (borosilicate)

usually contains a significant amount of B2O3, and

its amount must be accurately controlled by the

producer.

To aid in the analysis of such glasses, NBS issues

two SRM's certified for B2O3 content: SRM 92,

Low-Boron glass (0.70 wt. % B2O3) and SRM 93a,

High-Boron glass (12.6 wt. % B2O3).

Currently NBS issues 14 ores, 8 cements, 3

fertilizers, 9 minerals, 7 refractories, and 6 glasses

certified for chemical composition. Seven of the

eight cements are new SRM's issued since the 1973

Catalog. The analyses of the cements are given in

Appendix G. Besides these SRM's a special glass

was prepared and doped with 61 trace elements to form (nominal) concentration levels at 500, 50, 1,

and 0.02 ppm. These SRM's, called Trace Elements in Glass, were developed for use in caHbrating instruments and in checking analytical

techniques and procedures employed in the determination of trace elements in glass and glass-like

inorganic matrices (e.g., ores, ceramics).

NBS also issues ten glass SRM's certified for

viscosity, relative stress optical coefficient, softening point, annealing point, and strain point.

In addition to the standards available from NBS

a variety of similar standards are available from

other sources including:

Geological Survey of Canada

Department of Energy, Mines, and Resources

555 Booth Street

Ottawa 4, Ontario

Bureau of Analysed Samples, Ltd.

Newham Hall

Middlebrough, Yorkshire, England

Office National des Measurements

Budapest XH Hungary

The Iron and Steel Institute of Japan

No. 5 Otemachi 1-chome

Chiyoda-Ku-Tokyo 100

Japan

Schweizerische Arbeitsgemeinschaft

fuer Stein und Erden

Mineralogisch-petrographische Institut

Sahhstrasse 6

Bern, Switzerland

The American Society for Testing and Materials,

1916 Race Street, Philadelphia, Pennsylvania,

19013, is currently revising its DS 2 Publication:

Report on Available Standard Samples, Reference

Samples, and High-Purity Materials for Spectrochemical Analysis (1963). This pubhcation provides

a ready reference for sources and availability of

standards, reference samples, and high-purity

materials.

Methods of analysis, specifications or research

information related to several inorganic SRM's are given below. Standardization activities for most of

these materials are under the sponsorship of the

American Society for Testing and Materials

through one of its committees: C-1 on Cements,

C-8 on Refractories, C— 14 on Glass and Glass

Products, and E-16 on Samphng and Analysis of

Metal Bearing Ores and Related Materials. Additional standardization activities for cements are conducted by the Portland Cement Association.

Sources of Information

Glasses:

1. Napolitano, A., and Hawkins, E. G., Viscosity of a Standard

Soda-Lime-Silica Glass, J. Res. NBS 68A (Phy. and Cham.),

No. 5, 439-448 (Sep- Oct 1964).

2. Napolitano, A., and Hawkins, E. G., Viscosity of a Standard

Borosilicate Glass, NBS Spec. Publ. 260-23 (1970).

3. Napolitano, A., and Hawkins, E. G., Viscosity of a Standard

Lead-Silica Glass, NBS Misc. PubL 260-11 (1966).

4. Annual Book of ASTM Standards, Part 13 (1973), Chemical

Analysis of Glass Sand, ASTM Designation C 146-66; Chemical

Analysis of Soda-Lime Glass, ASTM Designation C169-69; Test

for Annealing Point and Strain Point of Glass, ASTM Designation

C 336—69: Test for Softening Point of Glass, ASTM Designation

C 338-57.

5. Gills, T. E., Thompson, B. A., and Masters, L. W., Deter- mination of Trace Elements in Glass by Activation Analysis

Using Hydrated Antimony Pentoxide for Sodium Removal,

Anal. Chem. 42 , No. 14 (1970).

6. Maienthal, E. J. , Determination of Trace Elements in Silicate Matrices by Differential Cathode Ray Polarography, Anal. Chem. 45, No. 4 (1973).

7. Barnes, J. L., Gamer, E. L., Gramlich, J. W., Moore, L. J., Murphy, T. J., Machlan, L. A., Shields, W. R., Tatsumoto, M.,

and Knight, R. J., Determination of Lead, Uranium, Thorium

and Thallium in Silicate Glass Standard Materials by Isotope

Dilution Mass Spectrometry, Anal. Chem. 45, No. 6 (1973).

Ores:

8. Annual Book of ASTM Standards, Part 32 (1973), Test for

Silica in Iron Ores and Manganese Ores, ASTM Designation

E 247-71; Test for Manganese in Manganese Ore by the Pyrophosphate (Potentiometric) Method, ASTM Designation E 248-

68; Test for Total Iron in Iron Ores by Stannous Chloride Reduction and Dichromate Titration, ASTM Designation E 278-71;

Test for Manganese in Iron Ores, ASTM Designation E 314-71;

Test for Iron in Manganese Ores, ASTM Designation E 316-71;

and Chromium Oxide in Chrome Ores, ASTM Designation

E 342-71.

Minerals:

9. Shultz,J. L,BeU,R.R., Rains T. C. and Menis, O. Me'i^^Hs

of Analysis of NBS Clay Standards, NBS Spec. Publ. 260-37

(1972).

10. Annual Book of ASTM Standards, Part 13 (1973), Chemical

Analyses of Chrome Containing Refractories and Chrome Ores,

ASTM Designation C 572-70; Chemical Analysis of Fire Clay

and High-Alumina Refractories, ASTM Designation C 573-70;

Chemical Analysis of Magnesite and Dolomite Refractories,

ASTM Designation C 574-70; and Chemical Analysis of Silica Refractories, ASTM Designation C 575-70.

Cements:

11. Annual Book of ASTM Standards, Part 9 (1973), Standard

Specification for Portland Cement, ASTM Designation C 150-70;

25
Test for Fineness of Portland Cement by Air Permeability Apparatus, ASTM Designation C 204-68; and Test for Fineness of Portland Cement by Turbidimeter, ASTM Designation C 115-70.

E. The Clinical Measurement System

The practice of medicine is still fundamentally an art in terms of measurement science, primarily

because of the extreme complexity of physiological

systems. Consequently, a physician still depends

to a large extent on his ability to build up a lore, and

to use that lore to diagnose and treat various

maladies. During the past 100 years, a number of

scientific disciplines have come into being in the

practice of medicine. These include: radiology,

histology, microbiology, hematology, chnical chemistry, and others. This section is devoted almost

entirely to clinicfd chemistry.

The functioning of the human body, and indeed

any biological system, depends upon the coordinated operation of almost countless chemical

subsystems. Because people are different, the

parameters defining these systems vary considerably among healthy people [1], but show marked

deviations in the case of many illnesses. In principle, by keeping records of levels of various

chemical and biochemical constituents in the

various body fluids, cells, and tissues, the onset

of illness could be predicted before physical

symptoms are apparent.

The accuracy, and sometimes even the precision,

of the data generated by clinical chemistry laboratories is in need of improvement. Many practicing

physicians use clinical chemistry data to support

diagnoses based on other medical or physiological

symptoms. Further, because of the lack of precision and accuracy in these data, the physician

using such data usually applies a large safety factor.

Thus, normal ranges for various chemical constituents have been developed based on large populations. This situation cannot noticeably change until

the accuracy and precision of clinical chemistry data

are greatly improved.

Many problems associated with the imprecision

and inaccuracy of today's clinical chemistry data

may be attributed to four sources:

(1) The lack of high-quaUty reference standards.

(2) The lack of well-designed and critically

evaluated test methods.

(3) The rapid turnover and/or lack of motivation

among many of the people actually making

the measurements.

(4) The change in the specimen from the time it

is removed from the body until the actual

measurement of the desired parameter is begun.

Only SRM's and reference methods (1 and 2,

above) will be considered in this section.

CUnical chemistry is basically no different from

any other type of analytical chemistry. It depends

upon acid-base reactions, oxidation-reduction

reactions, the weighing of a precipitate, the evaluation of a color, etc. For such reactions, several

groups of SRM's are available from NBS.

These SRM's designed for acid-base and oxidation-reduction reactions are usually high-purity

chemicals, which are evaluated in terms of the

desired reaction. These "primary chemicals" are

fisted in Appendix D.

The use of these "primary" chemical SRM's

permit the achievement of the most accurate data

presently possible in the areas of acid-base and

redox titrimetry. However, these SRM's are not an end in themselves. Samples of these SRM's must

be accurately weighed; they must be dissolved into

solution using clean glassware cafibrated according to NBS instructions [2]; consideration must be

given to the temperature, i.e., the solution must be

allowed to reach equiUbrium at room or other

specified temperature before diluting to the volumetric mark; the quafity of the distilled water must

be frequently evaluated; and, consideration must

be given to the environment in which the measure- ments take place. An excellent example of highaccuracy oxidation-reduction titrimetry has recently

been pubfished [3].

1 . Spectrophotometry.

One of the more frequently used techniques in the

clinical chemistry laboratory is spectrophotometry.

The importance of this technique is enchanced

because it is often used to obtain the final number

from which the required data is calculated. Spectrophotometers tend to fall under the general heading

of "semiautomatic" and as such their maintenance

and calibration are frequently neglected or over- looked [4]. Accurate spectrophotometry depends principally on five parameters: the stability of the light source;

the performance of the monochromator and accurate

knowledge of the wavelength of the light as it passes

through the sample; the pathlength through the

sample; the reduction of stray light to a minimum;

and, the performance of the detector (photometer).

The stability of the light source is more or less an

inherent feature of the construction of the instrument. A certain amount of instability can be over- come by the use of double-beam instruments. The

control of this parameter does not lend itself to the use of SRM's.

The performance of the monochromator is also a

structural feature of the instrument. However, the

wavelength accuracy must be determined periodically at several points. The knowledge of the ac- curacy of one wavelength does not guarantee a corresponding accuracy or lack of bias at another

wavelength. The mercury emission spectrum offers a number of accurately known wavelengths, table 1

[5], and is the method of choice in those cases where

26
it can be adapted to the spectrophotometer. In

cases where the use of the mercury lamp is not convenient, the absorption spectra of holmium oxide

and didymium glass [6] provide a useful alternative,

table 2.

Table 1. Mercury reference wavelengths useful for spectrophotometer wavelength calibration

Wavelength (nm) Wavelength (nm)

1. 253.65 7 . 404.66

2. 269.73 8. 407.78

3. 302.15 9. 435.84

4. 313.16 10. 546.07

5. 334.15 11. 576.96

6. 365.01 12. 579.07

Almost invariably, clinical spectrophotometry

measurements are made on liquids. The liquids are contained in rectangular-parallel pipedal cuvettes,

and the pathlength of the beam through the liquid

to be measured is defined by the dimensions of

this cuvette. Cuvettes are available from many

commercial sources, but. the quality varies widely.

Table 2. Characteristic wavelengths of didymium and holmium

glasses

nm nm

Holmium 241.5 0.20 Holmium 279.4 .30 Holmium 287.5 .35 Holmium 333.7 .55 Holmium 360.9 .75 Holmium 418.4 1.10 Holmium 453.2 1.40 Holmium 536.2 2.30 Didymium 573.0 3.00 Didymium 586.0 3.00 Holmium 637.5 3.80 Didymium 685.0 4.50

A standard reference cuvette (SRM 932) is available

from NBS. SRM 932 is an all-quartz cuvette provided with two optically transparent and parallel

windows (the Certificate for SRM 932 is Appendix

H [7]).

Stray light is light of other than the desired wavelength passing through or around the sample at the

time of measurement. Stray light may be estimated

by setting the instrument at 100 percent transmittance, inserting a sharp cutoff" filter, and measuring the photometer reading. Any indicated trans- mittance may be attributed to stray light. There are no Standard Reference Materials for stray light,

although such work has been proposed.

The photometric response of the spectrophotometer is the measurement upon which the calculation

of the desired result is based. Consequently, much

effort has been expended to develop standards in

this regard for the accurate calibration of spectrophotometers. For many years NBS has offered a

series of glass filters identified as SRM 2101-2105,

Color Standards for Spectrophotometer-Tristimulus

Integrator Systems [8]. Since a small error in the

determination of the wavelength will result in a gross error in the transmittance, the use of a more

neutral filter for this purpose is highly desirable;

that is, a filter whose transmittance shows relatively

small variations with wavelength. Such filters are issued as SRM 930a, Glass Filters for Spectrophotometry (see Appendix I). While glass filters provide a highly accurate

means of calibrating the photometric components

of a spectrophotometer, in many cases the use of

such filters is either inconvenient or nearly impossible. To alleviate this problem, NBS developed

SRM 931, Liquid Absorbance Standards for Ultraviolet and Visible Spectrophotometry (see Appendix

J). The filters in SRM 931 were prepared by dissolving high-purity cobalt and nickel in a mixture

of nitric and perchloric acids. The absorption

spectra of the resulting solution is shown in figure 1.

_l I I -J I 300 400 500 goo TOO

WAVELENGTH, nm

Figure 1. Relative absorbance spectra of NBS-SRM 931, Liquid Filters for Spectrophotometry.

27
The maxima at 302 and 512 nm is caused by absorption by NOiT and Co(H20)e^, respectively. The

maximum at 395 nm and the plateau at 650-700 nm

are caused by Ni(H20)6 ^. The pH of these solutions

is about 1. The absorption curve (fig. 1) of this SRM

is wavelength dependent [9]. The certified values for

"net absorbance" of SRM 931 are given on the

Certificate (Appendix J). The possibility exists that measurements could

be made with an instrument meeting all criteria

for accuracy and precision and still yield poor

results. This could be caused by faulty operator

techniques, problems with cuvettes, or improper

preparation of solutions. To test the entire spectrophotometric measurement system, a stable, easily

prepared solution having accurately known spectral characteristics is highly desirable. Several

such solutions have been proposed, some of which

are listed below:

(a) An alkaline solution of potassium chromate

[10].

(b) Solutions of potassium dichromate in 0.01

sulfuric acid at concentrations of 0.0500 and

0.1000 gram per liter [11].

(c) Solutions of cobalt ammonium sulfate in

dilute sulfuric acid [8]. (d) Potassium nitrate solution.

NBS has supplies of certain high-purity salts

that are currently being issued as SRM's for other

purposes. These include potassium hydrogen

phthalate (SRM 84h), potassium dichromate (SRM

136c), potassium nitrate (SRM 193), and benzoic

acid (SRM 350). NBS plans to measure the absorption spectra of solutions of each of these materials

and may issue a set of the four materials as an SRM with directions for preparing solutions of

known characteristics.

2. Ion-Selective Electrodes.

The use of the ion-selective electrode in the

clinical laboratory is not new. The determination

of acidity (pH) has been carried out for many years

[12]. More recently, the use of ion-selective electrodes has been extended to include various in- organic electrolytes [13]. Still more recently,

electrodes have been designed to measure organic

compounds [14]. As the areas of usefulness of the

various ion-selective electrodes are extended and

their miniaturization is achieved, their use in the

clinical laboratory should increase markedly.

It has long been recognized that the pH of blood

reflects the acid-base balance of the body. The pH

level is controlled between relatively narrow limits

by intricate mechanisms involving the production,

elimination and buffering of acids by the body. Its measurement is of great significance and may be

related to a large number of specific pathologic

conditions, including respiratory, gastro-intestinal,

and renal diseases [15].

In routine measurements of pH, the electrode

and emf measuring potentiometer used are not

highly stable and must be calibrated periodically

for accurate work. The subject of standards for

acidity measurements has been discussed [16],

and NBS has issued a series of SRM's defining the

pH scale from 1.7 to 10.0. The mixed phosphate

buffers (SRM 1861c, KH2PO4. and SRM 18611c,

Na2HP04) have been useful in the normal physiological range, although problems with their use,

such as precipitation of serum calcium, do exist.

To correct some of these problems, mixtures of

tris(hydroxymethyl)aminomethane, and its hydrochloride have come into use, and SRM's of these

materials are now available (see Appendix K).

The determinations of other electrolytes by ionselective electrode techniques are performed as

for pH. Again, careful calibration is necessary for

accurate results. At present, three SRM's are

available for calibration of ion-selective electrodes

(Appendix L). Each SRM is certified for the activity

coefficient of the cation and anion as well as the

p(Ion) value at several concentrations. Plans have

been made to prepare' calcium and silver SRM's for

ion-selective electrodes.

3. Clinical SRM's.

The generalized techniques described previously

and the SRM's that control them are used to determine many different constituents of body fluids.

Also available from NBS is a group of SRM's for

standardization of methods used for specific components (Appendix M).

AU of the materials listed in Appendix L are

highly purified. The serum constituent to which

they refer is obvious, except that of D-Mannitol

(SRM 920) which is used in the determination of

triglycerides. Each of the SRM's is dissolved in

the appropriate solvent and subjected to the same

manual or automatic treatment as the serum. The clinical SRM's have been criticized as being

"useless in this form . . . what the clinical laboratory

needs are SRM's in a serum or other matrix." However, analytical biological chemistry has not reached

the state-of-the-art where this can be done. Homogeneous sera samples could doubtlessly be

prepared, but at present this would lead only to improvement in precision, while the goal of standardization in clinical chemistry is compatibility through

accuracy. At this time, the only route to accuracy

is through the use of a reference method that will

give accurate results with both pure-material SRM's

and serum. When reference methods are perfected,

the preparation of accurately characterized serum

SRM's will be possible.

For electrolytes, the state-of-the-art is such that

most constituents can be determined directly in

serum using isotope-dilution mass spectrometry.

Certain other electrolytes can be determined by

"classical chemical" or carefully controlled spectro28
scopic methods. In cooperation with the Center for

Disease Control, NBS is studying the development

of SRM's containing sodium, potassium, calcium,

magnesium, lithium, and chloride at three concentrations in serum, and the development of

associated reference methods. Such development

will be a genuine break-through, but the certification of similar organic and biochemical-type SRM's

in serum must await the development and improvement of organic analytical methodology.

Research and development work on additional

SRM's is being pursued. In many cases this work

demands the development of new analytical competences. A priority list for clinical SRM's (Appendix N) was developed by the NBS Analytical

Chemistry Division Clinical Chemistry Advisory

Panel in 1973, and work on the higher priority

materials is underway. The Catalog of Standard

Reference Materials will list these materials when

they become SRM's.

4. Reagent Chemicals.

As equally important as the use of SRM's to control the quality of data, is the use of quality

reagents used to produce the data. These reagents

serve a variety of functions: reactant, stabilizer,

solvent, buffer, catalyst, cofactor, etc. Each reagent

chemical added to the chemical system introduces

some impurities, some of which may produce

unexpected and possibly undesirable results [17].

At present two major works deal with specifications for reagent chemicals. One is published in the

United States by the American Chemical Society

[18], the other in Great Britain by Analar Standards,

Ltd. [19]. Both of these sets of specifications require

minimum assay values and define in depth the

limits for impurities.

Throughout the world there are a number of

pharmacopeia] compendia. These primarily define

chemicals that are to be used as drugs so that there

will be no toxic or other deleterious effect. There

are two such volumes in the United States, the

United States Pharmacopeia [20] and National

Formulary [21].

A generalized scheme was devised by the

lUPAC Analytical Standards sub-committee to classify reagent chemicals [22]. This system

embraces only highest purity chemicals as follows:

Grade A: atomic-weight standard.

Grade B: ultimate standard— a substance which

can be purified to virtually atomicweight standard.

Grade C: primary standard — a commercially

available substance of purity 100 ± 0.02

percent.

Grade D: working standard — a commercially

available substance of purity 100 ±0.05

percent.

Grade E: secondary standard— a substance of

lower purity, which can be standardized against primary (Grade C)

material.

Obviously, a great many reagents in the area of

biological and organic chemistry do not fit into this

system. Radin has worked for several years to ex- tend this classification to include such materials.

However, his efforts have not been widely accepted

to date. The National Academy of Sciences has

attempted to fill this gap with "Specification and

Criteria for Biochemical Compounds" [23] which

provides specifications and criteria for about 500

biochemicals.

The most important, and possibly the least

studied, reagent in clinical chemistry is reagent

water. Nothing varies so much from laboratory to laboratory or with time. Generally, the quality of

reagent water is assessed on the basis of its conductivity or concentrations of inorganic salts. Only

the specialist thinks about possible organic and biological impurities. Specifications for reagent water

are given in the ACS Reagent Chemicals [18] and

in the ASTM Book of Standards [24]. The problems

and implications arising from the contaminants

in reagent water have been documented by

Winstead [25].

5. Kits.

In recent years the use of self-contained kits has

greatly increased in clinical chemistry, especially

in the physician's office or in the small laboratory.

A kit has been defined as any single reagent or combination of reagents and instruments packaged

together with instructions for use in a clinical

laboratory. This definition could be quite easily

interpreted to include everything from the simplest

reagent test tube ensemble intended for a specific

determination to a multichannel automated analysis

machine.

There are some 400 companies in the United

States engaged in the manufacture of clinical kits.

These manufacturers and the nature of their kits are described in the publication "List of Test Kits

for Clinical Laboratories" issued by the Center

for Disease Control [26]. For data on any specific

kit, the reader should write directly to the

manufacturer.

Studies have been made by J. E. Logan and his

associates at the Canadian Communicable Disease

Center on methods and instrument in use in

Canadian Clinical Laboratories [27, 28]. These

workers have also studied the merits of various

kits for the determination of calcium [29]. glucose

[30], urea nitrogen [31], and cholesterol [32].
6. Sources of Methodology.

In most of the sections of this publication, the

methods described as used in the laboratories are

of the standardized type. Such methods have been

developed by consensus procedures and often have

a quasi-legal status. Such methods are developed

by the American Society for Testing and Materials,

the International Standards Organization, and vari- ous special groups such as the Trade Association

of Pulp and Paper Industries (TAPPI) and the

American Association of Textile Chemists and

Colorists (AATCC).

In the clinical laboratory the situation is different.

No standard methods exist in the ASTM sense. In

general, the methods used in a given laboratory are chosen by the director of that laboratory from the

journal literature and textbooks. Many of the

methods used in clinical laboratories are the

methods adopted by a kit manufacturer or are those employed by the automated systems in use

in that laboratory.

The following textbooks and journals are suggested without comment as sources of laboratory

methods.

a. Textbooks

(1) Henry, R. J., Clinical Chemistry, Principles

and Technics, Hoeber Medical Division,

Harper and Row, 1964 (and later revisions).

(2) Todd-Sanford-Clinical Diagnosis by Laboratory Methods, 14th Edition, O. Davidson

and J. B. Henry, Ed.. W. B. Saunders Co.,

Philadelphia, Pa. 1969.

(3) Fundamentals of Clinical Chemistry, N. W.

Tietz, Ed., W. B. Saunders Co., Philadelphia, Pa. 1970.

(4) Reynolds, M. D., Clinical Chemistry for the

Small Hospital, C. C. Thomas, Springfield, lU. 1969.

b. Journals

(1) American Journal of Clinical Pathology

(monthly), American Society of Clinical

Pathologists, Chicago, lU.

(2) Clinical Chemistry (monthly), American Association of Clinical Chemists, WinstonSalem, N.C.

(3) Clinical Biochemistry (monthly), Canadian

Society of Clinical Chemists, Ottawa,

Ontario.

(4) Clinica Chimica Acta (monthly), Elsevier

Publishing Co., Amsterdam, Netherlands.

(5) Analytical Chemistry (monthly), American

Chemical Society, Washington, D.C.

Every second year, a review of analytical

methods used in the clinical laboratory

is published in this journal, usually in

the April issue, as was the case in 1973

[33].

c. Standard Methods in Clinical Chemistry.

In recent years, the American Association of

Clinical Chemists has published a periodical

treatise entitled. Standard Methods of Clinical

Chemistry. These volumes tend to foUow the

technique of the much older Organic Syntheses.

A chemist submits what he feels is a good method.

This method is then checked by two or more other

clinical people. If found satisfactory, it is published.

The following criteria have been established for

choosing a "Standard Method:"

(1) The method has been published previously,

therefore is not presented as new material.

(2) The method may offer an unusually advantageous feature for determining the un- known, such as the Berthelot reaction for

ammonia used in the analysis of urea

nitrogen.

(3) The method is "sound," has "stood the test of

time," and is "widely used."

(4) The method improves another one by over- coming objectionable features and by in- creasing desirable qualities such as

sensitivity, precision, and reliability.

(5) The method promotes the accuracy of

analysis, e.g.. by introducing a more nearly

pure standard.

7. Reference Methods.

NBS offers a number of SRM's for use in the steel

industry, which are usually in such a form that they can be used side by side with the steels being

analyzed. In clinical chemistry this is not the case. The NBS clinical SRM's are pure materials, whereas

in the clinical laboratory the substances being ana- lyzed are usually in a complex matrix such as serum,

urine, or other body fluid. The present state-of-the-art has not developed

sufficiently to enable NBS to issue an SRM in which

a clinically important component is incorporated

into serum. Thus, accurate methods are necessary

that will give equally accurate results for a given

component regardless of whether that component

is in the pure form or in serum. The need for such

accurate methods has lead to the development of

reference methods.

A reference method is defined as a method of

known and proven accuracy, that is, the principal

systematic errors or biases of the measurement

process have been found, mechanisms elucidated,

and these errors eliminated, or reduced to tolerable

levels, or when actual physical elimination is impossible, a correction applied [34]. A reference

method is not necessarily a legally imposed, a preferred, or a qualification method. It is not necessarily an easily used method, indeed, it may be

quite possible that relatively few scientists wiU be

capable of using it. The real value of a reference

method is its use in evaluating the more commonly

used daily routine methods.

30
One reference method. Calcium in Serum [34],

is an atomic absorption spectrometry analysis based

on the method published by Pybus, Feldman, and

Bowers [35]. This method was carefully worked

through several interlaboratory comparison exercises; the results were evaluated after each exercise

and the procedure was changed as necessary.

Fortunately, an absolute value for calcium in serum

could be obtained by the extremely accurate isotopedilution mass-spectrometric method. After five

exercises, it was felt that the state-of-the-art had

been reached. The reference method for calcium

is estimated to give a value for calcium that is within ±2 percent of the "true value."

Over the next few years a number of other refer- ence methods will be developed to bring greater

accuracy to clinical chemistry.

8. Standardization Activities.

A quality control system is essentially an intelli- gence or surveillance operation. It yields information

that can be used to bring about remedial action and

initiate a search for sources of error. A complete

system of quality surveillance includes two aspects.

The first is the measurement of internal precision

or reproducibility and is concerned with internal

comparability. The second is an estimate of comparability with other laboratories. One way to accomplish this inter-laboratory comparability is by the exchange of stable reference samples.

Proficiency Testing is a special type of external

surveillance of the laboratory. It is, usually limited

in scope, intermittent, and relatively slow in re- sponse. It is a method by which a large number of

laboratories may be intercompared, but it should

not be regarded as a substitute for internal quality

control [36].

Quality control systems involve the distribution of

samples to the participating laboratories. A goal for

precision and accuracy may or may not be established for a given exercise. The distributed samples

are analyzed by the participating laboratories, the

data returned to the evaluating group and statistically analyzed. Each laboratory is evaluated and,

if appropriate, rated on its relative attainment of

the predetermined goal. If necessary, remedial and

improvement measures are suggested.

At present there are two major interlaboratory

quality control operations in the United States.

Since 1949, the Standards Committee of the College

of American Pathologists has maintained this

service for its members. More recently, as a result

of the Medicare Act, the Center for Disease Control

has established a "Proficiency Testing" program to evaluate clinical laboratories operating in interstate

commerce and/or under the Medicare Program.

In addition to the above, two major activities are directed toward standardization of materials, re- agents, apparatus, and methods. The Standards

Committee of the American Association of Clinical

Chemists is engaged in development of SRM's,

the studies of control materials, and education of

clinical personnel. The group is divided into several

subcommittees including, e.g.. Organic Standards,

Spectrophotometry Standards, Protein Standards,

Inorganic Standards, and Enzymes. The National

Committee for Clinical Laboratory Standards consists of representatives of professional societies,

clinical laboratory supplies manufacturers, and

government. Its goal is a series of standard specifications and methods in areas of clinical chemistry,

hematology, instrumentation, and microbiology.

Additional activities are underway at the State

level in many places. As the importance of accuracy

in clinical chemistry is generally recognized, the

work of these various standardization activities

will assume added importance.

References

[1] Young, D. S., Factors Influencing the Concentration of Serum Constituents in Well Individuals, American Assoc.

of Clinical Chemists Meeting, Cincinnati, Ohio, August 24, 1972.

[2] Hughes, J. C, Testing of Glass Volumetric Apparatus, NBS

Circ. 602 (1959).

[3] Sappenfield, K. M.. Marinenko, G., and Hague, J. L., Comparison of Redox Standards, Nat. Bur. Stand. (U.S.), Spec. Publ. 260-24, 97 pages (Jan. 1972).

[4] Rand, R. M., Practical Spectrophotometric Standards,

Clin. Chem. 15,839 (1969).

[5] Manual of Recommended Practices in Spectrophotometry,

Amer. Soc. for Testing & Matls., Philadelphia, Pa. (1966).

[6] Edisbury, J. R., Practical Hints on Absorption Spectrometry

(Plenum Publishing Co., New York, N.Y., 1%7).

[7] Mavrodineanu, R., and Lazar, J. W., Standard Reference

Materials: Standard quartz cuvettes for high-accuracy

spectrophotometry, Clin. Chem. 19, No. 9, 1053 (Sept.

1973).

[8] Standards for checking the calibration of spectrometers

(200-1000 mix); NBS Letter Circ. No. 1017 (1955).

r91 Mavrodineanu, R., Nat. Bur. Stand. (U.S.), Tech. Note 584, O. Menis and J. I. Schultz, Eds. 175 pages (Dec. 1971).

[10] Haupt, G. W., An alkaline solution of potassium chromate

as a transmittance standard in the ultraviolet, J. Opt.

Soc. Amer. 42 , 441 (19.52). [11] PSG Collaborative test of 94 recording spectrometers,

Photoelec. Spectrometry Group Bull. 16,441 (1965).

[12] Bates, R. G., Determination of pH, Theory and Practice,

(John Wiley & Son, New York, New York, 1964).

[13] Ion-Selective Electrodes, R. A. Durst, Ed., Nat. Bur. Stand.

(U.S.), Spec. Publ. 314, 474 pages (Nov. 1969).

[14] Guilbault, G. G., and Shu, F. R., Enzyme electrodes based

on the use of a carbon dioxide sensor. Urea and 1 -Tyrosine

Electrodes, Anal. Chem. 44, 2161 (1972).

[15] Meinke, W. W., Standard Reference Materials for clinical measurements. Anal. Chem. 43, 28A (1971).

[16] Bates, R. G., Standardization of acidity measurements,

Anal. Chem. 40, (1968).

[17] Schaffer, R., Diagnostic Materials in Clinical Analysis,

International Conference on Standardization of Diagnostic

MaterieJs, Atlanta, Georgia, June 1973.

[18] Reagent Chemicals, 4th Edition, Am. Chem. Soc, Washington, D.C., 1968.

[19] Analar Standards for Laboratory Chemicals, 6th Edition,

Analar Standards, Ltd., London, 1%7.

31
[20] The Pharmacopeia of the United States of America, 18th

Revision, the United States Pharmacopeisil Convention,

Bethesda, Maryland, 1970.

[21] The National Formulary, 13th Edition, American Pharmaceutical Association, Washington, D.C., 1970.

[22] Report Prepared by the Analytical Standards Subcommittee, Analyst 90, 251 (1%5).

[23] Specifications and Criteria for Biochemical Compounds,

3rd edition, E. S. Tipson, Ed., National Academy of Sciences, Washington, D.C., 1972.

[24] Annual Book of ASTM Standards, Part 23 -Water; Atmosphere Analysis, Amer. Soc. for Testing & Matls., 1972.

[25] Winstead, M., Reagent Grade Water, Am. Soc. Med. Tech.,

The Steck Company, Austin, Texas, 1967.

[26] List of Test Kits for Clinical Laboratories (Clinical Chemistry, Hematology, Microbiology), Center for Disease

Control, Atlanta, Ga., 1972.

[27] Logan, J. E., A Survey of Methods and Instruments in Use

in Laboratories of Members of the Canadian Society of

Clinical Chemists Across Canada, Clin. Biochem. 2,215

(1969).

[28] Logan, J. E., Renton, H. M., and Allen, R. H., The use of diagnostic kits and reagents in the Clinical Laboratories

of Canada, Clin. Biochem. 3, 81 (1970).

[29] Sunderland, M. L. E., Weatherbum, M. W., and Logan, J. E., Observations on diagnostic kits for the determination of calcium in serum, Clin. Biochem. 4, 16 (1971).

[30] Logan, J. E., WaddeO, L. D., and Krynski, I. A., Observations on diagnostic kits for the determination of glucose, Clin. Biochem. 4, 16 (1971).

[31] Logan, J. E., Renton, H. M., and Eby, P. W., Observations

of diagnostic kits for the determination of urea nitrogen,

Clin. Biochem. 2, 189 (1969).

[32] Krynski, I. A., and Logan, J. E., Observations on Diagnostic

Kits for the Determination of Cholesterol, Clin. Biochem.

2, 105 (1%8).

[33] Gochman, N., and Young, P. S., Analytical Review— Clinical Chemistry, Anal. Chem. 45, IIR (1973).

[34] Cali, J. P., Mandel, J., Moore, L., and Young. D. S., A

referee method for the determination of calcium in serum,

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-36, 136 pages (May 1972).

[35] Pybus, J., Feldman, F. J., and Bowers, G. N., Jr., Measure- ment of Total Calcium in Serum by Atomic Absorption

Spectrometry, With Use of a Strontium Internal Reference,

Clin. Chem. 16,998 (1970).

[36] Boutwell, J. H., Quality Control— Clinical Chemistry, Center

for Disease Control, Atlanta, Ga., 1%8.

32
Appendices

Page A. List of NBS Publications in the Standard Reference Materials "260" Series 34

B. Certificate of Analysis, Standard Reference Material 8j, Bessemer Steel (Simulated),

0.1% Carbon 36

C. Summary of Compositions, 1200 Series Iron and Steel Standards 37

D. Primary, Working, and Secondary Standard Chemicals Available from the National

Bureau of Standards 38

E. Special Nuclear Standard Reference Materials Available from the National Bureau

of Standards 39

F. Isotopic Reference Standards Available from the National Bureau of Standards 40

G. Summary of Composition for Portland Cements, SRM's 633-639 40

H. Certificate, Standard Reference Material 932, Quartz Cuvette for Spectrophotometry. _ 41

I. Certificate, Standard Reference Material 930a, Glass Filters for Spectrophotometry. _ 43

J. Certificate, Standard Reference Material 931, Liquid Absorbance Standards for

Ultraviolet and Visible Spectrophotometry 46

K. Standard Reference Materials for pH Available from the National Bureau of

Standards 50

L. Ion-Selective Electrode Standard Reference Materials Available from the National

Bureau of Standards 50

M. NBS — Clinical Standard Reference Materials 51

N. Priority of Work on Chnical Standard Reference Materials 51

33
Appendix A. List of NBS Publications in the Standard Reference Materials

"260" Series

NBS Spec. Publ. 260, Standard Reference Materials:

1973 Catalog, April 1973, $1.25.*

NBS Misc. Publ. 260-1, Standard Reference Materials: Preparation of NBS White Cast Iron

Spectrochemical Standards, June 1964, NTIS

No. COM 74-11061, Springfield, Va. 22151.**

NBS Misc. Publ. 260-2, Standard Reference Materials: Preparation of NBS Copper-Base Spectrochemical Standards, October 1964, NTIS No.

COM 74-11063, Springfield, Va. 22151**.

NBS Misc. Publ. 260-3, Standard Reference Materials: Metallographic Characterization of an NBS

Spectrometric Low-AUoy Steel Standard, October

1964, NTIS No. COM 74-11060, Springfield, Va.

22151.**

NBS Misc. Publ. 260-4, Standard Reference Materials: Sources of Information on Standard Refer- ence Materials, February 1965, NTIS No. COM

74-11059, Springfield, Va. 22151.**

NBS Misc. Publ. 260-5, Standard Reference Materials: Accuracy of Solution X-Ray Spectrometric

Analysis of Copper-Base Alloys, March 1965.

(Out of print).

NBS Misc. Publ. 260-6, Standard Reference Materials: Methods for the Chemical Analysis of

White Cast Iron Standards, July 1965.

NBS Misc. Publ. 260-7, Standard Reference Materials: Methods for the Chemical Analysis of NBS

Copper-Base Spectrochemical Standards, October

1965, NTIS No. COM 74-11067, Springfield, Va.

22151.**

NBS Misc. Publ. 260-8, Standard Reference Materials: Analysis of Uranium Concentrates at the

National Bureau of Standards, December 1965,

NTIS No. COM 74-11066, Springfield, Va.

22151.**

NBS Misc. Publ. 260-9, Standard Reference Materials: Half Lives of Materials Used in the Preparation of Standard Reference Materials of Nineteen

Radioactive Nuclides Issued by the National

Bureau of Standards, November 1965, NTIS No.

COM 74-11065, Springfield, Va. 22151.**

NBS Misc. Publ. 260-10, Standard Reference Materials: Homogeneity Characterization on NBS

Spectrometric Standards II: Cartridge Brass and

> Low-Alloy Steel, December 1965, NTIS No.

COM 74-11064, Springfield, Va. 22151.**

NBS Misc. Publ. 260-11, Standard Reference Materials: Viscosity of a Standard Lead-Silica Glass,

November 1966. 25 cents.*

NBS Misc. Publ. 260-12. Standard Reference Materials: Homogeneity Characterization of NBS

Spectrometric Standards III: White Cast Iron and

Stainless Steel Powder Compact. September

1966, 20 cents.*

NBS Misc. Publ. 260-13, Standard Reference Materials: Mossbauer Spectroscopy Standard for the

Chemical Shift of Iron Compounds, July 1967.

40 cents.*

NBS Misc. Publ. 260-14, Standard Reference Materials: Determination of Oxygen in Ferrous Materials- SRM 1090, 1091, and 1092, September

1966. 30 cents.*

NBS Misc. Publ. 260-15, Standard Reference

Materials: Recommended Method of Use of

Standard Light-Sensitive Paper for Calibrating Carbon Arcs Used in Testing Textiles for

Colorfastness to Light, June 1967. Superseded by

SP260-41.

NBS Spec. Publ. 260-16, Standard Reference

Materials: Homogeneity Characterization of

NBS Spectrometric Standards FV: Preparation and Microprobe Characterization of W-20%

Mo AUoy Fabricated by Powder Metallurgical

Methods, January 1969, NTIS No. COM 74-11062,

Springfield, Va. 22151.**

NBS Spec. Publ. 260-17, Standard Reference

Materials: Boric Acid; Isotopic and Assay

Standard Reference Materials, February 1970.

65 cents.*

NBS Spec. Publ. 260-18, Standard Reference

Materials: Calibration of NBS Secondary Standard Magnetic Tape (Computer Amplitude

Reference) Using the Reference Tape Amplitude

Measurement "Process A," November 1969.

Superseded by SP260-29.

NBS Spec. Publ. 260-19, Standard Reference

Materials: Analysis of Interlaboratory Measurements on the Vapor Pressure of Gold (Certification of Standard Reference Materid 745),

January 1970. 30 cents.*

NBS Spec. Publ. 260-21, Standard Reference

Materials: Analysis of Interlaboratory Measurements on the Vapor Pressures of Cadmium and

Silver, January 1971. 35 cents.*

NBS Spec. PubL 260-22, Standard Reference^

Materials: Homogeneity Characterization of

Fe-3Si AUoy, February 1971. 35 cents.*

NBS Spec. Publ. 260-23, Standard Reference

Materials: Viscosity of a Standard BorosiUcate

Glass, December 1970. 25 cents.*

NBS Spec. Publ. 260-24, Standard Reference

Materials: Comparison of Redox Standards,

January 1972. $1.*

NBS Spec. Publ. 260-25, Standard Reference

Materials: A Standard Reference Material

Containing Nominally Four Percent Austenite,

February 1971. 30 cents.*

NBS Spec. Publ. 260-26, Standard Reference

Materials: National Bureau of Standards —

34
U.S. Steel Corporation Joint Program for Determining Oxygen and Nitrogen in Steel, February

1971. 50 cents.*

NBS Spec. Publ. 260-27, Standard Reference

Materials: Uranium Isotopic Standard Reference

Materials, April 1971. $1.25.*

NBS Spec. Publ. 260-28, Standard Reference

Materials: Preparation and Evaluation of SRM's

481 and 482 Gold-Silver and Gold-Copper Alloys

for Microanalysis, August 1971, NTIS No. COM

71-50365, Springfield, Va. 22151.**

NBS Spec. Publ. 260-29, Standard Reference

Materials: Calibration of NBS Secondary Standard Magnetic Tape (Computer Amplitude

Reference) Using the Reference Tape Amplitude

Measurement "Process A-Model 2," June 1971,

NTIS No. COM 71-50282, Springfield, Va.

22151.**

NBS Spec. Publ. 260-30, Standard Reference

Materials: Standard Samples Issued in the

USSR (A Translation from the Russian), June

1971, NTIS No. COM 71-50283, Springfield,

Va. 22151.**

NBS Spec. Publ. 260-31, Standard Reference

Materials: Thermal Conductivity of Electrolytic

Iron SRM 734 from 4 to 300 K, November 1971,

NTIS No. COM 71-50563, Springfield, Va.

22151.**

NBS Spec. Publ. 260-32, Standard Reference Materials: Standard Quartz Cuvettes For High Accuracy Spectrophotometry. December 1973. 55

cents.

NBS Spec. Publ. 260-33, Standard Reference Materials: Comparison of Original and Supplemental

SRM 705, Narrow Molecular Weight Distribution

Polystyrene, H. L. Wagner, May 1972, NTIS No.

COM 72-50526, Springfield, Va. 22151.**

NBS Spec. Publ. 260-34, Standard Reference Materials: Thermoelectric Voltage, April 1972, NTIS

No. COM 72-50371, Springfield, Va. 22151.**

NBS Spec. Publ. 260-35, Standard Reference Materials: Thermal Conductivity of Austenitic Stainless Steel, SRM 735 from 5 to 280 K, April 1972.

35 cents.*

NBS Spec. Publ. 260-36, Standard Reference Materials: A Referee Method for the Determination of

Calcium in Serum. SRM 915, May 1972, NTIS

No. COM 72-50527, Springfield, Va. 22151.**

NBS Spec. Publ. 260-37, Standard Reference Materials: Methods of Analysis of NBS Clay Standards,

June 1972, NTIS No. COM 72-50692, Springfield,

Va. 22151.**

NBS Spec. Publ. 260-38, Standard Reference Materials: Preparation and Calibration of Standards

of Spectral Specular Reflectar'-c, May 1972.

60 cents.*

NBS Spec. Publ. 260-39, Standard Reference Materials: The Eddy Current Decay Method for

Resistivity Characterization of High-Purity Metals,

May 1972. 55 cents.*

NBS Spec. Publ. 260-40, Standard Reference Materials: Selection of Thermal Analysis Temperature

Standards Through a Cooperative Study (SRM

758, 759, 760) August 1972. 65 cents.*

NBS Spec. Publ. 260-41, Standard Reference Materials: Use of Standard Light-Sensitive Paper for

Calibrating Carbon Arcs used in Testing Textiles

for Colorfastness to Light, August 1972, NTIS No.

COM 72-50775, Springfield, Va. 22151.**

NBS Spec. Publ. 260-42, Standard Reference Materials: The Characterization of Linear Polyethylene, SRM 1475, September 1972. 45 cents.*

NBS Spec. Publ. 260-43, Standard Reference Materials: Preparation and Homogeneity Characterization of an Austenitic Iron-Chromium-Nickel AUoy,

November 1972. 45 cents.*

NBS Spec. Publ. 260-44, Standard Reference Materials: Preparation and Use of Superconductive

Fixed Point Devices, SRM 767, December 1972.

75 cents.*

NBS Spec. Publ. 260-45, Standard Reference Materials: Powdered Lead-Based Paint, SRM 1579,

March 1973. 50 cents.*

NBS Spec. Publ. 260-47, Standard Reference Materials: Electrical Resistivity of Electrolytic Iron,

SRM 797, and Austenitic Stainless Steel, SRM

798, from 5 to 280 K. February 1974. 55 cents.*

NBS Spec. Publ. 260-48, Standard Reference Materials: Description and Use of Precision Thermometers for the Clinical Laboratory, SRM 933

and SRM 934, May 1974. 60 cents.*

*Send order with remittance to Superintendent of Documents. U.S. Government

Printing Office, Washington. D.C. 20402. Remittance from foreign countries should

include an additional one-fourth of the purchase price for postage. **Send order to: National Technical Information Service (NTIS), Springfield. Va.

22151.

35
Appendix B. Certificate of Analysis, Standard Reference Material 8j,

Bessemer Steel

Rational ^ureau of ^lanrfarcls

Certificate of ^nalgsis

Standard Reference Material 8j

Bessemer Steel (Simulated), 0.1% Carbon

C ivin P

c Si Cu Ni Pr V Mo

ANALYST

Combustionfhromatographic

s s

Cl> <

Spectrophotometric

c

o1 o = »

c ^ U H

Perchloric

acid

dehydration

Spectrophotonietric

Spectrophotometric

Spectrophotonietric

1 0.08P 0.50.5b 0.098 0.077"^ 0.059"' 0.022' 0.114 0.048f 0.0158 0.037

2 .080 .500 .095'' .076 .048<' .018* .110*

( .048j 1

\ .046''

)

.016'' .038

3 .082 .505

( .094"

\

\ .095h

1

.077<= (.056<i)

(.060'

)

.020'" (.116")

1-115 1 .046° .014P .041

4 .080 (.509 ) \.508'i)

( .095t>) (.094 )

.078"^ (.063<'l

( .065'

)

.020'" (.110")

(.113 i

.046^ .015t .038

Average 0.081 0.505 0.095 0.077 0.058 0.020 0.113 0.047 0.015 0.038

? l-g sample, tin-coated copper accelerator. "Potentiometric titration. „

""l-g sample burned in oxygen at 1425 C and sulfur dioxide absorbed in starch-iodide solution. Iodine liber- ated from iodide by titration, dunng the combustion,

.with standard KIO3 solution. "'Double dehydration. ^Diethyldithiocarbamate photometric method. Chromium separated from the bulk of the iron in a 10-g sample by NaHC03 hydrolysis, oxidized with peroxydisulfate, and titrated potentiometrically with ferrous ammonium sulfate. ^Vanadium separated as in (f), oxidized with HNO3 and

titrated potentiometrically with ferrous ammonium sul- fate.

^ A Ikali-molybdate.

} Same value obtained by gravimetric method,

i Diphenylcarbazide photometric method.

, Atomic absorption spectrometry.

' Gravimetric - tAg2P2^1-

'"Neocuproine photometric method.

"Dimethylglyoxime gravimetric method.

"Peroxydisulfate oxidation - Fe(NH4XS04)2 - KMn04

titration. PFeS04-(NH4)2 S;208-KMn04 titration. "IKI04 photometric method.

' Silicomolybdate photometric method. ^Chromium oxidized with peroxydisulfate and titrated amperometrically with ferrous ammonium sulfate. tKMn04-KN02-Urea-Fe(NH4)2(S04)2.

The material for the preparation of this standard was prepared at the Carpenter Technology

Corporation, Reading, Pennsylvania.

The overall direction and coordination of the technical measurements leading to certification were performed under the chairmanship of 0. Menis and J. 1. Shultz.

The technical and support aspects involved in the preparation, certification, and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference Materials by R. E. Michaelis.

Washington, D.C. 20234 J. Paul Cali, Chief

April 10, 1972 Office of Standard Reference Materials

List of Analysts

1. S. A. Wicks, R. K. Bell, and E. R. Deardorff, Analytical Chemistry Division, Institute for

Materials Research, National Bureau of Standards.

2. R. W. Bley, Inland Steel Company, East Chicago, Indiana.

3. R. H. Rouse, Bethlehem Steel Corporation, Sparrows Point, Maryland.

4. F. P. Valente, U. S. Army Materials and Mechanics Research Center, Watertown, Massachusetts.

36
Appendix C. Summary of Compositions, 1200 Series

Iron and Steel Standards

NBS No. 1261 1262 1263 1264 1265

Element Percent, by weight

C 0 . 382 0 16g 0.62 0 .87(5 0 0067 Mn . 66 1

.

0 4 1.50 • 255 0057

P .015 0

.

042 . 02g . Olp 0025

S .017 038 .008 .028 .0059

Sx . 223 39 . 74 . 067 .00 80

Cu . 042 50 •24g 0058

Ni 1 . 99 59 . 32 .142 041

Cr 0 . 69 30 1.31 . 065 0072

V .011 04i 0. 31 .IO5 0006 Mo .19 068 .030 .49 0050

W . OI5 21 . 045 .10 ('^ a 00004 )

Co .030 30 .0 48 . 15 007„

Ti .020 084 . 050 . 24 0006 As .017 092 . 010 •05, , , ( 0002)

Sn .Oil 016 (.095) [ .005]^/ 0002)

Al { I Ota i J .02^ 095 . 24 ( .008) ( 0007)

Nb . 022 29 . 049 .157 (< 00001) Ta .0 20 20 ( . 053) .11 - (< 00005)—

B .0005 0025 . 0009]_ .011 00013

Pb n n n n 0 00043 rt rt O 0 .024

Zr ,009 19 .0 49 .068 - (< 00001)

Sb . 0042 012 • OOlg ( 035) - (< A n A A C \ 00005

)

Bi. . 0004 1

\

.

002) (

. 0009) - (< A A A A T \ U 0 0 U i ) Tin . 0004 <

K • 0010) ( . 0038) (

. 00002) n 0 n n n 9

1

\J \J ij \J ^ / All (< . 00005) (.<. 00005) . 0005 0001 -(< 000002)

(< .0001) (. 0002) (< . 0001) « 0001) -« 00001)

Ma { .0001) {. 0006) (.0005) ( 0001) -(< 00002)

Se .004 [. 001] [.OOOIJ I 0003] -(< 00001) Te . 0006 (. 0005) (.0022) I 0002] - « 00001) Zn { .0001) (. 0005) (.0004) [ 001] {< 0001)^

Ce .OOI3 (. 0011) (.0016) ( .00025) -(< 000005) La .0004 0004 . 0006 . 00007 - {< 000005)

Nd . 0003 (. 0005) (.0007) (.00012) - (< 000005) Pr ( . 00014) (. 00012) C. 00018) ( 00003) - (< 000005)

Hf [. 0002] I. 006] 1.0015] I 005] - (< 00002)

N ( .0037)* u 0041)

*

(. 0041)* I 003]*, (-^ 0011)

0 ( .0009)

(. 0011) (.0007) , I 0017] (.-^ 0063)

»

H [< . 0005] K. 0005] I<.0005] I< 0005] (-v- 0001)

Ge I . 006] [. 002] I. 010] I .003] ('^' 0014)

Fe {by diff.) (95 .6) (95. 3) (94.4) (96 7) (99 9)

1/

_l* From gasometric certificates, SRM's 1095 through 1099

+ Isotopic dilution mass spectrometry - 0.00265; nuclear traclt - 0.00234

X Revised final value

a/ Values in parentheses are not proposed for certification. These values usually were obtained by a single analytical method of analysis,

b/ Values in brackets also are not proposed for certification. They are nominal or approximate values

from the heat analyses.

c/ Dash indicates "not detected." Values in parentheses following the dash are conservative "upper

limits" of detection by specific methods of analysis.

37
Appendix D. Primary, Working, and Secondary Standard Chemicals

Available from the National Bureau of Standards

SRM Type Certified Use Purity

17 Sucrose Polarimetric Value

40h Sodium Oxalate Reductometric Value 99.95 41a Dextrose (D-Glucose) Reductometric Value C)

83c Arsenic trioxide Reductometric Value 99.99 84h Acide potassium phthalate Acidimetric Value 99.99 136c Potassium dichromate Oxidimetric Value 99.98 350 Benzoic Acid Acidimetric Value 99.98 723 Tris(hydroxymethyl)aminomethane Basimetric Value 99.97 944 Plutonium Sulfate Tetrahydrate Assay 100 949d Plutonium Metal Assay 99.99 950a Uranium oxide (UsOs) Uranium Oxide Standard Value 99.94 951 Boric Acid Acidimetric and Boron Isotopic Value 100.00 960 Uranium Metal Assay 99.975 984 Rubidium Chloride Assay and Isotopic 99.90 987 Strontium Carbonate Assay and Isotopic 99.98 988 Strontium-84 Spike Assay and Isotopic 99.9

999 Potassium Chloride A c. J J r [Potassium Assay Standard ior 1/-,, , ., [Chloride

99.98

99.99

^ Sucrose — Moisture < 0.01 percent. Reducing Substances < 0.02 percent. Ash 0.003 percent.

' Dextrose— Moisture < 0.2 percent. Ash < 0. 0 1 percent.

38
Appendix E. Special Nuclear Standard Reference Materials Available from the

National Bureau of Standards

Number Description Certified for

^960

949d

944

945

946

947

948

Uranium oxide

Uranium metal

Plutonium metal

Plutonium sulfate

tetrahydrate

Plutonium metal

matrix

Uranium oxide content 99

Uranium content 99.975%

Plutonium content 99.99%

Plutonium content 47.50%

Trace elements

''Isotopic Abundance (Atom Percent

)

238 p„ 239p,j 240py 241p„ 242p„

Plutonium sulfate 0.247 83.128 12.069 3.991 0.565

tetrahydrate

Plutonium sulfate 0.296 75.696 18.288 4.540 1.180

tetrahydrate

Plutonium sulfate 0.011 91.574 7.914 0.468 0.0330

tetrahydrate

Isotopic Abundance (Atom Percent)

234 235u 236u 238U

U-0002 U3O8 Depleted 0.00016 0.01755 <0.00001 99.9823

U-005 U3O8 Depleted .00218 .4895 .00466 99.504

U-010 U3O8 Enriched .00541 1.0037 .00681 98.984

U-015 U3O8 Enriched .00850 1.5323 .0164 98.443

U-020 U3O8 Enriched .0125 2.038 .0165 97.933

U-030 U3O8 Enriched .0190 3.046 .0204 96.915

U-050 U3O8 Enriched .0279 .5.010 .0480 94.915

U-100 U3O8 Enriched .0676 10.190 .0379 89.704

U-150 U3O8 Enriched .0993 15.307 .0660 84.528

U-200 U3O8 Enriched .1246 20.013 .2116 79.651

U-350 U3O8 Enriched .2498 35.190 .1673 64.393

U-500 U3O8 Enriched .5181 49.696 .0755 49.711

U-750 U3O8 Enriched .5923 75.357 .2499 23.801

U-800 U3O8 Enriched .6563 80.279 .2445 18.820

U-850 U3O8 Enriched .6437 85.137 .3704 13.848

U-900 U3O8 Enriched .7777 90.196 .3327 8.693

U-930 U3O8 Enriched 1.0812 93.336 .2027 5.380

U-970 U3O8 Enriched 1.6653 97.663 .1491 0.5229

^ Available without license

^ Isotopic composition slowly changes due to radioactive decay.

39
Appendix F. Isotopic Reference Standards Available from the National

Bureau of Standards

SRM Isotopic Reference Standards Element Certified

951 Boric Acid Boron

952 Boric Acid. 95% enriched '"B Boron

975 Sodium Chloride Chlorine

976 Copper Metal Copper

977 Sodium Bromide Bromine

978 Silver Nitrate Silver 979 Chromium Nitrate Chromium

980 Magnesium Metal Magnesium

*981 Lead Metal, Natural Lead

*982 Lead Metal, Equal Atom (206/208) Lead

*983 Lead Metal, Radiogenic (92%- 206) Lead

984 Rubidium Chloride, assay and isotopic Rubidium

987 Strontium Carbonate, assay and isotopic Strontium

988 Strontium-84 Spiked, assay and isotopic Strontium

*Sold as a set of three only; 981, 982, and 983.

Appendix G. Summary of Composition for Portland Cements,

SRM's 633-639

SRM No. 633 634 635 636 637 638 639

Constituent P-e-r- c-e-n-t by w- e-i-g-h- t

2 3

3.7^ 5 . 2 6 . 2 3.1 3 . 3 4 . 5 4 .

3

CaO 64.5 62.^

6 59.3 63.5 66.

0

62.^ 65.3

MgO I.O4 3.4 1.25 4.0 0.72 3.84 1.2g

Ti02 0.24 0. 30 0.32 0.17 0.21 0.25 0.31

Si02 21.9 20.7 18.5 23.2 23.1 21.4 21.6

SO3 2.1g 2.1^

6

7.0 2.3 2.33 2.3 2.4

P2O5 0.24 0.10 0.17 0.09 0.25 0.06 0.08

M^2°3 0.04 0.28 0.09 0.12 0.06 0.05 0.08

O.I65 0.43 0.45 0.57 0.243 0.59 0.05

Na20 0.64 0. 14 0.07 0.10 0.13 0.12 0.63

SrO 0.31 0.12 0.22 0.04 0.10 0.07 0.15

Fe203 4.2 2.8.7 2.65 1.62 1.80 3.5g 2.42

Loss on

Ignition 0.75 1.6^ 3.25 1.6g 0.95 1.0

40
Appendix H. Certificate, Standard Reference Material 932,

Quartz Cuvette for Spectrophotometry

Rational ^ureau af ^imdntds

Certificate

Standard Reference Material 932

Quartz Cuvette for Spectrophotometry

R. Mavrodineanu and J. W. Lazar

This Standard Reference Material consists of a single, accurately calibrated cuvette that is issued

for use in the production of accurate spectrophotometric data on liquids. The design and dimensions of the all-quartz cuvette are shown in Figure 1. The pathlength of the cuvette is defined by the

distances between the two optically transparent windows taken at several heights within the

cuvette. Cuvettes issued as Standard Reference Material 932 range in pathlength between 9.97 and

10.03 mm. The inner surfaces of the windows are parallel within 0.002 mm. The pathlength and

parallelism are certified wdth an uncertainty of ±0.0005 mm as determined by measurements (at 20

C) taken at each 4 mm of height at positions within the cell from 2 mm of the bottom to within 6

mm of the window top.

Cuvette number 46 is issued with this certificate. For this cuvette the following measurements

were obtained.

Height Pathlength

mm (approx.) mm

37 (Top) 9 .9946

33 9 .9938

29 9 .9938

25 9 .9942

21 9 .9946

18 9 .9954

14 9 .9958

10 9 .9963

6 9 .9963

2 (Bottom) 9 .9967

The cuvette must be handled with great care and should be held only by the frosted-quartz side

windows. When not in use, it should be stored in the container provided for this purpose. Extended

exposure to laboratory atmosphere and dusty surroundings is to be avoided.

Washington, D. C. 20234 J. Paul Cali, Chief

November 5, 1973 Office of Standard Reference Materials

41
Appendix H. Certificate, Standard Reference Material 932,

Quartz Cuvette for Spectrophotometry — Continued

The cuvette was designed and produced at the National Bureau of Standards using special

techniques and non-fluorescent optical-quality fused silica. The transparent windows are attached to

the body of the cuvette by direct fusion, and the unit was stress-released by annealing. The overall

flatness of each surface of a transparent window is within two fringes (546 nm Hg line), and each

window's surfaces are parallel within one micrometer. The radiation pathlength measurements of

the cuvette were performed using electronic feeler-gauge type instruments capable of a resolution of

5 parts in 10^. The development and production of SRM 932 is a result of the combined efforts of

the Analytical Chemistry Division of the Institute for Materials Research, the Optical Physics

Division of the Institute for Basic Standards, and the Instrument Shops Division.

E. P. Muth and E. I. Klein designed and assembled the cuvette, respectively, and the actual

radiation pathlength measurements were performed by E. G. Erber.

The overall direction and coordination of the technical measurements leading to certification were performed under the chairmanship of 0. Menis and J. A. Simpson.

The technical and support aspects involved in the preparation, certification and issuance of this

Standard Reference Material were coordinated through the Office of Standard Reference Materials

by T. W. Mears.

I* 10- J

1—

r

T~ IlU

f —+1 10

RADIATION

PATHLENGTH'

.25-

6 TOP

SIDE

48

*-

(FROSTED)

FRONT

(TRANSPARENT)

BOTTOM

TOP

MEASUREMENT

LEVEL

1.25

1 BOTTOM

MEASURMENT

LEVEL

-M 12.5 KI

I

-10 -

12.5

"T"

12.5

INCIDENT RADIATION BEAM

42
Appendix I. Certificate, Standard Reference Material 930a,

Glass Filters for Spectrophotometry

^attunal "^mmn of ^tanrfardg

Certificate

Standard Reference Material 930a

Glass Filters for Spectrophotometry

R. Mavrodineanu and J. R. Baldwin

This Standard Reference Material is intended as a reference source for the calibration of the

photometric scale of spectrophotometers. It consists of three individual filters. Each filter bears an

identification number, and the upper left corner has been removed to indicate correct placing in the

metal holder.

Filter and Set

Identification

Number

ABSORBANCE (A) TRANSMITTANCE (%T)

Wavelength and (Bandpass)

nm

Wavelength and (Bandpass)

nm

440.0

(2.2)

465.0

(2.7)

590.0

(5.4)

635.0

(6.0)

440.0

(2.2)

465.0

(2.7)

590.0

(5.4)

635.0

(6.0)

1- 179

2-179

3- 179

1.027

0.770

0.511

0.954

0.716

0.4661

1. 052

0.788

0.514

1.003

0.752

0.501

9. 39

16.98

30.84

11.11

19.24

34 .19

8.87

16.29

30.60

9 .94

17.71

31.56

The transmittance values given are certified with a relative uncertainty of ±0.5 percent (example:

a nominal value of absorbance of 0.500 ± 0.0022). This uncertainty is the sum of the random error of ± 0.1 percent (2SD limit) and of estimated biases which are ± 0.4 percent. These biases are due to

possible systematic errors. Measurements were made at 25 °C. Room temperature variations within

several degrees Celsius of this temperature will not significantly affect the calibration of these

filters.

It is recommended that the filters be handled only by the edges and with soft plastic (polyethylene) gloves and optical lens tissue. When not in use they should be stored in their holders and

in the box provided for this purpose. Extended exposure to laboratory atmosphere and dusty sur- roundings should be avoided.

The overall direction and coordination of the technical measurements leading to certification were performed under the chairmanship of 0. Menis and J. I. Shultz.

The technical and support aspects involved in the preparation, certification, and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference Materials

by T. W. Mears.

Washington, D. C. 20234 j. Paul Cali, Chief

May 15, 1973 Office of Standard Reference Materials

43
Appendix I. Certificate, Standard Reference Material 930a,

Glass Filters for Spectrophotometry — Continued

The transmittance measurements were made with the high-accuracy spectrophotometer designed

and constnicted at the National Bureau of Standards [1]. The accuracy of this instrument was

established by hght-addition measurements.

The neutral NG-4 and NG-5 glasses for the filters were provided by Schott of Mainz, Germany,

and are designated as "Jena Colored and Filter Glass." Nominal transmittance for a filter 1.5 mm

thick is 20 percent at 400.0 nm wavelength and 32 percent at 700.0 nm wavelength. Between these

limits the transmittance varies in a monotonic manner [2]

.

The filter holder is black anodized aluminum alloy and has the following nominal dimensions:

height: 57 mm, width: 13 mm, depth: 13 mm. This holder and the size and shape of the filters were

selected to conform to the dimensions of the sample compartment of most conventional spectrophotometers. The filters are approximately 30.5 mm long, 11 mm wide, and 2.0 mm and 1.5 mm

thick for the NG-4 glass and 1.5 mm thick for the NG-5 glass. Corresponding to these thicknesses

are nominal transmittances of 10, 20, and 30 percent, respectively. These thicknesses v/ere selected

to |)rovide a means for calibrating the photometric scale at three different levels. The exposed

surface of the glass filter is approximately 29 mm by 8 mm, starting from a distance of 1..5 mm

from the filter holder base (see figure).

The transmittance of filters depends on the intrinsic properties of the material. Spectral bandpass, wavelength [2,3,5] , geometry of the optical beam, surface conditions, and positioning of the

filter also affect the transmittance values, and can lead to further biases. The certified data will be

reproduced when transmittance mf^asurements are made under similar conditions. The effective

spectral bandpasses used to determine the certified values are given on the face of the certificate and

the transmittance measurements are made by producing the vertical image of the slit (about 8 mm

by 0.5 mm), using a convergent beam geometry with an opening of f:10, in the middle of the

entrance face of the glass filter. (The filter should be positioned in the spectrophotometer as shown

in the figure, to obtain correct values.)

Prior to the certification measurements, each filter was examined for surface defects and then

thoroughly cleaned. If, through handling, the surface of the filter becomes contaminated with dust,

it may be cleaned with a small soft brush attached to a rubber tube connected to a vacuum source

[2,4]. If the surface becomes contaminated with fingerprints, they must be eliminated before

making measure ments. This may be accomplished by removing the filter from its holder, breathing

lightly on it, and rubbing the surface gently with optical lens tissue. The clean filter is then replaced

in its proper position in its holder. To remove and replace the filter in the metal holder, the

spring-loaded plate should be removed with care to prevent damage to the filter. As little handling

as possible is recommended.

NOTE: The check of the calibration of photometric scales defines only one of the parameters

required for obtaining accurate transmittance values and molar absorptivities. Other factors that

also must be estabhshed are wavelength accuracy, stray light, cell parameters, fluorescence, polarization, reflection, and temperature coefficient. Some of these variables are discussed in NBS publications [1,2,5]. It is planned to summarize various aspects of accurate spectropbrvmff ic iiK aoure- ments in an NBS-260 Special Publication that would provide additional data on specific Standard

Reference Materials. In the interim, SRM 930a, should be used as described in the certificate.

Consult the manufacturer of the instrument if differences are obtained that exceed those specified

by the manufacturer.

We wish to acknowledge the cooperation of George N. Bowers, Jr., M.D., of Hartford Hospital,

Hartford, Connecticut; Royden N. Rand, Ph.D., of the Hospital of the University of Pennsylvania,

Philadelphia. Pennsylvania;' and Donald S. Young, M.B., Ph.D., of the National Institutes of Health,

Bethesda, Maryland.

44
Appendix I. Certificate, Standard Reference Material 930a,

Glass Filters for Spectrophotometry — Continued

1. R. Mavrodineanu, An Accurate Spectrophotometer for Mcasfiring the Transmittance of Sohd

and Liquid Materials, NBS Journal of Research 76A, No. 5, 405-425 (1972).

2. R. Mavrodineanu, SoUd Materials to Check the Photometric Scale of Spectrophotometers, NBS

Tech. Note 544, O. Menis and J. I. Shultz, ed., pp 6-17, U.S. Government Printing Office,

Washington, D.C. 20402 (Sept. 1970), ibid NBS Tech. Note 584, pp. 2-21 (December 1971).

3. K. S. Gibson, Spectrophotometry. NBS Circ. 484 (Sept. 1949).

4. J. R. Edisbury, Practical Hints on Absorption Spectrophotometry, Plenum Press, New York

(1967).

5. Collected Papers from NBS Conference on Accuracy in Spectrophotometry and Luminescence

Measurements, NBS Journal of Research 76A, No. 5, 375-510 (1972).

METAL HOLDER FOR THE COLORED GLASS FILTERS

FRONT TOP

METAL HOLDER

57 X 13 X 13

EXPOSED FILTER SI 29 X 8

r

COLORED GLASS F

1.5

DIMENSIONS IN mm

45
Appendix J. Certificate, Standard Reference Material 93 1 , Liquid Absorbance

Standards for Ultraviolet and Visible Spectrophotometry

Rational ^urmu of ^tmdnxAs

C^rltftcat^

Standard Reference Material 931

Liquid Absorbance Standards for

Ultraviolet and Visible Spectrophotometry

R. W. Burke and E. R. Deardorff

These liquid filters are intended as reference solutions for checking the accuracy of the

photometric scale of narrow bandpass spectrophotometers. They are applicable for calibrating those

instruments that can provide an effective spectral bandpass of 1.5 nm or less at 302 nm, 2.0 nm or

less at 395 nm, 3.3 nm or less at 512 nm and 8.5 nm or less at 678 nm.

Net Absorbance

FUter

Wavelength and (Bandpass) , nm

302(1.0) 395(1.7) 512(2.0) 678(6.5)

"A"-"Blank" 0.307±0.003 0.304±0.003 0.303+0.003 0.11510.002

"B "-"Blank" 0.608±0.005 0.605±0.005 0.606±0.005 0.229±0.003

"C"-"Blank" 0.906±0.007 0.907±0.007 0.911±0.007 0.34510.003

Net ahsorbances ("A"—"Blank," "B"—"Blank," and "C"—"Blank") were determined using 10.00 mm cuvettes at 25.0 °C, see Instructions for Use.

Absorbance measurements were performed on a high precision double-beam spectrophotometer equipped with a double monochromator. The accuracy of the photometric scale of this instrument was established with the NBS high-accuracy spectrophotometer described by R.

Mavrodineanu [1] . The uncertainties of the certified values are twice the standard deviation,

commonly referred to as the "95 percent confidence level."

These filters are certified for absorbance at 25.0 °C. Absorbances at other temperatures in the

range 17 to 37 °C may be calculated using the formula on page 2.

While no long-term stability studies have been performed on this particular lot of filters,

studies of similar preparations have indicated that these filters should be stable for at least one year.

The overall direction and coordination of technical measurements leading to certification were performed under the chairmanship of 0. Menis and J. I. Shultz.

The technical and support aspects involved in the preparation, certification, and issuance of

this Standard Reference Material were coordinated through the Office of Standard Reference

Materials by T. W. Mears.

Washington, D.C. 20234 J. Paul Cali, Chief

January 17, 1972 Office of Standard Reference Materials

46
Appendix J. Certificate, Standard Reference Material 93 1 , Liquid Absorbance

Standards for Ultraviolet and Visible Spectrophotometry — Continued

Absorbance at various temperatures (17 to 35 °C)

At-A25[l+CA(t-25)]

where: Aj = Absorbance at temperature t (°C)

A25 = Absorbance certified at 25.0 °C

C;^ - Fractional change in absorbance per °C

The values of C^, at the four wavelengths, are given below. [NOTE: At wavelength 302 nm,

absorbance decreases with increasing temperature; at the other wavelengths, absorbance increases

with increasing temperature.]

Wavelength, nm C^^

302 -0.0014

395 +0.0014

512 +0.0018

678 +0.0014

Instructions for Use

1. Select two clean 10.00 mm cuvettes free of scratches. At least one should be fitted with a ground glass or Teflon stopper to minimize evaporation. Reserve it for all sample measurements.

2. Mark each of the cuvettes to assure the same orientation in the spectrophotometer.

3. Place the cuvettes in their respective holders and fill with distilled water. (Borosilicate

Pasteur-type pipettes fitted with rubber bulbs are recommended for transferring all solutions to and from the cuvettes. Soft glass pipettes, which are available commercially, contain residual

amounts of ultraviolet absorbing material, but may be used after proper cleaning. Several rinses,

first with isopropyl alcohol and then with distUIed water, are generally adequate.)

4. Obtain the optical mismatch of the cuvettes at 302, 395, 512, and 678 nm, using the spectral

bandpass limitations given on the face of the certificate.

5. Empty the cuvettes by suction without removing them from their holders, refill with distilled water and measure the absorbances again at each of the above wavelengths.

6. Repeat the emptying and refilling operation until constant absorbance readings are obtained.

7. Using the liquid filters provided, measure, in turn, the absorbances of the "Blank," "A," "B,"

and "C" against distilled water. Shake each ampoule before opening to remix any condensate

which may have collected in the neck (the ampoules have been prescored directly below the gold

band to facilitate opening).

8. Subtract the appropriate "Blank" reading from the absorbances obtained for "A," "B," and

"C." These net absorbances should agree with the certified values within the uncertainties

specified. Consult the manufacturer of the instrument if they do not.

The absorbances of these filters will depend not only on the accuracy of the photometric scale

but also on the wavelength accuracy and the spectral bandpass. A mercury lamp is recommended

for checking the wavelength scale. In addition, for those spectrophotometers having a hydrogen (H)

or deuterium (D) source, the two emission lines at 486.1 and 656.3 nm (H) or 486.0 and 656.1 nm

(D) may provide a convenient check at these wavelengths.

47
Appendix J. Certificate, Standard Reference Material 93 1 , Liquid Absorbance

Standards for Ultraviolet and Visible Spectrophotometry — Continued

To insure that the measured absorbances are not significantly different from the certified

values, the following restrictions are placed on the size of the spectral bandpass selected: To obtain

±0.1 percent of the true value, the effective spectral bandpass should not exceed 1.5,2.0, 3.3, and

8.5 nm at 302, 395, 512, and 678 nm, respectively. For ±0.2 percent, the respective bandpasses are

2.2, 2.9, 4.8, and 12.3 nm. Additional information on the effect of spectral bandpass on the

absorbances of these filters is given in the figure below. These curves are not to be used, however, to

correct the measured absorbances.

-5.00

2 4 6 8 10 12 14 16 18

EFFECTIVE SPECTRAL BANDPASS, nm

Preparation of filters

The filters were prepared by dissolving high purity cobalt and nickel in a mixture of nitric and

perchloric acids. The absorption spectrum of the resulting solution is shown in figure above. The

maxima at 302 and 512 nm are due to absorption by NO3 and Co(H20)6* , respectively. The

maximum at 395 nm and the plateau at 650-700 nm is due to Ni(H2 O)^* . The pH of these filters is about 1.

Reference

lU R. Mavrodineanu, NBS Technical Note 584, 0. Menis and J. I. Shultz, ed., pp. 2-21, U.S.

Government Printing Office, Washington, D.C. 20402 (Dec. 1971).

48
Appendix J. Certificate, Standard Reference Material 93 1 , Liquid Absorb

Standards for Ultraviolet and Visible Spectrophotometry — Continued

Absorbance at various temperatures (17 to 35 °C)

At=A2 5[l+CA(t-25)]

where: = Absorbance at temperature t (°C)

A25 = Absorbance certified at 25.0 °C = Fractional change in absorbance per °C

The values of C^, at the four wavelengths, are given below. [NOTE: At wavelength 302 nm,

absorbance decreases with increasing temperature; at the other wavelengths, absorbance increases

with increasing temperature. ]

Wavelength, nm

302 -0.0014

395 +0.0014

512 +0.0018

678 +0.0014

Instructions for Use

1. Select two clean 10.00 mm cuvettes free of scratches. At least one should be fitted with a ground glass or Teflon stopper to minimize evaporation. Reserve it for all sample measurements.

2. Mark each of the cuvettes to assure the same orientation in the spectrophotometer.

3. Place the cuvettes in their respective holders and fill with distilled water. (Borosilicate

Pasteur-type pipettes fitted with rubber bulbs are recommended for transferring all solutions to and from the cuvettes. Soft glass pipettes, which are available commercially, contain residual

amounts of ultraviolet absorbing material, but may be used after proper cleaning. Several rinses,

first with isopropyl alcohol and then with distilled water, are generally adequate.)

4. Obtain the optical mismatch of the cuvettes at 302, 395, 512, and 678 nm, using the spectral

bandpass limitations given on the face of the certificate.

5. Empty the cuvettes by suction without removing them from their holders, refill with distilled

water and measure the absorbances again at each of the above wavelengths.

6. Repeat the emptying and refilling operation until constant absorbance readings are obtained.

7. Using the liquid filters provided, measure, in turn, the absorbances of the "Blank," "A," "B,"

and "C" against distilled water. Shake each ampoule before opening to remix any condensate

which may have collected in the neck (the ampoules have been prescored directly below the gold

band to faciUtate opening).

8. Subtract the appropriate "Blank" reading from the absorbances obtained for "A," "B," and

"C." These net absorbances should agree with the certified values within the uncertainties

specified. Consult the manufacturer of the instrument if they do not.

The absorbar ^es of these filters will depend not orJy on the accuracy of the photometric scale

but also on the wavelength accuracy and the spectral bandpass. A mercury lamp is recommended

for checking the wavelength scale. In addition, for those spectrophotometers having a hydrogen (H)

or deuterium (D) source, the two emission lines at 486.1 and 656.3 nm (H) or 486.0 and 656.1 nm

(D) may provide a convenient check at these wavelengths.

49
Appendix K. Standard Reference Materials for pH

Available from the National Bureau of Standards

SRM Type

pH(S) Wt/Unit

(at 25 °C) , (grams)

185e Potassium Hydrogen Phthalate 4.004 60

1861c Potassium Dihydrogen Phosphate (6.863) 30

18611c Disodium Hydrogen Phosphate (7.415) 30

187b Borax 9.183 30

188 Potassium Hydrogen Tartrate 3.557 60

189 Potassium TetroxaJate 1.679 65

191 Sodium Bicarbonate

1 10.014

1

30

192 Sodium Carbonate 30

922 Tris|hydroxymethyl)aminomethane

1 7.699

1

25

923 Tris(hydroxymethyl)aminomethane Hydrochloride 35

Appendix L. Ion-selective Electrode Standard Reference Materials Available

from the National Bureau of Standards

SRM No. Name Molality

Activity

p(cation) p(anion) g/unit Ion a

2201 Sodium Chloride 1.0 Na+ 0.6956 0.157 125 ci- .620 0.208 2202 Potassium Chloride 1.0 .623 .206 160

ci- .586 .232 2203 Potassium Fluoride 1.0 F- .645 .190 125

50
Appendix M. NBS — Clinical Standard Reference Materials

SRM T1 ype Piiritv /mnl Unit u 1111

Ql71 1Laa Ljliolcstcrol 77.0 9a

Ql ') QO 7 913 1 n A r*i H 99.7 lUg

914 \-i iCdLlJllllC 99.8 IVg

^dlClUIIl V>d.l UUllalC 0077. 04- 7T 90IT 916 rti 11 ri 1 rti n 99.0 0u. 1ige

01 7 99^9 25g

7lO r^rf^f' Q C C 1 1 1 Tm I r1 1 ^ /I IT (JldsslUIIl 99.9 30g

017170 OULllUIIl ^jIllUllUC 99.9 30g

090 7Z.V n-\la nnJtol 99.8 50g

091 7^ 1

1 /"fc T"f 1 ClA 1 98.9 Ig

099 1 nc/ r) *7/ii"i"» V 17 m ri 17 1 1 Q vni f\t~\mt>t ri q n a 99.9 25g

09^ 1 i"i CI f 1 v/nT'ri V \7rri tfit ri \7 1 la FT» 1 rii"^rYi ri a Ti<i Hi 1 1 lioi^iiy Lii OA yiiicLiiyi^aiiiiiiuiiic Liidiiic i

x

VjI 99.7 35g

094. IjIIIIIUJII UUIidlc: 100.5 30g

925 VMA (4-hydroxy-3-methoxymanclelic acid) 99.4 Ig

926 Bovine-Serum Albumin powder IN PREPARATION

927 Bovine-Serum Albumin (7% solution) IN PREPARATION

928 Lead Nitrate IN PREPARATION

929 Magnesium Gluconate IN PREPARATION

930b Glass Filters for Spectrophotometry IN PREPARATION

931a Liquid Filters for Spectrophotometry IN PREPARATION

932 Quartz Cuvette for Spectrophotometry 1 each

933 Clinical Laboratory Thermometers Set of 3

934 Clinical Laboratory Thermometer 1 each

Appendix N. Priority of work on Clinical Standard Reference Materials

[1] Primary protein standard (Bovine serum albumin).

[2] Spectrophotometry liquids in UV-visible range. (Extension

of SRM 931 to 300 and 240 nm.)

[3] High-purity solid materials for spectrophotometry:

a. Potassium dichromate;

b. Potassium hydrogen phthalate.

[4] Standard cuvette for spectrophotometry.

[5] Cyanomethemoglobin — No SRM planned. NBS will co- operate in verification of the international standard.

[6] NADH (reduced-form of nicotinamide adenine denucleotide).

[7] Sodium pyruvate.

[8] Spectrofluorimetry standard — Quinine suKate.

[9] Ionic activity standards:

a. pH buffers in isotonic saline;

b. Ion-selective electrode standards Ca + + CaCl2'2H20),

F-(KF);

c. PCO2 and PO2.

[10] Toxicology standards- phenobarbital, pentobarbital, secobarbital, diphenylhydantoin.

[11] Abuse drug standards — morphine, methadone.

[12] Alcohol standards.

[13] Magnesium salt.

[14] Lead in blood.

[15] Saturated triglyceride — tripalmitin.

[16] UV spectrophotometry glasses.

[17] Human albumin.

[18] Temperature standardization in the clinical laboratory.

[19] PubUcations relating to techniques used in the clinical laboratory.

[20] Stray light standard for spectrophotometry.

[21] Particle size standards.

51

MBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS-MN 148

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Standard Reference Materials:

The Role of SRM's in Measurement Systems

5. Publication Date January 1975

6. Performing Organization Code

7. AUTHOR(S) J. P. Cali, T.W. Mears, R.E. Michaelis , W. P . Reed

R.W. Seward, C.L. Stanley, H. T. Holken, andH.H. Ku

j8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

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DEPARTMENT OF COMMERCE

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10. Project/Task/Work Unit No.

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12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as Number 9.

13. Type of Report & Period

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Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 74-600196

16. ABSTRACT (A 200-word or less factual summary of most sigpificant information. If document includes a significant

bibliography or literature survey, mention it here.)

This publication is a guide to the use of Standard Reference

Materials (SRM's) and should be useful to all users of SRM's,

particularly those in countries developing national measurement

systems. It is not intended to be an exhausti-ve description of the

NBS-SRM program, but rather a review of the role of SRM's play

in the measurement system, how SRM's are certified, and what

the certification means. To illustrate the use of SRM's several

selected industries are described in which SRM's have made significant contributions.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Certification; meaningful measurement; measurement;

measurennint system; precision; reference method; specificity; SRM; standard

reference material; systematic error.

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