Report
The NBS Standard Reference Materials Program: An Update It is interesting to note t h a t one of the first Standard Reference Materials (SRM) issued by NBS in 1906 is still in existence—albeit in its 14th renewal. This cast iron in chip form (SRM 4m) is one in the series of the gray iron SRM's t h a t are used widely by the Nation's foundries to determine, by chemical analytical methods, the composition of this still important industrial product. Over the past 65 years, more than one-half million of NBS-SRM's have been used to assure the accuracy of the analytical methods used. In these days of rapid technological innovation, it is interesting to note t h a t some things change only very slowly, and, further, t h a t N B S , recognizing the industrial importance of these basic products, has been willing to provide a continuity of service over this long period of time. Before 1966 the N B S - S R M program was almost entirely industry oriented. Of the 526 SRM's available in that year, only 65 were not directed toward industrial problems. These latter SRM's included a series of carbon-14 and tritium labeled sugars, thermal emittance standards, a Môssbauer SRM, a series of phosphors, and some radioactivity standards. Well over 90% of the N B S resources were directed to the production of SRM's for use in the metals, glass, ores, rubber, ceramics, cement, and chemical industries. A few basic primary SRM's (e.g., sucrose, acid potassium phthalate, and arsenic trioxide) and the p H SRM's were also available for basic standard-
Ten years ago, almost to the day, the National Bureau of Standards reported in Analytical Chemistry (1) the status of its Standard Reference Materials program. In view of the NBS 75th Anniversary of its founding coinciding, as it does, with our Nation's 200th Anniversary, it seems especially fitting that an update of that report be made. In this short 10-year period, more substantive changes have taken place than occurred over the first 60 years of the program's existence. It is the purpose of this report to recapitulate these events and to attempt to predict what the future holds in store. ization purposes in the Nation's analytical chemistry laboratories. It was with the establishment of the Office of Standard Reference Materials (OSRM) in 1965, under the dynamic leadership of W. Wayne Meinke, and the recognition by N B S
802 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
management of the S R M program as a major program in its own right, t h a t new thrusts and new directions were initiated. In 1969 J. Paul Cali, then deputy chief of the OSRM, became chief, and under his direction this momentum was continued and later accelerated with additional new work. Present SRM Inventory Examination of the S R M inventory is one way of demonstrating the changes t h a t have occurred in the program over the past 10 years. This is most easily done in tabular form (Table I). Making allowance for the 130 discontinued SRM's, there were 538 new SRM's produced. T h e annual production rate of 50-60 new SRM's is consistent with the level of financial input for production, which, when making allowance for inflation, has been relatively constant during this period. T h e most striking change has been the production of SRM's in 10 new categories. T h e most important are those involving health (clinical chemistry), environment, and several engineering categories including forensic, safety, and computer applications. Areas t h a t have been significantly strengthened include: ferrous metals, glasses, nuclear and isotopic, and radioactivity. Also significant is the increasing emphasis on SRM's characterized for various physical (as contrasted with compositional) properties. In 1966 there were 118 of 559 in this category (21%); currently, there are 314 of 967 (32%). Obviously, the
J. Paul Cali Institute for Materials Research National Bureau of Standards Washington, D.C. 20234
program has been broadened significantly to respond to national needs that have come to the fore over the past 10 years—health, environmental protection, nuclear power and safeguards, etc. The increase in physical property SRM's reflects, more than any other factor, the increasing interest of physicists, metrologists, and physical chemists in the SRM approach to measurement compatibility. The last two categories shown, Research Materials and General Materials, require some explanation. Research Materials represent a class of materials that can be used as reference materials. However, the property(ies) measured are not characterized with all the rigor and under the procedures required for a fully certified NBS-SRM. These materials are issued with a Report of Investigation signed by the scientist(s) involved in its characterization. Further, a Research Material does not carry the full weight—or guarantee, if you will—of the NBS; its scientific authority rests entirely with the individual scientists involved. The currently available 17 Research Materials consist of 14 phosphors, two ultrapure aluminum cubes and rods, and a scanning electron microscope resolution test specimen. General Materials (GM) are reference materials produced and certified or guaranteed by other government agencies, standards bodies, or other nonprofit organizations. When it is deemed to be in the public interest and when alternative national distribution channels do not exist, then NBS through the OSRM will undertake the distribution of such materials. Currently, NBS distributes two hydrogen in steel GM's, a nickel and vanadium in residual oil, and an attapulgus clay. Shortly to be added is a series of 34 soil profile GM's sponsored by the Soil Science Society of America, and a number of plastic materials sponsored by the Products Research Committee. This service is
Table I. S R M Inventory—1966 and 1 9 7 6 SRM's in inventory Category 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.
Metals, Ferrous Metals, Nonferrous Ores, Cements, Ceramics Organics, Metallo-organics H i g h - p u r i t y Metals Electron M i c r o p r o b e Primary and Secondary Chemicals Microchemicals Clinical L a b o r a t o r y Biological and Botanical Environmental—Gases E n v i r o n m e n t a l — L i q u i d s & Solids Forensic Fertilizers Glasses Nuclear and Isotopic p H , p D , Ion-Selective Mechanical Properties Heat Properties Magnetic Properties Optical Properties Radioactivity Properties Miscellaneous Physical Properties (incl.) Metallurgical, Môssbauer, X-ray Engineering Properties—Rubber Engineering Properties—Color Engineering Properties—Other Research Materials General Materials Totals
available to all organizations that qualify and have reference materials available or planned that would help solve a national measurement problem. Measurement Compatibility As NBS moved into new areas, especially those concerned with clinical chemistry and environmental measurements, it became increasingly
1966
1976
167 96 28 32 0 0 10 5 0 0 3 0 0 0 11 27 6 37 11 0 28 37
219 98 41 34 8 6 12 10 21 5 24 14 3 3 30 46 17 89 64 9 31 86
5 19 35 2 0 0
35 21 6 14 18 3
559
967
clear that many analysts working in these fields had either forgotten or never learned some basic, fundamental measurement concepts. NBS scientists also discovered that these concepts, almost taken for granted internally because of their closeness to measurement problems, had not been enunciated and passed along to the outside world of SRM users. As a result, some NBS scientists have re-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976 · 803 A
stated and published these basic ideas, since the production and issuance of reference materials are essentially of little practical value unless the user has firmly in mind their proper role within the context of the entire measurement process. In a series of papers by Cali and coworkers, addressed primarily to the practitioners of clinical chemistry, b u t more broadly applicable to many measurement systems, and including most of analytical chemistry, these ideas were laid out (for details see refs. 2-4). Briefly stated, without supplying t h e underlying arguments and rationale, measurem e n t compatibility (defined as the ability of all laboratories performing t h e same analysis to obtain, within some agreed-on limits, identical results) can be achieved when the measurement network is based on accuracy. Most scientists would agree t h a t if each laboratory in a measurement network were proceeding in such a way t h a t each operational step in the measurement process was related or traceable to the " t r u e value" (for a given property), and t h a t if each of these steps was performed without error, other t h a n the ever-present random errors, then measurement compatibility logically must ensue. This logic is based on the fact t h a t in a homogeneous, stable lot of material, there can exist at one point in time only one true value for the property under examination. By definition, an accurate measurement is one t h a t is both free of systematic error and precise and thus directly related to the " t r u e value" of the property being measured. T h u s , an accuracy-based measurement network automatically achieves compatibility. It was further argued, especially when chemical composition is the property being measured, t h a t three major components of the measurem e n t process must be attended to: an agreed-on system of measurement units—now in science the International System of Units (SI); methods of demonstrated accuracy,· whereby access to t h e base and derived measurem e n t units can be assured; and reference materials accurately characterized with regard to the property(ies) immanent within them. A hierarchy of reference methods and reference materials was needed to transfer accuracy throughout large and complex measurement networks such as exist in the areas of environmental protection and clinical chemistry. Inevitably, these complex and highly interactive networks require the assistance and cooperation of many diverse organizations—governmental agencies, professional societies, standards bodies, manufacturers, and, most importantly, the ulti-
mate user laboratory. Much time and effort, especially over the past five years, have been devoted by N B S scientists and managers working within the S R M program in helping to organize and implement these aspects of measurement compatible networks. I t was to this end t h a t N B S , through its OSRM, organized and sponsored the first large symposium in October 1973, dedicated entirely to this subject. T h e proceedings of t h a t conference were published in 1975 (5). T h e upcoming N B S sponsored symposium on Methods and S t a n d a r d s for Environmental Measurement (September 20-24, 1976) will further advance these ideas in areas associated with environmental measurements. It is fair to say t h a t in 1966, SRM's tended to be viewed in isolation as a measurement tool in and of themselves. T h i s view was predicated on an overall measurement knowledgeability, especially among nonindustrially oriented analytical chemists, t h a t subsequent events showed could not be supported by the facts. T h u s , the effort described in this section, largely educational in t h r u s t and content, should be considered an i m p o r t a n t and vital contribution to our understanding of how measurement systems m u s t be constructed and which meas u r e m e n t tools are required if measurement compatibility is to be achieved. This aspect of the N B S S R M program will continue strongly. Certification Process SRM's issued by N B S are certified. J u s t exactly what does t h a t mean? T h e r e are two aspects of certification, one legal, the other scientific. Legally, the certification process indicates t h a t an N B S certified S R M carries with it the full weight and legal authority of both the D e p a r t m e n t of Commerce and N B S in t h a t these are official materials authorized by appropriate federal laws and regulations. N B S - S R M ' s have been incorporated in the past by various government agencies (Departm e n t of Defense, Government Services Administration) as p a r t of purchase specifications or as a mechanism to provide traceability to the national measurement system. Today, the E n vironmental Protection Agency requires the use of three N B S - S R M ' s for the calibration of instruments in the determination of nitric oxides in air. Such requirements will undoubtedly increase in the future. Another illustration of the legal use of a N B S S R M involves the question of import duties on the mineral fluorspar. T h e basis for assessing these duties is a U.S. Customs method for the assay of fluorspar. This official method incorporates as p a r t of the analytical procedure N B S - S R M # 7 9 a , fluorspar.
804 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
Old, venerable classical techniques such as separation of arsenic, antimony, and tin prior to titrimetric or gravimetric determination using all-glass Scherer still as shown here are still widely used at NBS for very accurate determination of major and minor constituents in a wide variety of industrialtype SRM's However, of more direct interest to the analyst are the scientific aspects of certification. Where possible, N B S measures and certifies the numerical value of the property(ies) under investigation as accurate values, i.e., they are, within t h e stated overall uncertainty, " t r u e values". Now N B S does not claim to be infallible, and a proviso or two is called for. First, systematic errors in t h e measurement process leading to certification are always investigated, b u t with the realization t h a t advances in the state-of-the-art may uncover additional systematic errors t h a t were unsuspected a t the time of the original work. Therefore, a cautious, conservative estimate of residual and unknown systematic error is the rule, and this is always reflected in the final stated uncertainty. Second, every material is inherently unstable, given sufficient time. It would be theoretically desirable for N B S to test the stability of every issued S R M over a very long time period, say, 10 years or more. However, practical realities do (Continued on page 808 A)
Table I I . NBS Technical Divisions Involved in SRM Measurement and Production Division
213 221 232 243 275 310 311 312 313 316 401 425 491 600
Mechanics Heat Optical Physics A p p l i e d Radiation Cryogenics A n a l y t i c a l Chemistry Polymers Metallurgy Inorganic Materials Physical Chemistry Standards A p p l i c a t i o n & ,Analysis Electronic Technology Fire Science Institute f o r C o m p u t e r ScMences & Technology 650 C o m p u t e r Systems Engineering a
Total New plus renewal.
b
No. of SRM's produced (1973-1976)a
No. of scientists contributing to program
0 6 7 90 19 119 4 75 17 1 6 1 3
1 6 6 18 12 83 10 9 8 7 4 3 6
6 5
2 6
Technical Resources
359 Including three NBS staff statisticians.
not allow for such a luxury; consequently, N B S must issue its SRM's on a reasonable time scale. Where instability is known or suspected to be a problem, then an expiration date for the S R M is given (all clinical SRM's issued to date are valid for no more than 5 years from date of issuance.) Further, where instability is suspected, testing continues after issuance, and users are immediately notified if any value falls outside the uncertainty limits. Third, certified values are only valid when the S R M is used in the manner for which it is intended and with all stated precautions followed by the user. Misuse or mistreatment of the S R M by an uninformed analyst is not uncommon. N B S tries to minimize this problem, at least in part, by providing clear, full instructions to the user including such matters as storage conditions, minimum sample size to assure homogeneity, and degree of cleanliness in handling. Given these caveats, how then does NBS arrive at t h a t point where it feels as certain as human frailty allows to certify its SRM's? There are three principal measurement modes used: (1) Measurement by a method of known and demonstrated accuracy performed by two or more analysts working independently, OR (2) Measurement by two or more independent and reliable methods whose estimated inaccuracies are small, relative to accuracy required for certification, OR (3) Measurement via a qualified network of laboratories. (Note: a variation of 2, above, this mode is used at N B S only for the measurement and certification of renewal SRM's.)
additional information or values with smaller uncertainty limits, and not corrected values where additional work using more accurate methods has shown the original numbers to be incorrect. Recently, there has appeared in the literature work t h a t refers to material supplied by N B S to scientists for study. An example is a tuna fish material. Such materials are not SRM's until officially issued by N B S , and values reported in the literature should be considered as research findings only. In no way should they be viewed as carrying certified values— no m a t t e r how good the numbers a p pear to be.
184"
It is beyond the scope of this report to give all the precautions and specifics t h a t surround the choice and use of these three modes. Experience over many years, however, has validated this approach to certification. Users of SRM's often question the difference between a certified value and a value placed on the certificate under the heading "For Information Only". A certified value is one obtained tnrough the use of one of the measurement modes listed above. Values provided for information only have not been so obtained. Often, they are the result from one technique alone t h a t does not, for t h a t particular analysis, qualify as a reference method. Information only values are often subsequently upgraded to certified status as resources or additional techniques become available. Before 1973, many SRM's were issued on a "Provisional" basis. T h a t is, the values quoted were considered subject to some change to be based on work still to be done sometime in the future. Because of the pressures for new SRM's, t h a t work was often unduly delayed, and years might pass before the "final" values were arrived at. T h a t practice has now been discontinued, although many older SRM's continue even today to exist in "Provisional" condition. It is our intention to upgrade these SRM's to final status as quickly as time and resources permit. All values given on currently produced SRM's are either fully certified within the uncertainty limits expressed or else, as stated earlier, given "For Information Only". Revised certificates will continue to be issued, however. Hopefully, most of these will contain
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T h e current direct funding of t h e S R M program provided by the O S R M supports a level of effort equivalent to approximately 60 man-years of scientific, administrative, and support personnel time. In addition, a substantial effort, funded primarily and directly by various N B S technical divisions, supports the program through t h e development of new or more accurate measurement techniques which then are ultimately used in the characterization of SRM's when they are ready to go into production. T h e magnitude of this effort is estimated to be about one-half of the OSRM direct support, i.e., ~ 3 0 man-years. As indicated above, the entire program counting all sources of support is about 90 man-years. About 30 manyears are dedicated to administrative, preparation and processing of materials (packaging, labeling, etc.), distribution (billing, shipping, etc.), and to program direction activities. T h e remaining 60 man-years are used in measurement and certification activities in the technical divisions. However, it should not be concluded t h a t there are 60 N B S scientists working full time on SRM's. Most N B S scientists work on a variety of programs and projects, and this is the case with the S R M program. T h e actual number of scientists and technicians in the technical divisions who make major contributions to the program is shown as p a r t of Table II. As p a r t of a review done recently for the N B S Director, an actual count of the number of scientific competences (e.g., mass spectrometry, activation analysis, and all types of "absolute" radioactivity counting techniques) used in the measurement processes leading to certification was more t h a n 160. In the analysis of materials, more t h a n 60 distinct competences within the Analytical Chemistry Division alone were identified. These ranged from classical techniques of gravimetry and titrimetry to such complex
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procedures as isotope-dilution mass spectrometry involving teams of four to six scientists and the use of mass spectrometers similar to those used in atomic weight determination work. Precision and accuracies of the highest order are sometimes involved in the certification process. For exam ple, in the NBS X-ray Diffraction Sili con Powder (SRM 640), the standard error of the weighted average of the lattice parameter (ô = 5.43088 Â) was estimated to be 3.5 Χ 10 - 5 Â or less than 1 part in 10 000. The 15 NBS technical divisions, their SRM output over the past three years, and number of scientists in each division contributing to the program are shown in Table II. S R M Publications
Another area of the SRM program that has been given increased attention is that of the publication of information useful to SRM users. There is now established a NBS publication series reserved solely for information concerning all aspects of SRM work. It is called the NBS Special Publication 260 Series. Thus, all publications bearing the appellation NBS Special Publication 260, followed by a dash and a serial number, are members of this series (6). The sole exception, NBS Spec. Publ. 260 with no following number, is the code for the SRM Catalog, now published biennially. Since 1966 there have been 39 monographs issued; currently, more than 10 are in preparation. Subjects covered, together with the serial numbers of the monographs, are shown in Table III. The single most important SRM publication to date is the Proceedings of the Symposium on Standard Reference Materials and Meaningful Measurements, mentioned previously (5). At this conference over 250 scientists from 16 nations and 13 international
agencies met to discuss measurement problems existing in a wide and diverse selection of scientific and technological fields. Their findings and recommendations-are presented in 18 papers covering the plenary lectures. An additional seven papers were presented covering the SRM programs of six countries and the multinational program of the European Economic Community. Equally important as the plenary papers are the panel discussion reports. Fifteen discussion panels met covering topics ranging from health, environment, statistics, and quality control to industrial areas including metals, glasses, nuclear, plastics, etc. The 800+ pages provide a wealth of information on the SRM approach to measurement compatibility never before gathered in one volume. Specific information on each of the 967 SRM's currently available is given in the latest issue of the SRM Catalog (6). Shown separately in a supplement to the catalog are prices, quantities, and ordering information. The price supplement is issued when required to reflect changes in pricing. Reference Materials on the International Scene A note at this point is appropriate to clarify the use of the term "reference materials" in this section as contrasted with "standard reference materials". Historically, prior to the time that the modern program at NBS came into being in 1964, NBS reference materials were called "standard samples". Then in 1964 they were renamed Standard Reference Materials with the acronym SRM. This is still the term used by NBS. However, the word "standard" in this term creates serious problems in translation.. Thus, the International Union of Pure and Applied Chemistry (IUPAC) and the International Standards Organization
Table I I I . Guide to NBS Special Publications on SRM's" S R M subject matter
Serial numbers in 2 6 0 Series
Cast Iron Ferrous Metals Nonferrous Metals Information, Sources Nuclear and Radioactivity Glass and Ores Instrumental'Aspects Primary Chemicals Computer Physical Properties Polymers Clinical and Health
1, 6, 12 3, 10, 14, 26, 39, 43 2, 5, 7, 10, 39 4, 10, 30 8, 9, 27, 49 1 1 , 23, 37 13, 15, 16, 22, 25, 28, 32, 3 8 , 4 0 , 4 1 17, 24 18, 29 19, 2 1 , 3 1 , 34, 3 5 , 44, 47 33, 42 3 6 , 45, 48
" F o r example, # 3 0 in second c o l u m n , under I n f o r m a t i o n , Sources, first c o l u m n , refers t o NBS Special Publication 260-30 ( 1 9 7 1 ) . Its t i t l e is Standard Samples Issued in the USSR. Full titles are listed in current SRM Catalog, pp 85—87.
People
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1976
(e.g , International Atomic Energy Agency, World Health Organization, International Union of P u r e and Applied Chemistry, International Federation of Clinical Chemistry) m e t in Geneva and agreed t h a t ISO would provide secretariat services and coordinate work directed toward: questions of R M terminology and vocabulary; the gathering, collating, and dissemination of R M information; and questions with regard to the R M certification process. Working Groups to handle these activities are now being formed. Space does not permit a more detailed exposition of this subject except to point out t h a t interest in RM's is accelerating on the international scene. Significant programs are now being undertaken in several major international agencies. Examples of SRM's Issued in Past Two Years NBS designed and built high-accuracy spectrophotometer (absorbance can be measured to an accuracy of 1 part in 104) is used to certify Glass Filters for Spectrophotometry (SRM 930c) (ISO) recommended the adoption of the more general phrase "reference material" and its acronym R M to include all classes of such materials. NBS-SRM's, internationally, will become known as N B S - C R M ' s (i.e., certified reference materials from NBS). Therefore, in this section we will use the acronym " R M " as most appropriate to a discussion of international applications, while in the rest of the paper, when discussing the N B S program per se, we will use the more familiar " S R M " . This section on international developments is deemed appropriate in this review because many of the initiatives of these international R M activities have originated with and have been supported by N B S through its OSRM. Before 1969 there had never been an a t t e m p t to coordinate R M activities either between the industrial nations or international agencies. This lack of activity can be attributed to the fact t h a t prior to t h a t time essentially all RM's were produced for industrial applications where the guarding of proprietary information was often an overriding consideration. Further, the role of international standards agencies was just becoming to be recognized as having important implications for international trade. Few international standards incorporating R M ' s were extant. As early as 1969 it was becoming increasingly evident t h a t RM's were destined to play an important role in areas of emerging national and international concern: health, environmental protection, nuclear energy, increased industrial pro-
ductivity, and eventually in consumer protection and safety. T h u s , in 1969, N B S , together with the International Commission on Weights and Measures (CIPM) cosponsored the first international symposium on RM's. Thirty-three representatives from 15 countries and four international agencies agreed t h a t cooperation with regard to R M needs, the setting of priorities, and the gathering and dissemination of R M information would be a highly desirable activity. T h e C I P M was asked to explore the possibility of taking responsibility for these tasks. Subsequent to this meeting, the C I P M reported it could only be responsible for R M ' s impacting directly on basic metrological properties (e.g., platinum for the realization of the candela), t h a t it did not have resources to carry out successfully the larger responsibilities outlined a t the 1969 conference. T h e report of this symposium is found in ref. 7. There the m a t t e r rested until, in 1973, the International Organization for Legal Metrology (OIML) called a meeting at N B S t h a t followed the NBS-sponsored S R M symposium in October of t h a t year. At t h a t meeting, representatives from seven international agencies confirmed the recommendations of the 1969 conference with one important difference. Prior negotiations had indicated t h a t the much larger agency, ISO, would be willing to take initial responsibilities to get the entire project organized and to serve as the coordinating agency. As a result, in J a n u a r y 1976, representatives from 15 international agencies
812 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
This listing of SRM's and the one t h a t follows are not meant to be comprehensive or complete. Rather, they are given to provide a flavor of the current thrust of the overall S R M program. T h e entire inventory is given in the S R M Catalog (9). Radioactivity. T h e use of radioisotopes in chemical analysis continues to grow. In addition, the determination of radioactive species in the environment has led to the need for radioactivity SRM's in suitable matrices. Over the past 2 years, N B S has issued or renewed more than 30 specific radioisotopes of known purity and activity. Among these are several mixed radionuclides and a river sediment containing 12 radionuclides with certified values and 19 radionuclides whose values are given but not certified. Approximately 120 radioactivity SRM's are in stock (or for short half-life isotopes, available at specific times). These cover a large range of the table of radionuclides from tritium to americium-243 and encompass a wide range of alpha, beta, gamma, x-ray, electron capture, and photon energies. Many are of importance for radiopharmaceutical or nuclear medical diagnostic or t r e a t m e n t applications. Environmental (Air). A series of SRM's, nitric oxide in nitrogen, covering the concentration range from 50 to 1000 parts per million, completed the automotive emission gas series. In the prior year there were issued: five propane in air SRM's (2.8-475 ppm), three carbon dioxide in nitrogen SRM's (0.95-14.2%), and five carbon monoxide in nitrogen SRM's (9.7-957 ppm). Various environmentally harmful substances t h a t often are found in air are covered by the following SRM's: three lead in fuel SRM's t h a t cover
the range from 0.03 to 2.0 g of lead/gal of fuel, trace mercury in coal S R M at 0.13 ppm, and a nitrogen dioxide permeation device t h a t provides for the calibration of instruments used in determining concentration of this gas (permeation rates lie within the range 0.5-1.5 Mg NCVmin a t 25 °C). An important series of SRM's provides certified concentrations of several elements in coal, coal fly ash, and residual oil. As many as 13 elements are certified (e.g., zinc, vanadium, lead, and arsenic). Environmental (Water). Although several new "water pollution" SRM's are in process, to date only two mercury in water SRM's are currently available (other t h a n radioactivity related). One, a concentrate, is certified at 1.49 /tg of mercury/ml, a n d the other is certified at the very low level of 1.18 ng of mercury/ml. Industrial Hygiene. Currently, four SRM's are available to help calibrate instruments or to check methods used in industrial hygiene measurements: two freeze-dried urine SRM's, with fluorine at 0.8 and 7.1 mg of fluorine/1.; set of three of beryllium on filter media SRM's; and a set of four metals (Pb, Cd, Zn, Mn) on filter media SRM's. T h e metals of each set are characterized at detection, a t threshold, and above threshold levels. Instrumental Calibration. Some S R M ' s are specifically designed and produced to aid in the calibration of measuring instruments. T h e lattice parameter of the N B S silicon powder S R M , for example, has been measured and certified at 5.43088 Â with a stand a r d error of 3.5 Χ 10~ 5 Â. It is to be used as an internal standard for powder x-ray diffraction measurements where very high accuracy is required. T h e resolution and performance of scanning electron microscopes may be evaluated through the use of a T e s t Specimen—an alloy of aluminum and tungsten in fine dendritic structure formed on the surface of a 5-mm bead. Densitometers may be calibrated through the use of t h e N B S Photographic or X-ray Film Step Tablets. Optical density ranges from zero to four are covered in 21 steps for the photographic film and in 16 steps for the x-ray film. A Refractive Index Glass S R M in the form of a polished glass slab can be used to check the refractive index scale of refractometers. T h e refractive index is certified at 13 different wavelengths ranging from 0.4047 to 0.7065 Mm. Potassium fluoride has been provided as a S R M for the calibration of fluoride ion-selective electrodes. This S R M represents an addition to the series of SRM's certified for conventional single ion activities based on the Stokes-Robinson hydration theory, thus providing a
Portion of one of the radioactivity counting rooms at NBS reactor where SRM trace constituents are determined using high sensitivity radioactivation procedures
common-conventional ion-activity scale. Clinical Chemistry. More and more clinical determinations are being based on enzymatic reactions where the knowledge and control of temperature (typically to 0.1 °C or less) are critical. Two accurately calibrated thermometer S R M sets have been issued. T h e first set contains three thermometers. T h e ranges covered are from 24.00 to 26.00 °C, 29.00 to 31.00 °C, and 36.00 to 38.00 °C, respectively, all in divisions of 0.05 °C. Each thermometer also has an auxiliary scale from - 0 . 2 0 to +0.20 °C. Calibrated points are at 0, 25, 30, or 37 °C depending on the scale of the individual thermometer. T h e second thermometer S R M set consists of a single thermometer whose main scale extends from 24 to 38 °C, also in 0.05 °C divisions. There are now 21 distinct S R M ' s available for use in the clinical chemistry laboratory, all having been produced since 1969. In Meinke's report of this aspect of the S R M program (8), 11 clinical SRM's were reported available as of May 1971. Since t h a t time, SRM's added in addition to the clinical thermometers, above, are: sodium chloride, D-mannitol, Cortisol, lithium carbonate, VMA (a-hydroxy3-methoxymandelic acid), lead nitrate, liquid filters and quartz cuvettes for spectrophotometry. Industrial. By far the largest number of new and renewed SRM's fall in this category. In fact, since 1966, over 115 new industrial SRM's have been certified and issued. Industries served include: ferrous and nonferrous metals, ores and cements, transportation,
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fertilizers, glass, nuclear power, and rubber, among others. Of signal importance was the issuance of a series of five SRM's for use by the steel industry. This "1260 series" consists of four low alloy steels and an electrolytic iron containing a graded series of 40 elements. Certified values range from 0.15 p p m for lead in the electrolytic iron to 1.99% nickel in AISI # 4 3 4 0 alloy. This series represents one of the most comprehensive analytical measurement tasks ever performed. Well over 20 man-years and $500,000 were required to bring this series to fruition. E x a m p l e s of S R M ' s in P r o c e s s or P l a n n e d
As evident from the examples presented, the N B S - S R M program in the next 5-10 years will be heavily slanted toward SRM's in environmental, health, nuclear power and safeguards, and energy related areas. Environmental (Air). One import a n t measurement area where no reference materials are available is t h a t of particulate analysis. N B S is now working on an urban particulate SRM. Both particle size distribution and chemical composition will be characterized. Although, strictly speaking, not reference materials, a standard aerosol generator to produce aerosols of known size and distribution and an ozone generator are also under discussion. Other gaseous SRM's include ambient level pollutant reactive gases, SO2, NO2, NO, and various fréons. Environmental (Water). Probably the one single area of critical importance in water pollutant measure-
Bank of five "bag-houses" was operated near St. Louis, Mo., continuously for 18 months to collect urban dust particulate sample as candidate material for first wellcharacterized air particulate SRM. During this time, only 23 kg (50 lb) of material was obtained
ments is t h a t of the determination of trace organic substances. Under study now are SRM's containing known concentrations at very low levels of, for example, chlorinated hydrocarbons, selected pesticide residues, and polynuclear aromatic compounds. These substances will have to be studied and issued in various matrices: water, sediments, and tissues. In sediments, especially, organic constituents, the speciation of toxic materials (e.g., mercury as methyl mercury), PCB's, and toxic inorganic metals (Cd, Hg, P b , Cr, etc.) will be important constituents for characterization. Environmental (Energy Related). One important problem facing the Nation is to balance the need for greater energy production against unacceptable environmental degradation. T o this end, N B S , in cosponsorship with EPA, has just completed a series of eight workshops covering, for example, Oil Shale Processing, Power P l a n t Operation, Mine Drainage, Coal Gasification, etc. Some of the S R M needs expressed by the attendees were: trace elements in coal, fly ash, bottom ash; P N A ' s in coal; sulfur, by species, in coal; trace elements and trace organics in raw and spent oil shale and shale oil; constituents covering a wide range of inorganic and organic substances in various effluents, e.g., acid mine drainage, treated cooling water, process water; and various hydrocarbons in marine sediments and marine biological tissues. These are just a few of the S R M needs expressed, and considerable effort was expended in getting the attendees to give priority to their needs, since N B S will be able to respond to only the most critical and important. Health. Nearing completion are SRM's of bovine serum albumen, NADH, and the epileptic drugs—diphenylhydantoin, phénobarbital, primadone, and ethosuximid. In R&D stages are triglyceride (lipid analysis), dehydroepiandrosterone (steroid), and the other two components of the N A D H system, sodium pyruvate and NAD.
In clinical chemistry, N B S has been helping to develop reference (methods of demonstrated accuracy) methods of analysis. In view of the broadened scope of the program, explained earlier, reference method development is considered one important o u t p u t of the program. In 1973 t h e first reference method in clinical chemistry was completed, tested, and the work published (10, 11). Now nearing completion are reference methods for the determination of glucose, sodium, potassium, and chloride. Within one year of completion are reference methods for lithium, lead, and magnesium. Still in the development stages are reference methods for urea, uric acid, and cholesterol. Although obviously having applications in areas other t h a n health, several SRM's are under development for use in spectrophotometry and spectrofluorimetry. These include: an inconel on quartz filter having a flat response over the wavelength region from 200 to 1000 nm; a highly purified potassium dichromate solid salt certified at wavelengths in the ultraviolet; and quinine sulfate as a fluorescence emission standard. Still in planning stages or early development are a number of compounds to cover the entire range of fluorescence from 300 to 900 nm, while for wavelength standards under consideration is a series of rare-earth doped glasses, plastics, or solutions (200-900 nm). N u c l e a r Safeguards. N B S has just proposed a substantial program aimed toward providing S R M ' s (among other things) for use in the safeguarding and accountability of nuclear fissile materials. Measurements accurate to 0.1% are required if adequate safeguards are to be instituted and maintained. T o this end, the accurate measurement of a wide variety of nuclear materials in many complex matrices will have to be accomplished. Underway or in the planning stage are: spike solutions of 2 3 3 U, 2 3 5 U, and 2 4 4 Pu; thorium metal for chemical assay; a series of 2«8 P u /239p u SRM's of known absolute isotopic ratios; a variety of SRM's
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to be used with nondestructive assay techniques; SRM's for fuel b u r n u p predictions; and P u for calibration of calorimetric procedures. Industrial. One of the most import a n t S R M projects undertaken in many years is t h a t involving the certification of a series of over 20 SRM's for use by the copper industry. This series is to cover all aspects of the copper industry from ore beneficiation, to crude metal, to highly refined product. T h e series will consist of trace, minor and major constituents in both rod and chip form. Over 20 elements are to be certified, some a t levels as low as 0.1 /xg/g. Issued within the past year are four copper ore SRM's. Hopefully, three SRM's of unalloyed copper (in both chip and solid form) will be forthcoming in 1976, with six others to follow in 1977. T h e remaining three SRM's in the series are scheduled for issuance in 1978. Now in planning stages are some new nonferrous based alloy SRM's having applications in energy generation—cupro-nickel alloys; lead based materials for batteries; zirconium used for nuclear fuel rod cladding. For high-temperature applications, SRM's are planned for accurate determination of P b , Bi, Se, T e , and Tl in high-temperature alloys used as turbine blade materials both in power generating equipment and in jet engines. T o serve for aluminum producers, a series of bauxite ore SRM's is planned for the characterization for alumina plus other constituents, e.g., Fe, Mn, Cu, Si, and Ni. Bauxite ores typical of those from Arkansas, Surinam, Dominican Republic, and Jamaica will be examined. In addition to the ores, there are now in process three aluminum " b e n c h m a r k " SRM's consisting of two alloy and one pure aluminum members. T h e alloys are Aluminum Association numbers 6011 and 7075. Castings have been prepared, and homogeneity testing is underway. From 10 to 12 constituents will be certified. This series will be made available in both wrought and chill-cast forms, the former to be available by the end of 1976. Biological and Botanical. An unprecedented demand for the current Bovine Liver and Orchard Leaves SRM's ( # 1 5 7 7 and 1571) has provided evidence for the need for more SRM's in this category. Currently in various stages of preparation are: spinach, wheat and rice flours, and brewers yeast. T h e first three will be certified for a large number of trace elements of interest, both those of nutritional and toxic import. T h e yeast is especially important with regard to its chromium content, both total and organically bound.
Future N e w and Expanded S R M Areas. T h e role of the U.S. as the breadbasket for the world will certainly increase over the foreseeable future. International agricultural product standards now being developed and promulgated will require t h a t the quality of exported products be assured in terms of nutrients, cleanliness, moisture content, toxic and pesticide residues, etc. T h u s , SRM's to represent types of products must be developed, such as one grain to represent all grains, one vegetable to represent all legumes. Reference methods also need to be developed and implemented. Also assuming importance are nondestructive evaluation techniques (NDE), such as ultrasonic, dye penet r a n t (fluorescence), and radiography. N D E methods are now widely used in some industries, and predictions are t h a t these techniques may comprise more t h a n one-half of all quality control procedures by the 1980's. These methods require calibrating materials. Yet, few reference materials exist. N B S has just started an N D E program, and it is expected t h a t this program will include SRM's. D e m a n d vs. R e s o u r c e s . Since problems associated with national needs must be tackled first and dem a n d s are likely to exceed resources in the N B S - S R M program, some alternative modes of NBS-industry cooperation must be devised. An alternative to N B S doing the entire S R M job with only in-house resources already in operation is the Research Associate Program. Under this program, professional societies or trade or industry-wide associations, or standards bodies can sponsor scientists to work a t N B S , with N B S facilities, and in cooperation with N B S scientists on problems of mutual interest. In J a n u ary 1976, N B S and t h e American Society for Testing and Materials (ASTM) established such a program in the areas of SRM's. T h r e e major A S T M committees advise A S T M and the Research Associate as to the needs and priorities for metal and metal-bearing ore SRM's of importance to the metals industry. Cooperation permits the analytical work on S R M candidate materials to be done in private sector laboratories, with suitable safeguards. With this program, even in its early stages, 33 metal renewal S R M ' s are in process. This mode of support is open to other segments of U.S. industry, and it is hoped this avenue may strengthen and broaden the industrial component of NBS's S R M program. T e c h n i c a l Problems. Earlier SRM's were either pure substances or matrices of relatively simple composition, b u t newer demands call for characterization of matrices of great com-
plexity. T h e technical problems involved are immense. Problems of homogeneity, stability, and complex matrices are especially involved when reference materials related to natural substances as in clinical and biological areas are concerned. T o solve these problems, N B S and other laboratories who will be contributing knowledge and solutions to the complex problems involved will need to cooperate closely to characterize materials and develop reference methods. Reliable Measurements. T h e need for reliable measurements can perhaps best be understood by considering the economic and social costs of unreliable measurements. In clinical chemistry, the metal producing industries, in the environment, for instance, the need for repeated tests, the dumping of a "poor" product, the supplying of a more costly fuel t h a n is actually required are costs t h a t might result from " b a d " measurements. Even under very conservative estimates, costs resulting from unreliable measurements run into billions of dollars per year. In the three areas of greatest national concern—environmental protection, health, and nuclear energy— where regulatory agencies are required by law to operate, it is imperative t h a t regulations governing the measurement process be based on sound scientific measurement principles. Where composition is the property being regulated, as is often the case in each of these three areas, the S R M plus reference method approach appears to be a good first step in assuring measurem e n t compatibility and integrity.
References (1) "The NBS Standard Reference Materials Program", Anal. Chem., 38, 27A (1966). (2) J. P. Cali, et al., "The Role of Standard Reference Materials in Measurement Systems", NBS Monograph 148, GPO, Washington, D.C., 1975. (3) J. P. Cali and C. L. Stanley, Ann. Rev. Mat. Sci., 5, 329 (1975). (4) J. P. Cali, Fed. Proc, 34, 2123 (1975). (5) R. W. Seward, Ed., Proceedings of the Symposium on Standard Reference Materials and Meaningful Measurement, NBS Spec. Publ. 408, GPO, Washington, D.C., 1975. (6) Various SRM topics and titles are specifically given in Catalog of NBS Standard Reference Materials, NBS Spec. Publ. 260, pp 85-87, GPO, Washington, D.C. (ordering information is also given), 1975-76 ed. (7) A. V. Astin, Metrologia, 6, 33 (1970). (8) W. W. Meinke, Anal. Chem., 43, 28A (1971). (9) Catalog of NBS Standard Reference Materials, NBS Spec. Publ. 260, GPO, Washington, D.C, 1975-76 ed. (10) J. P. Cali, et al., "A Referee Method for the Determination of Calcium in Serum", NBS Spec. Publ. 260-36, GPO, Washington, D.C, 1973. (11) J. P. Cali, G. N. Bowers, Jr., and D. S. Young, Clin. Chem., 19,1208 (1973).
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dard Reference Materials program. He is a graduate of Brown University (1949) and the author of over 35 journal articles, the earlier ones dealing with the analysis of trace constituents using radiochemical techniques. His later publications deal with various means of achieving measurement compatibility stressing the use of reference materials and methods. He is also editor and author of the book, "Trace Analysis of Semiconductor Materials". Mr. Cali is a member of the American Chemical Society, Sigma Xi, Chemical Society of Washington, D . C , and the American Association of Clinical Chemists. He is the U.S. representative to the Council Committee on Reference Materials of the International Standards Organization as well as two reporting secretariats on reference materials within the International Organization for Legal Metrology. He has served as an officer and on various committees or as consultant to the International Union of P u r e and Applied Chemistry, World Health Organization, International Atomic Energy Agency, and the International Federation of Clinical Chemistry. In U.S. standards work, Mr. Cali is a m e m b e r of the American National S t a n d a r d s Institute, American Society for Testing and Materials, and is chairman of the subcommittee on reference methods of the National Committee for Clinical Laboratory Standards. Mr. Cali has received several special achievement or outstanding performance awards while a t N B S , as well as the D e p a r t m e n t of Commerce Silver Medal Award for his contributions to the S R M program. In 1975 he received a DOC award in recognition of his efforts in the advancement of equal opportunity at N B S .
