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The role of reference materials in quality assurance for chemical measurement.

This article informs Canadian scientists about the variety and role of Certified Reference Materials available from NRC and CANMET

Information from chemical measurements is often needed for critical decisions relating to health, the environment, or the safe and reliable production of useful goods and services. For the measurement data to be credible, they must be supported by evidence of precision and accuracy. Such evidence is best provided through a well-managed program of quality assurance [1,2]. Proper evaluation of the performance of a chemical measurement system requires the analysis, along with the samples under study, of known materials that are of composition as similar as possible to the samples. These known materials should be homogeneous, representative of the samples under analysis, and certified for average concentration and variability of the relevant constituents. Use of internally produced reference materials of this kind, along with split samples, spikes, and surrogates, allows a laboratory to estimate with confidence the precision, and to some extent the accuracy, of the measurements that it makes. The danger exists, however, of bias in the measurement process that affects both test samples and internally produced standards. The use of external reference materials gets around this problem. It is best, therefore, to check overall measurement operations by periodically analyzing externally certified reference materials that match test sample matrices as closely as possible. Users of commercial analytical laboratories often require data on analyses of certified reference materials as evidence of analytical quality control.

Reference materials

Reference materials range in composition from simple mixtures or solutions to complex natural materials that have been collected, homogenized, and packaged so as to provide a stable, representative matrix. The value of a reference material depends on the extent to which inferences made from measurements of it are valid. If the reference material and test sample matrices are closely matched, the analyst can reasonably expect a small number of measurements on the reference material to provide useful information on the quality of the measurement process. Certified reference materials do more than provide a way of monitoring the quality of current methodologies, however. They are also useful for the evaluation of new or revised analytical methods.

The preparation of reliable reference material is expensive and time consuming, and generally only a few organizations have the resources and experience to do this demanding job well. Reference materials produced and certified by reputable agencies are commonly called certified reference materials (CRMs) while those produced without certification are called simply reference materials (RMs). [ISO Guide 30:1992 [3] defines a certified reference material (CRM) as a "reference material, accompanied by a certificate, one or more of whose property values are certified by a procedure that establishes its traceability to an accurate realization of the unit in which the property values are expressed, and for which each certified value is accompanied by an uncertainty at a stated level of confidence."] In Canada, several organizations produce CRMs. These include the National Research Council (NRC) of Canada through their Marine Analytical Chemistry Standards Program (MACSP), and the Canada Centre for Mineral and Energy Technology (CANMET) through the Canadian Certified Reference Materials Project (CCRMP). The analytical standards program of NRC Canada produces CRMs suitable for the evaluation of analytical methods for trace organic compounds and trace metals in matrices such as natural waters, sediments, and biological tissues, while the CANMET program produces CRMs for the determination of levels of elements in rocks, soils, ores, and related materials.

Outside of Canada, one of the oldest and best known producers of reference materials is the U.S. National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards. The materials which NIST certifies for chemical analysis are usually prepared to their specifications by user groups interested in the availability of a standard, but the necessary analyses for certification are carried out by NIST scientists in house. These RMs are called standard reference materials, or SRMs. (The term 'SRM' is a registered trademark whose use is controlled by NIST.) Other organizations that produce CRMs include the National Institute for Environmental Studies of Japan, the Community Bureau of Reference of the Commission of European Communities, and the International Atomic Energy Agency.

Canadian CRMs currently available

A listing of CRMs currently available from NRC Canada is given in Table 1, and from CANMET in Tables 2 and 3. Some of these CRMs are pure substances that can serve directly as standards or as staring materials for the preparation of standards; examples include solutions of PCBs (CLB-1 and CLB-2) and of domoic acid (DACS-1B) for instrument calibration.

Taylor [3] points out that analytical laboratories, to the extent that they can produce acceptable measurement results on reference materials, can be said to be intercalibrated not only with other laboratories obtaining acceptable results on the same reference materials, but also with the certifying agency in so far as the material is concerned. This is called traceability. In this context, traceability is defined by the International Standards Organization (ISO) as a "property of the result of a measurement or the value of a standard whereby it can be related, with a stated uncertainty, to stated references, usually national or international standards, through an unbroken chain of comparisons" [4]. To obtain acceptable data, analysts should not only have their analytical measurement environment under a state of statistical control, but should use CRMs correctly. Valuable information on the correct use of individual CRMs can be obtained from the certificate accompanying each of them. The certificate normally includes, in addition to certified values and their uncertainties, the minimum amount of the CRM to be taken per analysis to avoid heterogeneity effects, along with supplementary information on expiry dates where applicable, optimum storage conditions, drying procedures, and so on.

Because of the expense of preparation and characterization, CRMs should not be used as routine check samples to determine whether an analytical procedure is in a state of statistical quality control. Organizations should develop their own in-house RMs for this purpose, and use CRMs as a way of evaluating new methods, laboratories, and in-house RMs. Also, CRMs frequently may not be certified for all of the substances the analyst wishes to determine, or may be certified for the total amount of a substance instead of for the species in which the analyst is interested. A recommended source of information for those interested in the proper use of reference materials is the discussion by Taylor [5]. Also useful is a brief publication on the use of NIST SRMs for decision making by Becker, et al [6], and several guides by the International Standards Organization (ISO) [4,7-10].

Keeping up with reference materials

The development and evaluation of reference materials is expanding rapidly, and many new materials are appearing from a variety of producers. This makes keeping up with present and forthcoming availability difficult. One way to find out about new RMs is to follow the column on reference materials produced by Stan Rasberry of NIST in American Laboratory. Another is to consult the computer database COMAR (for Code Reference Materials). Initiated in Europe some 20 years ago, and now sponsored by eight countries [11], this database contains information on some 7000 reference materials from 18 countries, and includes classifications according to certified properties, physical form, addresses of producers and areas of application. Information on materials in the database is available from the Office of Standard Reference Materials, National Institute of Standards and Technology, Building 202, Gaithersburg, MD 20899, USA. It is also possible to buy a disk containing the database in IBM PC-compatible format.

In the area of marine science and related fields, a valuable compilation of information is the third edition of Standard and Reference Materials for Marine Science [12]. This publication provides data on sources, availability, and use of some 2000 reference materials from 16 producers, and contains indices for organic compounds, elements, and isotopes. It is published independently by both the Intergovernmental and the U.S. National Oceanic and Atmospheric Administration (NOAA), and is also available from NOAA in electronic form.

Additional useful information can be obtained from regular international meetings on reference materials. The proceedings of the most recent meeting, the 5th International Symposium on Biological and Environmental Reference Materials, held in May 1992 in Aachen, FRG, appeared in January 1993 [13].

Summary

Almost all chemical measurements are made by relating the result of a measurement of an unknown to that of a standard. Both standard and unknown are carried through an analytical procedure, including preliminary operations such as drying, dissolution, separation, and so on, and the results compared. In effect, the standard is analyzed to calibrate the system or procedure. The quality of the analytical result reported for the unknown is, therefore, affected by the quality of the standard and the degree to which the standard matches the unknown, as well as by the extent to which other sources of error in the measurement system are in a state of statistical control. Factors affecting the quality of an RM include homogeneity, stability, and the accuracy of the analyses that led to the certified values. RMs certified for many elements or substances in a variety of organic and inorganic matrices are available from a number of reputable sources. Used properly, they provide analytical laboratories with a vital link in the production of quality results.

References

1. Taylor, J.K., Quality Assurance of Chemical Measurements, Lewis Publishers, Chelsea, MI, 1987.

2. Dux, J.P., Handbook of Quality Assurance for the Analytical Chemistry Laboratory, Van Nostrand Reinhold, New York, NY, 1986.

3. International Standards Organization, 'Terms and Definitions Used in Connection with Reference Materials'; ISO/REMCO Guide 30, ISO Central Secretariat, Geneva, Switzerland, 5 pp., 1978; See also Potts, P.J., and Kane, J.S., Geostandards Newsletter, 16: 333-341, 1992.

4. International Standards Organization, 'Terms and Definitions used in Connection with Reference Materials', ISO/REMCO Guide 30, 2nd ed., ISO Central Secretariat, 1 rue de Varembe, Case postale 56, Ch-1211, Geneva, Switzerland. 8pp., 1992.

5. Taylor, J.K., 'Handbook for SRM Users', NIST Special Publication 260-100, National Institute of Standards and Technology, Gaithersburg, MD 20899, 85pp., 1985. NIST publications can be ordered from Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402, USA.

6. Becker, D., Christensen, R., Currie, L., Diamondstone, B., Eberhardt, K., Gills, T., Hertz, H., Klouda, K., Moody, J., Parris, R., Schaffer, R., Steel, E., Taylor, J., Watters, R., and Zeisler, R., 'Use of NIST Standard Reference Materials for Decisions on Performance of Analytical Chemical Methods and Laboratories', NIST Special Publication 829, 27pp., 1992.

7. International Standards Organization, 'Contents of Certificates of Reference Materials', ISO/REMCO Guide 31, ISO Central Secretariat, Geneva, Switzerland, 8pp., 1981.

8. International Standards Organization, 'Uses of Certified Reference Materials', ISO/REMCO Guide 33, ISO Central Secretariat, Geneva, Switzerland, 12pp., 1989.

9. International Standards Organization, 'Certification of Reference Materials - General and Statistical Principles', ISO/REMCO Guide 35, ISO Central Secretariat, Geneva, Switzerland, 32pp., 1989.

10. International Standards Organization, 'Linear Calibration using Reference Materials', ISO/REMCO Guide 240, ISO Central Secretariat, Geneva, Switzerland, 31 pp., 1992.

11. Klich, H., and Walker, R., Fresenius J. Anal. Chem., 345:104-106, 1993.

12. Standard and Reference Materials for Marine Science, Revised edition, Intergovernmental Oceanographic Commission Manuals and Guides 25, UNESCO, 1993. NOAA publications may be ordered from NOAA, National Status and Trends Program, 6001 Executive Blvd., WSC-1, Rm 312, Rockville, MD 20852, USA.

13. Fresenius J. Anal. Chem., 345:179-350, 1993.

RELATED ARTICLE: Table 1. Reference materials available from NRC Canada

From Institute for Environmental Research and Technology(a)

Marine sediment reference materials for trace metals, matrix, and minor constituents:

BCSS-1 Coastal marine sediment from Gulf of St. Lawrence MESS-2 Estuarine sediment from Beaufort Sea PACS-1 Harbour marine sediment from British Columbia(b)

Water reference materials for trace metals:

SLEW-2 Estuarine water from St. Lawrence River estuary SLRS-3 Riverine water from Ottawa River CASS-3 Coastal sea water from near Nova Scotia NASS-4 Open ocean sea water from North Atlantic

Biological tissues for trace metals and other elements:(c)

TORT- 1 Partially defatted lobster tomalley (hepatopancreas) LUTS-1 Non-defatted lobster tomalley (hepatopancreas) DORM-2 Partially defatted dogfish muscle DOLT-2 Partially defatted dogfish liver

Biological tissue for trace organochlorine contaminants (dioxins, furans):

CARP-1 Fish tissue

From NRC Institute for Marine Biosciences(d)

Reference materials and standards certified for polychlorinated biphenyls:

CS-1 Coastal marine sediment HS-1, HS-2 Harbour marine sediments (HS-2 has higher concentrations) CLB-1 Set of 4 instrument calibration solutions covering 51 individual congeners CLB-2 Set of 4 GC/MS internal standard solutions of a 13C-labelled congener

Reference materials and standards certified for polycyclic aromatic compounds:

HS-3, HS-4, HS-5, HS-6 Harbour marine sediments SES-1 Spiked estuarine sediment, certified for 15 PACs DPAC-1, DPAC-2 Solutions of deuterated PACs

Reference material and standards certified for shellfish toxins:

MUS-1 Mussel tissue (Mytilus edulis), certified for domoic acid DACS-1B Domoic acid calibration solution OACS-1 Okadaic acid calibration solution MUS-2 Mussel tissue (mytilus edulis), certified for okadaic acid and DTX-1 toxin PSP-1 Calibration solutions for four paralytic shellfish poisoning toxins

a Ordering address: Marine Analytical Chemistry Standards Program, Institute for Environmental Science and Technology, Montreal Road, Ottawa, ON, K1A 0R6.

b Also certified for organotin compounds.

c All biological tissues are certified for organomercury as well as total mercury content.

d Ordering address: Marine Analytical Chemistry Standards Program, Institute for Marine Biosciences, 1411 Oxford Street, Halifax, NS, B3H 3Z1.

RELATED ARTICLE: Table 2. Environmental and geochemical reference materials available from CANMET under the Canadian Certified Reference Materials Project(a)

Environmental

CLV-1 and CLV-2 Spruce twigs and needles LKSD-1 to LKSD-4 Lake sediments SO-2 to SO-4 Soils STSD-1 to STSD-4 Stream sediments UM-2 and UM-4 Sulfide-bearing ultramafic rocks UTS-1 to UTS-4 Uranium railings

Geochemical FER-1 to FER-4 Iron formation rocks MRG-1 Gabbro rock SY-3 Syenite rock TDB-1 Diabase rock WGB-1 Gabbro rock WMS-1 Massive sulfide rock WPR-1 Altered peridotite rock

Radioactive ores

BL-1 to BL-4 Uranium-thorium ore BL-2a, BL-4a, BL-5 Uranium ores CUP-1 Uranium ore CUP-2 Uranium ore concentrate DH-1a Uranium-thorium ore DL-1a Uranium-thorium ore RL- 1 Uranium ore

Metals and alloys

NZA-1 to NZA-7 Zinc-aluminum foundry alloy discs for spectrographic purposes SSC-1 to SSC-3 Commercial-purity copper rods for spectrographic purposes

a Ordering address: Coordinator, CCRMP, CANMET (NRCan), 555 Booth Street, Ottawa, ON, K1A 0G1.

RELATED ARTICLE: About the Marine Analytical Chemistry Standards Program

As a historical, note, the Marine Analytical Chemistry Standards Program of the National Research Council of Canada originated in 1972 when several scientists in the Chemical Oceanography DMs on of the Bedford Institute of Oceanography, Department of Fisheries and Oceans, became concerned with increased needs for analyses of marine materials for inorganic and organic constituents (partly to satisfy regulatory requirements) in the absence of, among other things, appropriate reference standards. Following discussions in 1973 with the Canadian Committee of Oceanography, a center was established in the Atlantic Research Laboratory of the National Research Council in Halifax to develop reference materials, improve analytical methodologies, and carry out interlaboratory comparisons. With the inclusion of the inorganic expertise of the analytical laboratories in the NRC Division of Chemistry in Ottawa, an integrated program to improve the quality of chemical analyses was incorporated in 1976 as a recognized component of the scientific programs of NRC. Although inititiated in response to concern with marine analytical data, even at the outset the need in Canada for reference materials designed to satisfy wider environmental requirements was recognized, and the current list of materials reflects that need.

For the first decade or so program activities were divided about equally, with the NRC Atlantic Laboratory focused on reference materials certified for organic constituents and the Ottawa Laboratory concerned primarily with materials certified for trace elements. Early NRC materials included coastal marine, harbour marine, and estuarine sediments (certified for trace elements, PCBs, and PAHs), the first open ocean seawater (certified for trace metals), and biological tissues from lobstr and dogfish (also certified for trace metals).

In recent years, the Atlantic Laboratory, now the NRC Institute for Marine Biosciences, has devoted considerable effort to the preparation and certification of standards and reference materials for marine toxins such as domoic acid, the compound responsible for amnesic shellfish poisoning, and for polycyclic aromatic and organochlorine compounds in sediments and biological tissues. The measurement science group in Ottawa, now part of the NRC Institute for Environmental Research and Technology, has broadened its scope to include preparation and certification of biological tissue reference materials for dioxins and furans in addition to continuing its certification work for trace elements.

The expertise developed in NRC through the MACSP program is now recognized world-wide, and the advice and assistance of NRC measurement scientists is frequently sought internationally. What is more important the program provides a reservoir of knowledgeable chemists who can be called on to assist with the myriad of environmental contamination problems that confront Canadians today.

RELATED ARTICLE: Table 3. Ores and mineral processing products available from CANMET under the Canadian Certified Reference Materials Project

Ores BH-1 Tungsten ore CD-1 Antimony ore CH-3 Gold-bearing sulfide ore CT-1 Tungsten ore HV-1 Copper-molybdenum ore KC-1a Zinc-lead-tin-silver ore MA-1b Gold ore MA-2b Gold ore MA-3 Gold ore MP-1a Zinc-tin-copper-lead ore MP-2 Tungsten-molybdenum ore MW-1 Iron ore OKA-1 Niobium ore OKA-2 Rare-earth - thorium ore PR-1 Molybdenum ore PTA-1 Platiniferous black sand SCH-1 Iron ore SU-1a Nickel-copper-cobalt ore TAN-1 Tantalum ore TLG-1 Tungsten ore

Mineral Processing Products CCU-1B Copper concentrate CPB-1 Lead concentrate CZN-1 Zinc concentrate GTS-2 Gold tailings HCC-1 Pyrometallurgical feed INM-1 Pyrometallurgical feed PC-1 Petroleum coke PC-2 Petroleum coke PC-3 Petroleum coke PD-1 Non-ferrous dust PTC-1a Noble metals-bearing sulfide concentrate PTM-1a Noble metals-bearing nickel-copper matte RTS-1 Sulfide ore tailings RTS-2 Sulfide ore tailings RTS-3 Sulfide ore tailings RTS-4 Sulfide ore tailings SL-1 Blast furnace slag UMT-1 Ultramafic ore tailings

Byron (Ron) Kratochvil, FCIC, is the Chairman, Department of Chemistry, University of Alberta, Edmonton, AB, T6G 2G2. Alan Walton is the Director, Centre oceanographique de Rimouski, Rimouski, QC, G5L 3A1.
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Author:Kratochvil, Byron; Walton, Alan
Publication:Canadian Chemical News
Date:May 1, 1995
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