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Quality of Chemical Measurements

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Quality of Chemical Measurements Quality of Chemical Measurements International Evaluation Program Prof. De Bièvre as coordinator. Interest was immedi- Reveals True Situation ately demonstrated by UNESCO (Dr. A. Pokrovsky), and cooperative information dissemination efforts began. IUPAC’s Committee on Chemistry and Industry (COCI) Although a brief summary of the concerns, “Me- has submitted the following article, prepared by Dr. trology in Chemistry”, had been published in Chemis- Philip D. P. Taylor (Joint Research Centre-European try and Engineering News (C&EN, 31 May 1999, Commission, Institute for Reference Materials and Mea- p. 29), COCI encouraged Prof. De Bièvre to submit a surements, Retiesweg, B-2440 Geel, Belgium; E-mail: more detailed summary to Chemistry International (CI). [email protected]), Dr. Ioannis Papadakis This article provides “snapshot” pictures of chemical ([email protected]), Mrs. Lutgart Van Nevel measurement (un)reliability, with many practical, so- ([email protected]), Dr. Ciaran Nicholl (nicholl@ cietal implications. irmm.jrc.be), and Prof. Paul De Bièvre (Duineneind 9, B-2460 Kasterlee, Belgium; E-mail: paul.de.bievre@ Dr. A. Nelson Wright skynet.be). COCI Chairman Dr. A. Nelson Wright Chairman, IUPAC Committee on Chemistry and (12539 Ranger, Montreal, Quebec H4J 2L7, Canada; Industry (COCI) E-mail: [email protected]) contributed the Introduc- tion. Introduction Background The wide variation in chemical trace measurements was Chemical measurements form the basis of many im- first brought to my attention by Prof. De Bièvre at the portant economic, political, environmental, medical, 1998 meeting of the Committee on Chemistry and In- and legal decisions. Each day, millions of such mea- dustry (COCI) in Johannesburg, South Africa. During surements are carried out throughout the world. our 1999 meeting in Berlin, Germany, he again pre- The real basis for decision-making and implement- sented data demonstrating a huge (±50%) variation in ing regulations depends on the comparability and reli- trace element (Pb, Cd, Fe, and Zn) concentrations in ability of the results of these chemical measurements. water even from “accredited” laboratories. Similar prob- The European Commission requires that measurements lems were demonstrated for analysis of catalyst met- performed in one Member State must be acceptable to als (Pt) in car exhaust material. Because such data are all other Member States in the Union. This process re- increasingly used in decision-making areas of indus- quires that such reliability be demonstrated. Moreover, trial concern, our committee adopted “Reliability of the results of European chemical measurements must Chemical Measurements” as a new, formal project, with also be transparent and clearly understood by Europe’s trading partners in the whole world and vice versa. Globalization of commerce and the need for fair trade require knowl- edge of the degree of equivalence of the measurement results as they affect the value of traded goods. The Problem The comparability of chemical measure- ment results demands, where possible, that they should be traceable to “stated references” (i.e., values) and preferably expressed in values of the International System of Units (SI—Système Interna- tional d’Unités). Traceability to such val- ues ensures comparability (results can be validly compared). Many modern analytical methods are Chemistry International, 2001, Vol. 23, No. 1 1 number of atoms of an element (or molecules of a com- pound) in a sample through direct comparison to a known number of atoms of the same element (or mol- ecules of the same compound) in an amount of a spike added. IRMM’s International Measurement Evaluation Program (IMEP) In 1988, IRMM founded the International Measurement Evaluation Program (IMEP) as an awareness program and as a tool to show the true state of chemical mea- surement results, both to practitioners and to end-users of chemical measurements. The IRMM is part of the European Commission’s Joint Research Centre, which is made up of eight re- search institutes spread over five sites. The mission of A selection of certified isotopic reference materials. the IRMM is to promote a common European mea- surement system supporting the implementation and based on comparative techniques that rely on measur- monitoring of Community policies, by ing the ratio of an instrumental signal for an unknown sample with that of a known “standard”. Very few • developing and performing specific reference mea- chemical measurements are being performed with a surements, clearly stated traceability of their results. • producing Certified Reference Materials (CRMs), Accreditation schemes can improve the quality of • organizing International Measurement Evaluation measurements by making them comply with formal Programs, procedures, but the problem of the reliability and de- • establishing community databases, gree of equivalence of the results must be addressed, • performing prenormative research related to Euro- and this issue has been tackled only recently. pean norms or standards, and The essence of the problem lies in applying the prin- • offering special high-level training in measurement ciples of Metrology in Chemistry (MiC). Metrology is sciences. still rather new in chemistry but will grow according to IRMM has developed its expertise and facilities to the increasing emphasis on establishing the degree of be able to deliver the highest quality of primary mea- equivalence and the need for demonstrating the true surements. Host to the largest isotope mass spectrom- reliability of the measurement results. The BIPM Gets Involved The International Consultative Committee on Amount of Substance (CCQM—Comité Consultatif pour la Quantité de Matière) was set up in 1995 by the Paris- based International Bureau of Weights and Measures (BIPM—Bureau International des Poids et Mesures). Key tools of MiC are primary methods of measurement, defined by the CCQM as methods for which a com- plete uncertainty statement can be written down in terms of SI units and whose results are accepted without ref- erence to a standard of the quantity being measured. So far, the CCQM recognizes five such methods, namely, titrimetry, coulometry, gravimetry, determina- tion of freezing point depression, and—most versatile and widely used—isotope dilution mass spectrometry (IDMS), which has been applied and refined at the In- stitute for Reference Materials and Measurements (IRMM) for over 30 years. IMEP-10 bottle for the determination of Cd, Cr, Hg, The basis of IDMS is that one measures an unknown Pb, As, Cl, Br, and S in polyethylene. 2 Chemistry International, 2001, Vol. 23, No. 1 etry facility in Europe, IRMM also has extensive tech- ditions, and they may choose their own techniques, nological material preparation facilities. This unique procedures, and instrumentation (preferably routine combination yields Certified Test Samples (CTS) in conditions). They are requested to report their re- “real life” matrices fully characterized for homogene- sults with a best estimate of combined uncertainty ity and stability. according to ISO/BIPM guidelines. IMEP runs in adherence to the IRMM mission. • IMEP is open to all laboratories, and full confiden- Today IMEP also runs under the auspices of IUPAC, tiality with respect to results and participant’s iden- Analytical Chemistry in Europe (EURACHEM), tity is guaranteed. European Organization of Metrology (EUROMET), • IMEP focuses on “real life” samples that represent and Cooperation on International Traceability in highly important areas (food, environment, health, Analytical Chemistry (CITAC). etc.). IMEP enables field laboratories to compare their • IMEP graphically displays reference values and re- results against SI-traceable reference values. CTS with sults from participating laboratories, but the conclu- undisclosed values are sent to interested participants. sions are to be drawn by the laboratories themselves. The participants are asked to return values together with Additional support can be offered by IRMM after uncertainty statements claiming in writing to contain the measurement round. so-called “true” values. The undisclosed reference • IMEP is coordinated by IRMM. The establishment values are established by measurement procedures of reference values with their uncertainty (which based (where possible) on IDMS applied as a primary gives a reference range) is a time-consuming and method of measurement. Thus, IMEP basically expensive task. It is accomplished in collaboration demonstrates the degree of equivalence of results of with several partners—worldwide “IMEP reference chemical measurements for individual laboratories on laboratories” that can demonstrate a track record of the international scene as evaluated against results of successful applications of IDMS as a primary primary methods of measurement. method of measurement. • IMEP is complementary to collaborative studies and • Laboratories evaluate themselves under normal con- proficiency testing schemes; the latter are more fre- Table 1 Completed IMEP rounds. IMEP Title Time Period Publication 1 Li in human serum 1988 Fres. Z. Anal. Chem. 332, 718–721 (1988) 2 Cd in polyethylene 1990–91 Fres. Z. Anal. Chem. 345, 310–313 (1993) 3 Trace elements in water 1991–93 Accred. Qual. Assur. 1, 71–82 (1996) 4 Trace elements in bovine serum 1991–95 Accred. Qual. Assur. 3, 447–458 (1998) 5 Fe in human serum 1991–94 Scand. J. Clin. Lab. Invest. 53, Suppl. 212, 38 (1993) 6 Trace elements in water 1994–95 Accred. Qual. Assur. 3, 56–68, (1998)
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