Title: SI-Traceable Certification of Methylmercury Amount Content in a Tuna Material
Publication Year: 2005
JRC N°: JRC32591
URI: http://publications.jrc.ec.europa.eu/repository/handle/JRC32591
Type: Articles in Journals
Abstract: Speciation analysis has an ever-increasing relevance in environmental studies, for risk assessment and eventually, for legislation. There is a concomitant need for reliability and comparability of speciation measurements made at different places and at different times. To assist in the comparability and accuracy of speciation measurement results, the concepts of traceability (to a stated system of reference) and combined uncertainty budgeting can be applied. Traceability can be visualised as a chain, or a combination of chains, between a measurement result and a reference where each link in the chain(s) is a mathematical expression representing our best understanding of a measurement process, within a declared level of uncertainty. The uncertainties associated to each parameter of the traceability chain are used to calculate the combined uncertainty of the measurement result. The uncertainty budget can also help method optimisation and in the diagnosis of results in error or those with high deviation, by highlighting components that contribute significant uncertainty, provided that each parameter affecting the result in is included in the budget and carries an appropriate uncertainty. This work shows how a methylmercury, MeHg, measurement was made, and how understanding of the measurement process could make the result reliable and comparable to results form different laboratories. In addition, this work also shows how the MeHg measurement process was validated according ISO17025 through the use of reference materials for calibration, comparison of results with different methods, a systematic assessment of factors influencing the result, by calculating the combined uncertainty and through participation in inter-laboratory comparisons. A tuna material was distributed as a test sample for the laboratory inter-comparison studies Comité Consultatif pour la Quantité de Matière (CCQM) pilot study P39 and International Measurement Evaluation Programme (IMEP) comparison 20, on pollutant measurement. MeHg was measured using species-specific isotope dilution, SSID, by blending samples with an isotopic certified reference material, ERM® AE670, which contains CH3HgCl isotopically enriched in 202Hg, followed by extraction and derivatisation of Hg species and detection by gas chromatography – inductively coupled plasma mass spectrometry. SSID was applied to provide mass fractions traceable to the SI through the comparison of isotope ratios measured in blends prepared by metrological weighing, with reference materials, carrying values traceable to the SI. Uncertainties associated with each of the experimental steps of the measurement were investigated and combined uncertainties were calculated according to ISO guidelines. For submission to the CCQM P 39 comparison, the mass fraction of Hg in the form of MeHg in the tuna sample carried an expanded combined uncertainty of 5.6 %. The level of equivalence was demonstrated in that the standard deviation of mass fractions reported by 8 participating laboratories that applied SSID was 1.9 % and the IRMM value lay 0.4 % from the mean. From the uncertainty budget, it was evident that the greatest contributions to uncertainty on the result stem from the degree to which sample + spike equilibration could be proven, the certified MeHg concentration of the spike material and where appropriate, sample homogeneity. Uncertainty contributions from the known natural Hg isotopic composition, isotope ratio measurement and instrumental correction factors were minor in comparison.
JRC Institute:Institute for Reference Materials and Measurements

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