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Methylmercury inTtuna: Demonstrating Measurement Capabilities and Evaluating Comparability of Results Worldwide From the CCQM P-39 Comparison

Published

Author(s)

C Quetel, J P. Snell, Y Aregbe, L Abranko, Z S. Jokai, C Brunori, R Morabito, W Hagan, S. Azemard, E Wyse, V Fajon, M. Horvat, M. Logar, O.F. X. Donard, E Krupp, J Entwisle, R Hearn, Michele M. Schantz, K Inagaki, A Takatsu, P Grinberg, S. N. Willie, H Hintelmann, E Bjorn, E Blanco Gonzalez, G Centineo, J Ignacio Garcia Alonso, A Sanz-Medel

Abstract

Six Metrology Institutes (NMIs), representing at the Comit International des Poids et Mesures (CIPM) 4 Member States of the Metre Convention and 2 international organisations, and 8 expert laboratories selected outside CIPM have compared their capabilities to quantitatively measure methylmercury (MeHg) in a prepared tuna material. This comparison was the object of the CIPM Comit Consultatif pour la Quantit de Mati re (CCQM) Pilot Study 39. The Institute for Reference Materials and Measurements (IRMM), from the European Commission Joint Research Centre offered to organise it and it was agreed and launched during the April and October 2002 CCQM meetings as an activity of the Inorganic Analysis Working Group. The participants were free to apply the measurement strategy of their choice. Beside the test material itself, a bottle of the BCR-464 tuna Certified Reference Material (CRM) and an ampoule of IRMM 670, a 202Hg isotope enriched MeHg candidate isotopic CRM, were distributed to all. Four, including 1 NMI, relied on external calibration or the method of standard additions, whereas the other 10 implemented an isotope dilution mass spectrometry (IDMS) approach and choose to use the IRMM-670 for their measurements. Alkaline digestion at room temperature (with mechanical shaking) or high temperature (under sonication, oven or hot plate conditions) was employed by most participants with hydrochloric acid leaching the second most popular choice. Alkylation (4 phenylations, 4 ethylations and 3 propylations) in the aqueous phase was preferred by a large majority over butylation by the Grignard reaction. One participant applied a unique method with protease digestion of the sample with liquid chromatography for separation of Hg species. All the other participants used capillary gas chromatography. The 4 non-IDMS participants used cold vapour atomic fluorescence spectrometry for the detection step, whereas 7 of the others used inductively coupled plasma MS and the remaining 3 electron impact MS. All participants were requested to estimate the uncertainty associated to their results and 9 out of 14 stated relative combined uncertainties below 6 % (k=2). Despite this apparent homogeneity, the perception of which factor caused the largest contribution to this estimation differed among participants because of the differences in the analytical methodologies deployed but also because of wide differences of concepts for the uncertainty estimation. For the 14 participants the average was 1.93 0.57 10 5 mol Kg-1 (95 % confidence) and the mixture mode median, calculated also from the measurement uncertainties stated by the participants, was 1.967 0.204 10 5 mol Kg-1 (95 % confidence). Twelve of these results were regrouped within a range of less than 0.3 10 5 mol Kg-1, they nearly all (1 exception) overlapped with each other within k=2 stated uncertainties, and for this sub-group the average value was 1.962 0.155 10-5 mol Kg-1 (95 % confidence) and the mixture mode median was 1.967 0.162 10 5 mol Kg-1 (95 % confidence). For the other 2 results the uncertainty seemed to have been particularly underestimated as they lied respectively at more than 20 % above and less than 40 % below the overall average. A posteriori these two laboratories identified problems that seemed to have been the cause of significant unrecognised measurement biases, and the results of their new sets of measurements agreed with those of the other participants. The relative standard deviation of the results of 9 laboratories out of the 10 that applied IDMS was about 2.6 %. Among these laboratories there was also less than 3 % difference between those that applied direct IDMS and the 3 that applied double IDMS. Incidentally, this is also a validation of the IRMM-670 draft certificate values that had been provided to the participants. Although it could be argued that the Hg content in this tuna material was close to the legal limits and rath
Citation
Journal of Analytical Atomic Spectrometry

Keywords

CCQM, chemical metrology, dilution, laboratory intercomparison, mercury, methylmercury, reference material, species-specific isotope, traceability

Citation

Quetel, C. , Snell, J. , Aregbe, Y. , Abranko, L. , Jokai, Z. , Brunori, C. , Morabito, R. , Hagan, W. , Azemard, S. , Wyse, E. , Fajon, V. , Horvat, M. , Logar, M. , Donard, O. , Krupp, E. , Entwisle, J. , Hearn, R. , Schantz, M. , Inagaki, K. , Takatsu, A. , Grinberg, P. , Willie, S. , Hintelmann, H. , Bjorn, E. , Blanco Gonzalez, E. , Centineo, G. , Ignacio Garcia Alonso, J. and Sanz-Medel, A. (2021), Methylmercury inTtuna: Demonstrating Measurement Capabilities and Evaluating Comparability of Results Worldwide From the CCQM P-39 Comparison, Journal of Analytical Atomic Spectrometry (Accessed April 18, 2024)
Created October 12, 2021