Uncertainty contributions to single and double isotope dilution mass spectrometry with HPLC-CV-MC-ICP-MS for the determination of methylmercury in fish tissue

Abstract

A full uncertainty budget has been used to compare the performance of species specific single and ‘approximate matching’ double isotope dilution mass spectrometry (IDMS). Two fish tissue reference materials were employed in this study, DORM-2 (NRC, Canada) and BCR464. Methylmercury was separated from inorganic mercury by HPLC, which was coupled to multicollector ICP-MS via a cold vapour generation system for the measurement of isotope amount ratios. For each CRM the mass fraction of methylmercury determined by the two IDMS methods was not statistically different, within the limits of uncertainty, from the certified values. Double IDMS was found to be more precise than single IDMS when a full uncertainty budget was estimated, the converse applied when the method precision was estimated only by the standard deviation of replicate analysis. For single IDMS the major uncertainty contribution was derived from the standard uncertainty of each analytical replicate, Uwithin. The between blend standard uncertainty, Ubetween, was the major contributor to the expanded uncertainty for approximate matching double IDMS.