Uncertainty contributions to species specific isotope dilution analysis. Part 2. Determination of methylmercury by HPLC coupled with quadrupole and multicollector ICP-MS

Species-specific isotope dilution analysis (IDA) has been used to determine the mass fraction of methylmercury in a fish muscle certified reference material, DORM-2. Two spike solutions were prepared and their mass fractions determined by coupling high-performance liquid chromatography (HPLC) with both quadrupole (Q) and multicollector (MC) inductively coupled plasma mass spectrometry (ICP-MS) and uncertainty budgets calculated. For the Q-ICP-MS determination the relative standard uncertainty of the spike was 4.1%. The major uncertainty contribution (62%) arose from the uncertainties associated with the relative isotopic abundances of Hg isotopes as given by IUPAC. The measured isotope amount ratios (Hg-200:Hg-199 and Tl-205:Tl-203) contributed 37% of the combined uncertainty. For the multicollector instrument the relative standard uncertainty of the spike solution mass fraction was 0.7%. In this case the uncertainty budget was dominated by the Hg isotopic uncertainties, with a contribution of 99%. The uncertainty contributions from the measured isotope amount ratios were reduced to 0.7% of the total when using MC-ICP-MS. The two spike solutions were separately used to determine the mass fraction of methylmercury in DORM-2 CRM by species-specific IDA, using HPLC coupled with quadrupole ICP-MS. The values found were 4.45 +/- 0.90 mug g(-1) and 4.25 +/- 0.47 mug g(-1) using spikes characterised by Q- and MC-ICP-MS, respectively (the uncertainties quoted are the expanded uncertainty, k = 2), compared with a certified value of 4.47 +/- 0.32. The major uncertainty contribution for each determination was from the measured isotope amount ratios (70%) with lesser contributions from the uncertainties associated with the natural isotopic abundance of Hg and the spike mass fraction.