Results of the international aqueous mercury speciation intercomparison exercise

Abstract

Twenty-seven laboratories from around the world agreed to participate in an intercomparison exercise for total Hg (Hgt) and methyl Hg (MMHg) in pristine lake water. Unfiltered samples from a remote brown water lake in northern Wisconsin (USA) were collected into acid cleaned Teflon® bottles using ultra-clean sample handling techniques. The samples were acidified in the field with 0.4% by volume of pre-analyzed HQ (12N; <5 pg Hg/mL), and sent to the primary reference laboratory (PRL) by overnight mail. Within one week of receipt, the samples were randomized, and 10% analyzed for Hgt and MMHg at the PRL to verify the homogeneity of the set. Each participating laboratory was then sent 3 randomly selected 1 L bottles, while the PRL retained 30, and the secondary reference laboratory (SRL) retained 12 samples. The participating laboratories were asked to analyze each bottle in triplicate for both Hgt and MMHg, reporting all QA data including blanks, spike recoveries, and detection limits. The PRL analyzed samples in triplicate at both the beginning and the end of the analytical window, to provide a controlled estimate of any changes in concentration or speciation over that time. Of the 23 laboratories that returned results, 18 utilized Br CI oxidation, gold trapping, and cold vapor atomic fluorescence (CVAFS) detection for Hgt-Four laboratories reported similar techniques, varying either in detector (cold vapor atomic absorption), or wet chemistry. Only 16 laboratories reported MMHg results, with 15 using a variation of the aqueous phase ethylation, GC separation, and CVAFS detection technique. The results show good convergence between the participating labs for both Hgt and MMHg. For Hgt 18 of 23 labs reported means within 20% of the consensus value and PRL results (1.27 ± 0.18 ng/L and 1.27 ± 0.14 ng/L respectively). For MMHg, 13 of the 16 labs reported results within 20% of either the consensus value (0.420 ± 0.055 ng/L) or the PRL value (0.446 ± 0.041 ng/L).