A new 202Hg isotopically enriched methylmercury spike material with SI-traceable reference values for isotope dilution measurements in biological and environmental samples

HgO enriched in 202Hg was used for the preparation of a solution of 202Hg enriched CH3Hg. The CH3HgCl was synthesised by reaction with a Grignard reagent and a subsequent comproportionation reaction between dimethylmercury, (CH3)2Hg, and HgCl2, that was optimised to give a high yield of the product, pure from other Hg species and by-products of the synthesis reaction. To prepare the CH3HgCl for use as an IDMS spike, it was dissolved in 2 % ethanol. The spike was thereby maintained in a relatively reactive form without solution preservatives that might alter its chemical behaviour compared to incipient CH3Hg in a sample. Aliquots were sealed in quartz ampoules and a 1-year stability study was undertaken by storing a series of ampoules under different temperature conditions to all be measured on the same occasion (an isochronous study) and by retaining a portion of the solution in a closeable bottle under recommended storage conditions, with measurements at 3-month intervals. The Hg amount content in the form of CH3Hg was obtained by subtraction of the inorganic Hg amount content (determined by gas chromatography inductively coupled plasma mass spectrometry, GC-ICP-MS) from the total Hg amount content (determined by blending with IRMM-639, a natural Hg isotopic certified reference material, ICRM, and isotope dilution mass spectrometry of the digested material). Only CH3Hg and inorganic Hg were detectable in the reference material with inorganic Hg in <2 % of the total amount. GC-ICP-MS was also used to confirm that the isotopic composition of Hg in the form of CH3Hg was identical to that of IRMM-640, an inorganic Hg ICRM prepared from the same 202Hg enriched HgO, within enlarged uncertainty statements. These processes allowed the SI-traceable certification of both the amount content of CH3Hg and its isotopic composition, accompanied by combined uncertainty statements estimated according to ISO/GUM. The final uncertainty on the Hg amount content in the form of CH3Hg (3.5 % relative, k=2) included a contribution covering for potential changes over 2 years of shelf-life. No degradation of the CH3Hg content or isotopic composition was statistically observable over the storage period, so this contribution was extrapolated from the uncertainty in measurement repeatability of the isochronous study. Ampoules are now available for distribution as an ICRM, named IRMM-670, with the certified reference values