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

摘要

HgO enriched in ~(202)Hg was used for the preparation of a solution of ~(202)Hg enriched CH_3Hg. The CH_3HgCl was synthesised by reaction with a Grignard reagent and a subsequent comproportionation reaction between dimethylmercury, (CH_3)_2Hg, and HgCl_2, 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 CH_3HgCl for use as an IDMS spike, it was dissolved in 2 percent ethanol. 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 CH_3Hg 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 CH_3Hg and inorganic Hg were detectable in the reference material with inorganic Hg in <2 percent of the total amount. GC-ICP-MS was also used to confirm that the isotopic composition of Hg in the form of CH_3Hg was identical to that of 1RMM-640, an inorganic Hg ICRM prepared from the same ~(202)Hg enriched HgO, within enlarged uncertainty statements. These processes allowed the Si-traceable certification of both the amount content of CH_3Hg and its isotopic composition, accompanied by combined uncertainty statements estimated according to ISO/GUM.