Preparation and certification of ERM-AE670, a 202Hg enriched methylmercury isotopic reference material

Abstract

The need for comparability of methylmercury, noted MeHg, measurement results and for the reduction of their associated uncertainties is hampered by the lack of suitable calibration materials. An isotopic certified reference material, ICRM, of MeHg containing isotopically enriched Hg has the potential to answer these problems, in combination with the isotope dilution mass spectrometry, IDMS, strategy. The objective of this work was to produce such a material, of demonstrated stability and carrying certified values traceable to the SI accompanied by combined uncertainty statements. Mercury oxide enriched in ²⁰²Hg was used for the preparation of a solution of ²⁰²Hg enriched CH₃HgCl. The starting material had previously been employed for the preparation of ERM-AE640; a ²⁰²Hg enriched inorganic mercury ICRM. The CH₃HgCl was synthesised by reaction with a Grignard reagent and a subsequent comproportionation reaction between (CH₃)₂Hg and HgCl₂. The process was optimised to give a high yield of the product, minimise contamination with ^naturalHg and additional steps were applied to purify the material from other Hg species and by-products of the synthesis reaction. Aliquots of 5 g of the solution were sealed in quartz ampoules for later distribution as a reference material, named ERM-AE670. Using gas chromatography inductively coupled plasma mass spectrometry, only MeHg and inorganic Hg was detectable in the finished reference material with inorganic Hg in <2% of the total amount. The isotopic composition of Hg in the form of MeHg was confirmed to be identical to that of ERM-AE640, within enlarged uncertainty statements. The Hg amount content in the form of MeHg was obtained by subtraction of the inorganic Hg amount content from the total Hg amount content (determined by IDMS in the digested material). The final uncertainty on the Hg amount content in the form of MeHg (3.5% relative, k = 2) included a contribution covering for potential changes over 2 years of shelf-life. This contribution was estimated from the results of a 1 year isochronous stability study.