Production of artifact methylmercury during the analysis of certified reference sediments: Use of ionic exchange in the sample treatment step to minimise the problem

Production of artifact methylmercury (MeHg+) during the analysis of two certified reference sediments, CRM-580 and IAEA-405, was investigated. Leaching of the analyte from the solid sample was achieved by ultrasound assisted acidic extraction. The aqueous leachate was either ethylated (NaBEt4) or phenylated (NaBPh4) using acetic/acetate or citric/citrate to buffer the solution. Preconcentration of the volatile compounds was carried out by extraction with an organic solvent (n-hexane) or solid phase microextraction (SPME). MeHg+ was finally separated and detected by gas chromatography with atomic emission or mass spectrometry detection (GC–MIP-AED or GC–MS). In all the cases the concentrations obtained for MeHg+ in the CRM-580 were significantly higher than the certified value. For the IAEA-405, however, the MeHg+ concentration found was always statistically indistinguishable from the certified value. Experiments were also conducted with synthetic samples, such as aqueous mixtures of MeHg+ and inorganic mercury (Hg2+) or silica-gel spiked with both compounds. The methylation rates found (defined as the percentage of Hg2+ present in the sample which methylates to give artifact MeHg+) ranged from not observable (in certain synthetic aqueous mixtures) to 0.57% (analysis of CRM-580 under certain conditions). As the amount of Hg2+ available in the sample seems to be the main factor controlling the magnitude of the artifact, several experiments were conducted using an ionic exchange resin (Dowex M-41) in order to minimise the concentration of this chemical in the reaction medium. First, a hydrochloric leachate of the sample was passed through a microcolumn packed with the exchanger. Second, the resin was mixed with the sample prior to extraction with HCl. In both cases, the predominant Hg2+ species, HgCl42−, was adsorbed on the resin, whereas MeHg+, mainly as MeHgCl, remained in solution. Following the second option, a new method to analyse MeHg+ in conflictive matrices like certain sediments was proposed. This approach produced better results for the CRM-580, but a MeHg+ concentration slightly, but statistically significant, higher than the reference value was still obtained.