Low pressure inductively coupled plasma ion source for atomic and molecular mass spectrometry: Investigation of alternative reagent gases for organomercury speciation in tissue and sediment

A low pressure ICP has been operated in ‘atomic' and ‘molecular' modes. Atomic mass spectra for methylmercury were obtained when the plasma was sustained in pure helium between 9 and 12 W power, and molecular spectra were obtained at 5 W power in the presence of a reagent gas. Methane, isobutane and ammonia were investigated as reagent gases, with ammonia yielding the best stability and sensitivity. A detection limit of 8 pg was obtained for Me2Hg in ‘atomic' mode, and detection limits of 18, 48 and 20 pg were obtained for MeHgEt, MeHgCl and Me2Hg, respectively, in ‘molecular' mode. Four extraction methods were compared for the determination of methylmercury spiked into sediment, with the best extraction efficiencies of 70–90% being obtained by microwave extraction, both with and without an ethylation step. Two extraction methods were compared for the determination of methylmercury in TORT-2 certified reference material. Microwave extraction with derivatization resulted in a found value of 0.171 ± 0.018 µg g−1 compared to the certified value of 0.152 ± 0.013 µg g−1.