Measurement of mercury methylation in sediments by using enriched stable mercury isotopes combined with methylmercury determination by gas chromatography–inductively coupled plasma mass spectrometry

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Abstract
A novel technique for the calculation of mercury methylation rates in sediments by using enriched stable mercury isotopes is described. The method takes advantages of the ability of an inductively coupled plasma mass spectrometry (ICP-MS) instrument to measure individual isotopes. An ICP-MS instrument was used as a detector for the determination of methylmercury compounds after separation by gas chromatography (GC). CH3Hg+ was isolated from sediments by distillation, converted to methylethylmercury by sodium tetraethylborate and analysed after purge-and-trap pre-collection on a Tenax adsorber and thermodesorption onto the GC column. Detection limits were found to be ≈ 1 pg (as Hg) absolute or 0.02 ng g–1 dry sediment. The precision was ≈ 4% relative standard deviation when 250 pg of methylmercury were processed. The accuracy of the GC–ICP-MS technique was demonstrated by analysis of an International Atomic Energy Agency certified reference material (IAEA CRM 356) Harbor Sediment, giving a concentration of 5.40 ± 0.40 ng g–1, compared with the certified value of 5.46 ± 0.38 ng g–1. Mercury methylation was investigated by spiking sediments with stable enriched mercury isotopes at in situ mercury concentrations not perturbing the system. More than 3% of the mercury added to a lake sediment was methylated during a 21 d incubation period. Isotope ratios of total mercury differed significantly from isotope ratios of methylmercury at the end of the experiment, suggesting that the system was still not in equilibrium after 21 d.

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