Abstract

An isotope dilution (ID) based GC-ICP-MS method, able to simultaneously determine incipient concentrations of methylmercury as well as the degree of methylation of inorganic mercury and de-methylation of methylmercury in brackish water sediments, was developed. Methyl- and inorganic mercury were extracted from the sediments using a solution containing KBr–H₂SO₄–CuSO₄. CH₃²⁰⁰Hg⁺ was added as a species-specific internal standard and ¹⁹⁹Hg²⁺ to monitor artefact formation of methylmercury during sample work-up. Extracted mercury species were treated with NaBEt₄ to form ethyl derivatives that were trapped on Tenax. The typical precision of the method was in the range of 2–3% RSD for methylmercury concentrations of 2–5 ng g⁻¹ sediment (dry weight). The detection limit of the method based on 3 times the standard deviation of repeated sediment analysis was 0.3 ng g⁻¹ sediment as the metal. When using bromide for extraction, degradation of methylmercury into elemental mercury and the probable formation of methylmercury bromide during sample pretreatment and/or analysis were observed. For investigations of mercury methylation and de-methylation during sample incubation, ²⁰¹Hg²⁺ and CH₃¹⁹⁸Hg⁺ were added to sediment layers at 20% of the ambient concentrations of the two species. Mercury methylation and de-methylation activities differed by a factor of four between depths and were highest in the top 3 cm of the sediment. In this region 9.5% of the added ²⁰¹Hg²⁺ tracer was methylated and 79% of the CH₃¹⁹⁸Hg⁺ tracer was de-methylated. Detection limits for methylation of ²⁰¹Hg²⁺ and de-methylation of CH₃¹⁹⁸Hg⁺ were 0.1 ng g⁻¹ and 0.2 ng g⁻¹, respectively. Methylmercury concentrations changed about 5 times over the depth profile, which implies that samples have to be taken over a well-defined depth when assessing representative data.