Applications of enriched stable isotope tracers in combination with isotope dilution GC-ICP-MS to study mercury species transformation in sea sediments during in situ ethylation and determination

An isotope dilution (ID) based GC-ICP-MS method, able to simultaneously det ermine incipient concentrations of methylmercury as well as the degree of m ethylation of inorganic mercury and de-methylation of methylmercury in brac kish water sediments, was developed. Methyl- and inorganic mercury were ext racted from the sediments using a solution containing KBr-H2SO4-CuSO4. CH3 Hg-200(+) was added as a species-specific internal standard and Hg-199(2+) to monitor artefact formation of methylmercury during sample work-up. Extra cted mercury species were treated with NaBEt4 to form ethyl derivatives tha t were trapped on Tenax. The typical precision of the method was in the ran ge of 2-3% RSD for methylmercury concentrations of 2-5 ng g(-1) sediment (d ry 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 m etal. When using bromide for extraction, degradation of methylmercury into elemental mercury and the probable formation of methylmercury bromide durin g sample pretreatment and/or analysis were observed. For investigations of mercury methylation and de-methylation during sample incubation, Hg-201(2+) and CH3 Hg-198(+) were added to sediment layers at 20% of the ambient conc entrations of the two species. Mercury methylation and de-methylation activ ities differed by a factor of four between depths and were highest in the t op 3 cm of the sediment. In this region 9.5% of the added Hg-201(2+) tracer was methylated and 79% of the CH3 Hg-198(+) tracer was de-methylated. Dete ction limits for methylation of Hg-201(2+) and de-methylation of CH3 Hg-198 (+) were 0.1 ng g(-1) and 0.2 ng g(-1), respectively. Methylmercury concent rations changed about 5 times over the depth profile, which implies that sa mples have to be taken over a well-defined depth when assessing representat ive data.