DETERMINATION OF METHYLMERCURY IN SEDIMENTS USING SUPERCRITICAL-FLUIDEXTRACTION AND GAS-CHROMATOGRAPHY COUPLED WITH MICROWAVE-INDUCED PLASMA-ATOMIC EMISSION-SPECTROMETRY

A method employing supercritical fluid extraction (SFE) and GC coupled with microwave-induced plasma atomic emission spectrometry (MIP-AES) is presented for the determination of methylmercury in sediments. Buty lmagnesium chloride was used to derivatize the target compound to buty lmethylmercury which is amenable to GC. Using a commercially available reference sediment (PACS-1, National Research Council of Canada) as t he model sample, a factorial design was utilized to investigate the ef fect of three variables; density, temperature and flow rate, on the ex traction efficiency. An extraction efficiency of 49 +/- 0.5% could be obtained for a 37.5 min dynamic extraction, corresponding to 25 thimbl e volumes of supercritical CO2, and using purified support sand. Studi es on the efficacy of SFE for another sediment matrix as a function of time have also been undertaken. Repeated pressure reductions in combi nation with support sand were found to increase the extraction efficie ncy of methylmercury from PACS-1 but not from a sediment issued by the Community Bureau of Reference (BCR) as part of an interlaboratory com parison. For PACS-1 this resulted in an increase in the average extrac tion efficiency to 96% for duplicate determinations following 50 thimb le volume sweeps. Distillation was used as a reference method for isol ation of methylmercury from sediments. Parallel extractions of the BCR sediment, using GC-MIP-AES for the final determination, gave results that were in good agreement and corresponded well with data submitted during the intercomparison exercise. The detection limit for the methy lmercury in sediment using the described SFE GC-MIP-AES method is esti mated to be 0.1 ng g(-1) based on a 20 mu l injection, 0.5 g of sample and three times the blank level. It is proposed that the co-extracted sulfur from the sediment mediates the transport of methylmercury and, to some extent, inorganic mercury from the sediments. This is support ed by the strong correlation between the concentrations of butylmethyl mercury and dibutylsulfide found in the toluene extract. Using a stabl e isotope tracer, Hg-199, and ICP-MS, evidence for the spurious format ion of methylmercury during SFE under certain conditions is also prese nted.