Determination of methylmercury in sediments using supercritical fluid extraction and gas chromatography coupled with microwave-induced plasma atomic emission spectrometry

Abstract

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. Butylmagnesium chloride was used to derivatize the target compound to butylmethylmercury which is amenable to GC. Using a commercially available reference sediment (PACS-1, National Research Council of Canada) as the model sample, a factorial design was utilized to investigate the effect of three variables; density, temperature and flow rate, on the extraction efficiency. An extraction efficiency of 49 ± 0.5% could be obtained for a 37.5 min dynamic extraction, corresponding to 25 thimble volumes of supercritical CO₂, and using purified support sand. Studies on the efficacy of SFE for another sediment matrix as a function of time have also been undertaken. Repeated pressure reductions in combination with support sand were found to increase the extraction efficiency of methylmercury from PACS-1 but not from a sediment issued by the Community Bureau of Reference (BCR) as part of an interlaboratory comparison. For PACS-1 this resulted in an increase in the average extraction efficiency to 96% for duplicate determinations following 50 thimble volume sweeps. Distillation was used as a reference method for isolation 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 methylmercury in sediment using the described SFE GC–MIP–AES method is estimated to be 0.1 ng g^–1 based on a 20 µl 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 supported by the strong correlation between the concentrations of butylmethylmercury and dibutylsulfide found in the toluene extract. Using a stable isotope tracer, ¹⁹⁹Hg, and ICP-MS, evidence for the spurious formation of methylmercury during SFE under certain conditions is also presented.