Evaluation of different sample pre-treatment and extraction procedures for mercury speciation in fish samples

Investigations are described to select the most suitable drying and extraction procedures for mercury speciation in fish samples. The criteria for selection were good recovery and preservation of the original species information. Three different drying techniques were evaluated: oven-dried, microwave oven-dried and freeze-dried. Best results were obtained when oven-dried, but microwave dried or freeze-dried showed mercury losses. The four extraction methods investigated were: HCl leaching, alkaline-methanol extraction with tetramethylammonium hydroxide (TMAH) and with KOH, and sodium dodecyl sulfate (SDS) extraction. The extraction efficiency was determined by comparing the total amount of mercury in the samples after microwave digestion with the amount extracted by the different methods described. Mercury measurements were carried out by flow injection-cold vapour-atomic fluorescence spectrometry (FI-CV-AFS). Quantitative mercury extraction was only achieved with HCl leaching (97%). Species integrity after extraction was evaluated by investigating the extracts with capillary gas chromatography-atomic fluorescence spectrometry. Artificial formation of dimethylmercury occurs when TMAH is used as extractant. HCl leaching-GC-AFS was finally applied to methylmercury determination in tuna fish and swordfish samples, and gave results in the range 0.78-1.93 mg kg(-1). The detection limit for monomethylmercury (MMHg) was found to be 1.2 pg.