Determination of monomethylmercury in low- and high-polluted sediments by microwave extraction and gas chromatography with atomic fluorescence detection

The analysis of monomethylmercury (MMHg) in sediments is a difficult task because both very low concentrations and interconversion between species especially in highly polluted samples are frequent. This work features a general strategy for real sediment analysis with preconcentration and/or clean-up steps for both low- and high-polluted sediments to control these specific problems. The extraction conditions have been optimized using closed-vessel microwave-assisted heating with acidic extractants. The analysis has been carried out by the injection of ethylated derivatives of the analytes into a capillary gas chromatographic system coupled to fluorescence spectrometry. When using 6 M HNO3, the most labile inorganic mercury fraction as well as MMHg were extracted from the sediment but there was still some inorganic mercury that remained un-extracted. MMHg was stable and quantitatively recovered by applying this procedure. The role of the labile inorganic fraction on artifact MMHg generation has been evaluated and it has been found to be highly variable depending on the sediments’ geochemical characteristics. Therefore, for high-polluted sediments (inorganic mercury concentration above 500 ng g−1) a clean-up step with dichloromethane has been used before ethylation, whereas for low content samples, preconcentration under nitrogen stream at room temperature has been optimized. Both steps can be combined if necessary. MMHg content has been found in good agreement with the certified value for the reference materials (IAEA-405 and ERM-CC580).