A simple field sampling strategy for determination and speciation of inorganic mercury and methylmercury in river waters is described. River water samples are collected in situ by passing them with a pump through a minicolumn packed with C18 modified with sodium diethyldithiocarbamate. This packing is able to isolate and retain the desired mercury species. Minicolumns loaded with the analytes are then returned to the laboratory where mercury species are eluted with 500 μl of 5% thiourea in 0.5% HCl. A detailed study of the construction of these minicolumns and the stability of the mercury species retained in the packing is presented. The separation of the two mercury species was carried out by liquid chromatography using a cysteine solution as mobile phase and final detection was accomplished by inductively coupled plasma mass spectrometry. Between-column relative standard deviations of 12% at the 200 ng l−1 Hg level and limits of detection of 5.2 ng l−1 for inorganic mercury and 5.6 ng l−1 for methylmercury are obtained. The method has been applied to mercury speciation in clean river waters of the Asturias region of Spain.
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