Speciation of mercury at ng/ml concentration levels by capillary electrophoresis with amperometric detection

A technique based on capillary electrophoresis and amperometric detection (CE-AD) has been developed for the speciation of mercury. This technique has the unique capability to detect only cationic mercury species that are electrochemically active. Capillary electrophoresis with electrokinetic injection allows efficient separation of inorganic mercury and organomercury cations in 8 min. Selective detection of these electrochemically active species is attained by controlling the reduction potential applied on the micro-electrode. For Hg2+, an optimal potential of −0.2 V can be used to prevent interference by less electroactive toxic metals and other substances found in complex environmental samples. The amperometric signal is linearly proportional to the Hg2+ concentration over three orders of magnitude, with a detection limit of 0.2 ng/ml. The mass detection limit corresponds to 8 fg of Hg2+ in an injection volume of 40 nl. For CH3Hg+, the detection limit is 3 ng/ml when a potential of −0.5 V is used. These detection sensitivities are attractive for environmental monitoring of contaminated sediments in ecosystems. Steam distillation is evaluated for the extraction of CH3Hg+ from the sediment matrix. It yields an unknown mercury species which is unsuitable for CE-AD determination under the specified conditions.