Extraction of monomethylmercury from seawater for low-femtomolar determination

Humans are exposed to toxic monomethylmercury (MMHg) principally by the consumption of marine fish. However, and due in part to analytical limitations, little is known about the distribution, sources, and biogeochemical cycling of MMHg in the ocean, where aqueous concentrations are in the femtomolar range and often less than current limits of detection. Here, we present a simple method for extraction and analysis of MMHg in seawater that has a detection limit of about 2 fM for a 2-L sample, which is a 10-fold improvement over current approaches. The technique, which is readily adaptable to a shipboard laboratory, involves purging and quantification of dimethylmercury (DMHg) from an unaltered 2-L water sample followed by acidification to 1% with H(2)SO(4) for > 6 h, pH neutralization, derivatization of MMHg in the seawater matrix with an ethylating agent, and purge-and-trap analysis with gas-chromatographic cold-vapor atomic fluorescence spectrometry. The method was developed and validated with analyses of seawater from the North Atlantic and Pacific Oceans, in addition to two fresh waters. This technique can be used to quantify, differentiate, and develop an improved understanding of the biogeochemistries of MMHg and DMHg in the ocean.