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The extraction efficiency of several metal cations (50 mM CoCl 2 , 20 mM NiCl 2 , 30 mM Pb(NO 3 ) 2 and 50 mM SrCl 2 ) and chelating agents (30 mM dimercaprol, 10 mM EDTA and 50 mM D-penicillamine) was evaluated in order to determine the optimal extracellular extractant for extracting Hg from the moss Pseudoscleropodium purum, by the sequential elution technique (SET). Moss samples were incubated with a high concentration of Hg (40 mg L 21 ) for one hour, and after the intercellular fraction was removed, the samples were subjected to the SET with the above-mentioned extractants. The chelating agents dimercaprol and D-penicillamine displaced most extracellular Hg, probably because both contain -SH groups, which have a high affinity for Hg. Dimercaprol was chosen as the best extractant because it displaced up to 93% of the extracellular metal. This compound was then used to extract Hg in an industrial gradient of Hg contamination, and the concentration of extracellular Hg was calculated relative to the total amount (80%). A new modification to the SET was also proposed. In the new version of the technique, the metals were determined in solid moss samples rather than in extracts. One advantage of the modification is that all of the determinations are carried out with the same analytical technique (direct mercury analysis), thereby minimizing the errors associated with the use of different techniques as in the previous version of the method. It was found that the cell compartment where the metals are located is not always accurately reflected by the SET, and therefore the use of other complementary techniques that show the correct location of the element under study is recommended.

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