On the Chemical Form of Mercury in Edible Fish and Marine Invertebrate Tissue

Total mercury, monomethylmercury (CH 3 Hg), and dimethylmercury ((CH 3 ) 2 Hg) in edible muscle were examined in 229 samples, representing seven freshwater and eight saltwater fish species and several species of marine invertebrates using ultraclean techniques. Total mercury was determined by hot HNO 3 /H 2 SO 4 /BrClldigestion, SnCl 2 reduction, purging onto gold, and analysis by cold vapor atomic fluorescence spectrometry (CVAFS). Methylmercury was determined by KOH/methanol digestion using aqueous phase ethylation, cryogenic gas chromatography, and CVAFS detection. Total mercury and CH 3 Hg concentrations varied from 0.011 to 2.78 μg∙g −1 (wet weight basis, as Hg) for all samples, while no sample contained detectable (CH 3 ) 2 Hg (<0.001 μg∙g −1 as Hg). The observed proportion of total mercury (as CH 3 Hg) ranged from 69 to 132%, with a relative standard deviation for quintuplicate analysis of about 10%; nearly all of this variability can be explained by the analytical variability of total mercury and CH 3 Hg. Poorly homogenized samples showed greater variability, primarily because total mercury and CH 3 Hg were measured on separate aliquots, which vary in mercury concentration, not speciation. I conclude that for all species studied, virtually ail (>95%) of the mercury present is as CH 3 Hg and that past reports of substantially lower CH 3 Hg fractions may have been biased by analytical and homogeneity variability.