Chemical form and distribution of mercury and selenium in canned tuna

The content, chemical form and distribution of mercury and selenium were determined for several samples of canned tuna. Samples represented albacore, bluefin, skipjack and yellowfin species, and were packed in either water or vegetable oil. Recently-processed samples (less than one year old) and much older samples (8-29 years old) were examined. Methylmercury was the predominant mercury form in all samples, comprising 57.4-94.7% of the total mercury content. For these samples, 7.6-44.8% of the total selenium content was present as selenate. There was no apparent effect of packing medium, species or sample age on the methylmercury and selenate percentages. Although the total selenium content exceeded that of total mercury, there was no apparent correlation between the total levels of both elements. Sample age influenced the level of water-extractable mercury and selenium in canned tuna. For recently canned samples, an average of 27.0% of the total mercury and 55.6% of the total selenium content were water-extractable. However, for the older samples, the corresponding respective average extractable levels were 46.4 and 48.2%. For both categories, inorganic mercury and selenate were more extractable, on an average percentage basis. In addition, an average of 7.2% of the total mercury content was benzene-extractable.