Percent methylmercury and organic mercury in tissues of marine mammals and fish using different experimental and calculation methods

Muscle and liver tissues of marine mammals and fish were extracted with methylene chloride–hexane (DCM‐hexane) or toluene and the extracts were analyzed for organic mercury by cold vapor atomic absorption spectroscopy (CVAAS) and for methylmercury by gas liquid chromatography with an electron capture detector (GC‐ECD). Total mercury in tissues was determined by CVAAS. Methylmercury and organic mercury concentrations in muscle agreed with each other and with total mercury for both marine mammals and fish, indicating that on average 100% of the total mercury in this tissue was in the form of methylmercury. Either total mercury or organic mercury determined by CVAAS was found to be a valid measure of the average methylmercury concentration in muscles of marine mammals and fish. In liver, the CVAAS method produced higher mercury values (15% organic mercury) than the GC‐ECD method (6% methylmercury), methylmercury being only 38% of organic mercury. From this, the presence of organic mercury compound(s) other than methylmercury was inferred. The CVAAS method produced a biased estimate of methylmercury in liver; an accurate measure of methylmercury in this tissue was obtained by GC‐ECD, either with DCM‐hexane or with toluene extraction. The DCM‐hexane extract could be directly analyzed by CVAAS for organic mercury while toluene required an additional back‐extraction step for such a determination. In calculating the average percentage of methylmercury and organic mercury in a sample, three different calculation methods were used. Only one of them, linear, robust regression analysis produced acceptable results when the variables (MeHg, organic Hg, total Hg) were significantly correlated. The other two methods overestimated the mean percentage under these conditions.