Speciation of mercury in fish samples by solvent extraction, methyl-mercury reduction directly in the organic medium and cold vapour atomic absorption spectrometry

Abstract

The extraction of CH₃Hg⁺ as bromide from fish samples by chloroform and its direct determination in an organic medium by cold vapour atomic absorption spectrometry (CVAAS) is proposed. Parameters studied include the nature and concentration of the reductant and the acid stirring time. No matrix effect was observed. Inorganic Hg was determined in the residual aqueous phase by reduction with NaBH₄. Spiking studies showed good recoveries of 102±2%. The procedure was applied to fish samples and the ratio between Hg as CH₃Hg⁺ and total Hg was in the range 88–98%, usually found in the literature. The sum of the organic and inorganic Hg concentrations matched the total Hg value found by CVAAS after acid digestion. The detection limit (3σ) for the total analytical procedure was 25 ng of Hg (as CH₃Hg⁺) per gram of wet sample. The proposed procedure was also applied to two proficiency testing materials and to a standard reference material and the summation of organic and inorganic Hg concentration values were in good agreement with the reference values, according to a student t test for a significance level of 99%, n= 5.