Sequential cloud point extraction for the speciation of mercury in seafood by inductively coupled plasma optical emission spectrometry

A novel nonchromatographic speciation technique for the speciation of mercury by sequential cloud point extraction (CPE) combined with inductively coupled plasma optical emission spectrometry (ICP-OES) was developed. The method based on Hg2+ was complexed with I- to form HgI42-, and the HgI42- reacted with the methyl green (MG) cation to form hydrophobic ion-associated complex, and the ion-associated complex was then extracted into the surfactant-rich phase of the non-ionic surfactant octylphenoxypolyethoxyethanol (Triton X-114), which are subsequently separated from methylmercury (MeHg+) in the initial solution by centrifugation. The surfactant-rich phase containing Hg(II) was diluted with 0.5 mol L-1 HNO3 for ICP-OES determination. The supernatant is also subjected to the similar CPE procedure for the preconcentration of MeHg+ by the addition of a chelating agent, ammonium pyrrolidine dithiocarbamate (APDC), in order to form water-insolvable complex with MeHg+. The MeHg+ in the micelles was directly analyzed after disposal as describe above. Under the optimized conditions, the extraction efficiency was 93.5% for Hg(II) and 51.5% for MeHg+ with the enrichment factor of 18.7 for Hg(II) and 10.3 for MeHg+, respectively. The limits of detection (LODs) were 56.3 ng L-1 for Hg(II) and 94.6 ng L-1 for MeHg+ (as Hg) with the relative standard deviations (RSDs) of 3.6% for Hg(II) and 4.5% for MeHg+ (C = 10 μg L-1, n = 7), respectively. The developed technique was applied to the speciation of mercury in real seafood samples and the recoveries for spiked samples were found to be in the range of 93.2-108.7%. For validation, a certified reference material of DORM-2 (dogfish muscle) was analyzed and the determined values are in good agreement with the certified values