Dual-cloud point extraction for speciation of mercury in water and fish samples by electrothermal atomic absorption spectrometry

Highlights: • A d-CPE was developed for mercury speciation in freshwater fish samples by ETAAS. • Method was applied to the speciation of Hg in water and freshwater fish samples. • This method showed reproducibility and reliability. -- Abstract: A dual-cloud point extraction (d-CPE) technique was established for preconcentration and speciation of mercury ions (Hg(II) and CH₃Hg) in water and fresh water fish samples by electrothermal atomic absorption spectrometry (ETAAS). In the first step of d-CPE, Hg(II) was complexed with Dithizone (DTZ) and transferred into a nonionic surfactant (Triton X-114) solution. In the second step, the complexes of Hg(II)-DTZ in the surfactant phase were back-extracted with an aqueous solution of thiourea in nitric acid combined with heating and ultrasonication. Then, final amount of Hg(II) was detected by ETAAS. The CH₃Hg concentration was calculated by subtracting the concentration of Hg(II) from the total mercury concentration after the conversion of CH₃Hg to Hg(II) by acid microwave-assisted digestion. The conditions affecting the extraction of Hg(II) were investigated in detail and fully optimized. The enrichment factor for this method was 17.8, with a linear range of 0.40–15.0 μg L^-1 and a correlation coefficient of 0.9997 along with a detection limit of 0.23 μg L^-1. Relative standard deviations (RSDs, when concentration = 1.0 μg L^-1 and n = 6) was 4.85%. Standard reference materials (SRM-1566b, oyster tissue) was used to find out the accuracy of the proposed method, and the developed method was successfully applied for speciation and determination of mercury within several water and freshwater fish samples.