Rapid Determination of Inorganic Mercury and Methylmercury in Biological Reference Materials by Hydride Generation, Cryofocusing, Atomic Absorption Spectrometry After Open Focused Microwave-assisted Alkaline Digestion

Abstract

A rapid and simple microwave-assisted digestion method with alkaline solution (tetramethylammonium hydroxide or methanolic KOH solution) for speciation analysis of inorganic mercury and methylmercury (MeHg⁺) in biological tissues was developed. Extracts with quantitative recoveries of mercury species after microwave-assisted alkaline dissolution of the sample were directly analysed by an automated on-line hyphenated system incorporating aqueous hydride generation, cryogenic trapping, gas chromatography and detection by atomic absorption spectrometry. Optimum conditions allow sample throughput to be controlled by the instrumental analysis time (≈10 min per sample) and not by the sample preparation step. At a power of 40–60 W, sample preparation time is only 2–4 min, which is much faster than previous literature methods. The proposed method was validated by the analysis of three biological certified reference materials, CRM 463, DORM-1 and TORT-1, and one BCR sample from an interlaboratory study, Tuna Fish 2. The detection limit of the overall procedure was found to be 3 ng per gram of pulverised dried biological tissue, for both labile Hg²⁺ and MeHg⁺. After alkaline extraction, a mean recovery of 102% with a relative standard deviation of 7% was obtained for methylmercury concentrations ranging from 0.128 to 3.464 µg g^-1 as Hg in the four reference fish tissues mentioned above. Total mercury, calculated as the sum of determined inorganic mercury and methylmercury, was also in agreement with the certified values. The methylmercury extraction efficiency of five extractants, viz., nitric, hydrochloric and acetic acid, and tetramethylammonium hydroxide and methanolic KOH solution, the temperature profile of these extractants under microwave irradiation and the behaviour and stability of methylmercury in a focused microwave field were also investigated.

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