Application of speciated isotope dilution mass spectrometry to evaluate methods for efficiencies, recoveries, and quantification of mercury species transformations in human hair

Abstract

Nine different analytical extraction methods commonly used for mercury speciation in human hair samples were evaluated by analyzing hair certified reference material (IAEA-085) certified for the content of monomethyl mercury and total mercury. Extraction efficiencies, recoveries, as well as their potential of species transformations during sample processing, were evaluated by applying speciated isotope dilution mass spectrometry (SIDMS) as both a quantitative and diagnostic tool. The extraction methods evaluated in this study were based on acid leaching with HCl and HNO₃. Detection of total mercury and mercury species from all extraction methods were carried out by inductivley coupled plasma mass spectrometry (ICP-MS) and high performance liquid chromatography-ICP-MS, respectively. Microwave-assisted extraciton (4 mol l⁻¹ HNO₃, at 100 °C for 10 min) and mechanical shaking (2 mol l⁻¹HCl, at room temperature for 24 h) were found to be the most efficient protocols that demonstrated approximately 100% mecury recovery with less species transformations (6% or less). Ultrasound-assisted extraction (2% HCl + 10% ethanol, at 60 °C for 7 min) or extraction with 2 mol l^−lHCl in a water bath (at 100 °C for 5 min) resulted in the second highest extraction efficiency (∼90%) with little or no mercury species transformations. Demethylation as high as 22% using 5 mol l⁻¹ HNO₃ in a water-bath heating (at 100 °C for 2 h) and 90% with cold acid digestion using concentrated HNO₃ and H₂O₂ (room temperature for 24 h) were observed, however, all the extraction methods tested produced accurate quantitation when corrected in accordance with the SIDMS method standardized in the US Environmental Protection Agency's method 6800.