Determination of inorganic selenium species in rain and sea waters by anion exchange chromatography-hydride generation-inductively-coupled plasma-dynamic reaction cell-mass spectrometry (AEC-HG-ICP-DRC-MS)

Abstract

The inorganic selenium (Se) species selenite, selenate and selenocyanate in waters are determined by anion exchange chromatography-hydride generation-inductively-coupled plasma-dynamic reaction cell-mass spectrometry (AEC-HG-ICP-DRC-MS) with instrumental detection limits of 0.15, 0.27 and 0.19 ng Se L⁻¹, respectively. Sea water has to be diluted ten-fold prior to analysis to overcome chromatographic interferences, so practical method detection limits for this matrix are around 2–3 ng Se L⁻¹. The species are separated by gradient elution with NaOH and converted to SeH₂ by prereduction with iodide in HCl at 100 °C, followed by subsequent reaction with KBH₄, before the SeH₂ is introduced into the plasma after aerosol and water vapor removal. Quantification using the main Se isotope ⁸⁰Se was possible by employing, for the first time, a mixture of two reaction gases: methane for eliminating the ⁴⁰Ar₂⁺ dimer, and ammonia for eliminating a significant interference from HBr⁺ caused by bromine present in the employed reagents. Oxidation of Se⁺ to SeO⁺ using O₂ as the reaction gas was also attempted, but yielded incomplete (10–25%) conversion. In addition, the HBr⁺ interference was not eliminated by this approach, because the interfering molecular ion was also oxidized to HBrO⁺. The optimized method was successfully applied to the determination of Se speciation in uncontaminated sea water and in rain water. In rain water, an unidentified Se species was detected, which we believe to be a monomethylated Se species.