Capabilities and limitations of Se isotopic analysis using hydride generation coupled to MC-ICP-MS†

This study presents a comprehensive methodological investigation aimed at optimizing selenium (Se) isotopic analysis using MC-ICP-MS. Fundamental aspects of the plasma were revisited through spatial profiling, enabling detailed characterization of the distribution of Se⁺ and ArAr⁺/ArArH⁺ species within the plasma. Increasing the sampling depth (sampling further upstream in the plasma) proved more effective than the commonly employed methane addition, offering a more effective suppression of the Ar-based species, although at the cost of some loss in the sensitivity for Se. Under these conditions, precision values (expressed as 2SD) of 0.03‰ and 0.17‰ were obtained for δ^82/78Se and of 0.08‰ and 0.38‰ for δ^82/76Se, at 100 and 25 μg L⁻¹, respectively. Moreover, the method proved robust, with a long-term reproducibility of 0.07‰ (2SD, n = 120) and high accuracy, even at up to 30% sample-standard concentration mismatch. However, the method's relatively high hydride formation rate (∼7 × 10⁻³) limits its applicability to samples with As/Se post-isolation ratios ≤0.05, beyond which mathematical corrections lead to biased results. Finally, the method was validated using the SELM-1 reference material, for which the δ^82/78Se and δ^82/76Se values were in excellent agreement with published data, and was subsequently applied to a set of tuna fish organs (liver, spleen, kidney, and intestine). This study demonstrates that the method that was developed, optimized and validated forms a solid basis for further investigating Se metabolic pathways in marine fish and for elucidating its role in Hg detoxification.