Determination of selenium isotopic ratios by continuous-hydride-generation dynamic-reaction-cell inductively coupled plasma-mass spectrometry

Abstract

The goal of this study was to develop an accessible method for the determination of selenium isotopes within selenium-rich geological samples and examine the influence of sample introduction, instrumental parameters, column separation and the application of standard-sample bracketing for mass bias and drift correction. Quantitative selenium separation and enrichment of samples was achieved by a column separation using 0.2 g thioglycollic impregnated cotton fiber prior to introduction of the sample to an ICP-DRC-MS. 0.6 ml min⁻¹ premixed Ar (95%) + H₂ (5%) was favored over CH₄ and NH₄ as a reaction cell gas and was used within the DRC coupled with optimized DRC rejection parameters RPa (0) and RPq (0.65) to effectively reduce the signal to background ratio of all measured selenium isotopes (m/z 76, 77, 78, 80 and 82). Ion signal intensity of all measured selenium isotopes were increased 100 fold over classic nebulization by mixing of 1% NaBH₄ and acidified sample digestions in a membrane-less computer-controlled continuous hydride generator. Transient hydride ion signals were time-averaged for five readings and three replicates to produce an in-run precision (2σ) of ±0.45‰ δ^82/76Se_Merck (relative to a Merck titrosol ICP-MS standard) and ±0.85‰ δ^82/76Se_Merck over an 18 month period. In the absence of a selenium isotopic standard, the accuracy of the method was determined using four interlaboratory solutions and five geological standard reference materials covering 0 to −4.5‰ δ^82/76Se_Merck. Our results indicate excellent reproducibility within method precision. The minimum mass of Se required for isotopic ratio determination was 3 μg (>100 000 cps at m/z 82 and 76).