Abstract

The performance and validation characteristics of different single collector inductively coupled plasma mass spectrometers based on different technical principles (ICP-SFMS, ICP-QMS in reaction and collision modes, and ICP-MS/MS) were evaluated in comparison to the performance of MC ICP-MS for fast and reliable S isotope ratio measurements. The validation included the determination of LOD, BEC, measurement repeatability, within-lab reproducibility and deviation from certified values as well as a study on instrumental isotopic fractionation (IIF) and the calculation of the combined standard measurement uncertainty. Different approaches of correction for IIF applying external intra-elemental IIF correction (aka standard-sample bracketing) using certified S reference materials and internal inter-elemental IIF (aka internal standardization) correction using Si isotope ratios in MC ICP-MS are explained and compared. The resulting combined standard uncertainties of examined ICP-QMS systems were not better than 0.3–0.5% (uc,rel), which is in general insufficient to differentiate natural S isotope variations. Although the performance of the single collector ICP-SFMS is better (single measurement uc,rel = 0.08%), the measurement reproducibility (>0.2%) is the major limit of this system and leaves room for improvement. MC ICP-MS operated in the edge mass resolution mode, applying bracketing for correction of IIF, provided isotope ratio values with the highest quality (relative combined measurement uncertainty: 0.02%; deviation from the certified value: <0.002%).

Highlights

  • Natural S consists of four stable isotopes: 32S, 33S (0.75%), 34S (4.25%) and 36S (0.01%).[1]

  • We focus on the 34S/32S ratio as the mostly investigated S isotope ratio

  • The deviation of the obtained ratios from the certi ed value (IAEA-S-1) is displayed in Fig. 4 and summarized in a table form in Electronic supplementary information (ESI) S2.† The MC Inductively coupled plasma mass spectrometry (ICP-MS) operated in the edge mass resolution mode proved to be most suitable for S isotope ratio measurements

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Summary

Introduction

Natural S consists of four stable isotopes: 32S (relative abundance: 94.99%), 33S (0.75%), 34S (4.25%) and 36S (0.01%).[1] The variation of the relative abundance of the S isotopes in nature is a result of kinetic and thermodynamic effects during e.g. Earth's core–mantle differentiation,[2] uptake and metabolism of S compounds by fauna and ora[3] or crystallization and evaporation of seawater.[4] Analysis of 33S/32S, 34S/32S and 36S/32S isotope ratios reveals processes that cause both mass-dependent and mass-independent isotopic fractionation. We focus on the 34S/32S ratio as the mostly investigated S isotope ratio

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