A simple and rapid ICP-MS/MS determination of sulfur isotope ratios (³⁴S/³²S) in complex natural waters: A new tool for tracing seawater intrusion in coastal systems

Conventional sulfur isotope measurements in complex natural liquid or solid samples via GS-IRMS are complicated, time consuming and relatively expensive. Here we assessed a novel ‘collision cell’ based ICP-MS/MS approach which can determine the sulfur isotope abundances (i.e., ³⁴S/³²S ratios, expressed as δ³⁴S) in complex coastal waters rapidly, accurately and with minimal sample preparation. The approach was validated via repeated ICP-MS/MS measurement of S isotope certified reference materials (CRM) providing accurate and reproducible results, with a typical uncertainty on δ³⁴S of around 1.1–1.5‰ (1SD). This novel approach is suitable for water samples with sulfur concentrations at or above 2 μg/mL (ppm). Matrix matching between samples and the CRM was necessary when seawater-like solutions were analysed addressing common matrix related errors. The ICP-MS/MS approach was used to investigate δ³⁴S signature of porewaters from a variety of coastal systems in South Australia (including acid sulfate soils), and how they responded to progressive seawater inundation. Importantly, inundation induced a shift in S isotope ratio in affected porewaters in which δ³⁴S approached that of seawater. The simple sample preparation, with rapid and accurate δ³⁴S determination of complex natural waters using the ICP MS/MS approach, greatly increases the applicability of sulfur isotope tracing studies to identify and monitor sources and bio-geochemical pathways of S in coastal and near-surface environments.