Effects of instrumental parameters and mass loading on isotopic fractionation in sulfur isotopic ratio analysis of sulfide using femtosecond laser ablation MC-ICP-MS

The sulfur isotope ratio serves as a crucial geochemical tracer, providing key insights into geological processes. However, the accurate determination of sulfur isotope ratios by LA-MC-ICP-MS is affected by complex isotopic fractionation. To study the effects of instrumental parameters on isotopic fractionation, several experiments were performed to study how the sample gas flow rate influences the peak shape, polyatomic interference, signal intensity, isotope ratio. Moreover, we systematically adjusted the sampling depth and sample gas flow rate to investigate the axial ion distribution in the plasma, which reveals the local ionization efficiency and isotopic fractionation behavior. Compared to the sample gas flow rate of 1.20 L min^-1 that provides maximum sensitivity (maximum sensitivity condition), a higher flow rate not only reduces signal intensity, but also increases polyatomic interferences. In contrast, a slightly lower flow rate (1.16 L min^-1) is recommended to stabilize the sulfur isotope ratio and reduce polyatomic interferences (robust condition). Although this robust condition resulted in an approximately 2% signal reduction, it provided a 6.76-fold improvement in ³⁴S/³²S ratio stability while maintaining comparable precision. Furthermore, the mass loading effect was evaluated by changing the ablation diameter using different laser ablation systems and instrumental conditions. The results showed that while both laser ablation systems exhibited a mass loading effect, it was less pronounced with femtosecond laser (fs-LA) than with nanosecond laser (ns-LA). The pronounced mass loading effect observed under maximum sensitivity condition were significantly reduced under robust condition using fs-LA. The improved stability of isotopic ratio and the reduction of mass loading effect under the optimized instrumental conditions demonstrate the importance of instrumental optimization of LA-MC-ICP-MS for achieving high-precision and accurate S isotopic analysis.