Measurement of sulfur in environmental samples using the interference standard method with an O₂ -pressurized...

Precise and accurate analysis of trace sulfur in environmental samples is essential to understand its biogeochemical cycle. Yet it remains a challenging analytical task for single-quadrupole inductively coupled plasma mass spectrometry (ICP-MS) instrumentation because of the presence of significant isobaric interferences.

We evaluated the effect that different instrumental parameters have on the formation of SO⁺ species, and show that, under our instrumental configuration, it is not possible to fully remove all spectral interferences. Consequently, our instrumental optimization is focused on minimizing their contribution while obtaining the best possible sensitivity. We assessed the performance of the interference standard (IFS) method, where the signal corresponding to the SO⁺ ions formed in the ICP is normalized to that of ³⁶ Ar⁺ .

We found that, by pressurizing the cell with O₂ , there is a significant improvement in sensitivity and detection limits, associated with a more effective production and focalization of SO⁺ ions. Moreover, by applying the systematics of the IFS with pure O₂ injected into the cell (IFS-O₂ ) we observed a significant improvement in accuracy and detection limits, making this the method of choice for the quantification of trace sulfur in environmental samples by ICP-MS.

The method described here represents an effective and affordable alternative for the accurate and precise determination of sulfur using single-quadrupole ICP-MS in environmental samples, with LODs and LOQs comparable with those obtained with more expensive instrumentation.