A comparison of sulfur isotope measurements of geologic materials by inductively coupled plasma and gas source mass spectrometry

Sulfur isotope measurements by multi collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) require up to one hundred times less sulfur than traditional measurements via elemental analyzer isotope ratio mass spectrometry (EA-IRMS). Here, we report an improved MC-ICP-MS protocol for measuring delta 34S values in different natural samples, specifically, calcium-sulfate minerals, carbonate associated sulfate, and dissolved sulfate in water. Sample preparation and subsequent anion exchange chromatography quantitatively recovers sulfur as sulfate. However, for samples with high and complex anion budgets, such as meteoric water, a stronger retention of sulfate on the anion exchange resin is observed relative to samples that contain sulfur as pure sulfate such as calcium-sulfate minerals and carbonate associated sulfate. An extended elution scheme with HNO3 is therefore required to achieve full recovery of sulfur, and thus accurate and reproducible delta S-34 values for the different materials investigated. Sulfur isotope measurements were performed on a Neptune Plus MC-ICP-MS coupled to an Apex-Omega desolvator, resulting in a dry plasma, negligible level of isobaric interferences, a sensitivity of > 40 V ppm-1 sulfur, and a precision of 0.2 parts per thousand (2 S.D.) for delta S-34 for as little as 70 ng sulfur of a pure sulfate solution. The accuracy of the procedure was tested against duplicate delta S-34 EA-IRMS measurements for eleven different geologic samples. The EA-IRMS and MC-ICPMS values generally agree well within analytical uncertainty. However, procedural blank levels are on the order of a few tens of nanogram sulfur, and are thus the main limiting factor for accurate low-level sulfur isotope measurements with MC-ICP-MS.