MC-ICP-MS measurement of δ34S and ∆33S in small amounts of dissolved sulfate

• Sulfur isotopes are crucial in understanding biogeochemical cycles.
• The use of the Neptune Plus MC-ICPMS requires very small amount of sulfates, down to 5 nmol S.
• Isotopic ratios are measured with high precision (0.1 to 0.2‰ 2sd).
• 32S, 33S and 34S can be measured.

Over the last decade, the use of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has significantly lowered the detection limit of sulfur isotope analyses, albeit typically with decreased precision. Moreover, the presence of isobaric interferences for sulfur prevented accurate analysis of the minor isotopes 33S and 36S. In the present study, we report improved techniques for measuring sulfur isotopes on the MC-ICP-MS Neptune Plus (Thermo Fischer Scientific) using a heated spray chamber coupled to a desolvating membrane (Aridus, Cetac). Working at high mass resolution, we measured δ34S values of natural samples with a typical reproducibility of 0.08–0.15‰ (2sd) on 5 to 40 nmol sulfur introduced into the instrument. We applied this method to two seawater profiles, using 25 μl of sample (700 nmol of sulfate). The average δ34SVCDT value is 20.97 ± 0.10‰ (2sd, n = 25). We show that the amount of sulfate required for an analysis can be decreased to 5 nmol. Because the plasma is sustained by Ar, measurement of 36S is impossible at the current mass resolution due to the presence of 36Ar+, but a reproducibility of 0.1–0.3‰ (2sd) is achieved on the measurement of mass independent fractionations (Δ33S). This is the first time such precision has been achieved on samples of this size.