LAUSR

Atmospheric oxidation of dimethyl sulfide (DMS, CH3SCH3) to sulfate (SO42-) and methanesulfonate (MSA, CH3SO3-) is a key process for the atmospheric sulfur cycle, particularly in remote marine regions. To better understand the DMS oxidation chemistry, oxygen isotope signatures (δ18O and Δ17O) serve as potential tracers for investigating the oxidation of DMS to MSA. However, conventional gas-source isotope ratio mass spectrometry (IRMS) is rarely used to measure the natural abundance isotopic compositions of MSA due to the requirement for micromole-level sample sizes and the complexity of the analytical steps. Here, a new method measuring δ18O and Δ17O values of MSA was developed using electrospray ionization Orbitrap mass spectrometry (ESI-Orbitrap-MS) with a dual-inlet system (4 reference and 3 sample blocks for 98 min on ∼13 nmol samples). Performing Δ17O measurements directly on CH3SO3- ions requires a high mass resolution of 120,000 (FWHM at m/z 200) to separate 17O and 13C isotopologues, which tends to increase analytical uncertainty compared to a lower mass resolution of 60,000. Therefore, we measured SO3- fragment ions derived from MSA while eliminating the -CH3 group, enabling Δ17O analysis using a lower mass resolution setting of 60,000 and a higher scan speed (4 scans/s). A three-point calibration between our ESI-Orbitrap-MS measurements and conventional IRMS measurements shows that the standard deviations of the residuals from the regression lines were 1.0 and 0.4‰ for δ18O and Δ17O values, respectively. Overall, this new analytical method is more direct and sensitive than earlier approaches, providing access to more natural atmospheric and/or ice core samples.