Triple Oxygen Isotope Analysis of Methanesulfonate Using the SO₃^– Fragment in ESI-Orbitrap-MS

Atmospheric oxidation of dimethyl sulfide (DMS, CH₃SCH₃) to sulfate (SO₄^2-) 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 <i>m</i>/<i>z</i> 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.