Insights on the Interpretation of the DART Mass Spectra of Organosulfur Compounds

Detection and characterization of organosulfur compounds (OSCs) and their intermediates can be rapidly accomplished by direct analysis in real-time – high resolution mass spectrometry (DART-HRMS). However, some classes of OSCs such as sulfides, when exposed to the metastable helium (He*) DART gas stream under soft ionization conditions, undergo several reactions to produce new organosulfur species which complicate interpretation of their spectra. In this study, these new entities were characterized and the mechanisms of their formation explored. For example, when MeSMe (C2H6S, m/z 62.0190) was analyzed by DART-HRMS in positive-ion mode at 350 °C, the molecular ion at m/z 62.0185 corresponding to [MeSMe]+• and its protonated precursor at m/z 63.0262 corresponding to [MeSMe + H]+ were detected, along with: m/z 79.0213, consistent with protonated dimethyl sulfoxide (DMSO) [MeSOMe + H]+; m/z 109.0145, consistent with [Me3S2]+; m/z 123.0308, consistent with [C4H11S2]+; and m/z 157.0376, corresponding to the protonated dimer of DMSO [MeSOMe + H]+. Tracer studies using isotopically labelled compounds were performed to investigate the adduct formation pathway. High-level ab initio/DFT calculations using the CCSD(T)/cc-pVTZ//M06-2X/aug-cc-pV(T+d)Z level of theory, revealed that several of these peaks are artifacts of reactions occurring with He* and OH•. The MeS• and •OH formed in the DART gas stream serve as key intermediates in subsequent reactions that lead to the various products detected by DART-HRMS analysis. The complete reaction mechanism and the implications of the formation of the identified intermediates are presented. The results provide valuable insights into the systematic interpretation of the DART mass spectra of sulfide compounds, including various natural products of agricultural and culinary importance, and demonstrate how reactions occurring in He* might be harnessed to perform synthesis of molecules in the gas phase.