Experimental and Computational Investigation of Sulfur Chemistry in the DART-MS Gas Stream: Implications for the Interpretation of the Mass Spectra of Organic Disulfides

Direct analysis in real time-high-resolution mass spectrometry (DART-HRMS) has proven useful for the detection and faithful representation of labile organosulfur compounds (OSCs). Nevertheless, it has been found that when exposed to the metastable helium (He*) of the DART gas stream under soft ionization conditions, some classes of OSCs such as disulfides, undergo several reactions to produce new organosulfur species, which complicate interpretation of their spectra. In this work, these new entities were characterized and the mechanisms of their formation explored. DART-HRMS analysis of diphenyl disulfide (PhSSPh) exhibited peaks consistent with [PhSSPh]+•, [PhSSPh + H]⁺, [PhSPh]+•, and [PhSOSPh + H]⁺ and various fragments and adducts including [PhSS]⁺ and [Ph₃S₃]⁺. Ab initio/DFT calculations coupled with variational transition state theory revealed that several of these peaks are artifacts of reactions occurring with He* and •OH. Branching ratio analysis showed the dominant decomposition pathway of [PhSSPh]+• to be [PhSSPh]+• → PhS• + PhS:, rather than [PhSSPh]+• → PhS⁺=S + Ph•. The PhS•, Ph•, and •OH formed serve as key intermediates in subsequent reactions that lead to the m/z values observed. From the results, a systematic approach for the interpretation of the DART mass spectra of organic disulfides was developed and successfully applied to various disulfide compound classes (e.g., aryl, alkyl, benzyl, and phenyl). Organic solvents were also observed to influence the ability to detect the compounds. Benzene; dichloromethane; hexane; and with some exceptions, ethyl acetate and tetrahydrofuran were found to be suitable, while disulfide signals in dimethylformamide were totally absent.