Selective Molecular Characterization of Petroleum Thioethers by tert -Butyl Derivatization Coupled with High-Resolution Mass Spectrometry

Thioethers are important sulfur-containing constituents of petroleum, but their selective molecular characterization in complex petroleum matrixes remains challenging. Here, we developed a selective method for petroleum thioethers based on tert-butyl derivatization using pivaloyl chloride/AlCl3, followed by positive-ion electrospray ionization (ESI) high-resolution mass spectrometry. Model-compound experiments revealed distinct reaction pathways for different sulfur classes: thiophenic compounds predominantly underwent aromatic alkylation, thiols formed corresponding thioesters, and thioethers were converted into positively charged tert-butyl sulfonium salts that could be selectively detected by ESI high-resolution mass spectrometry. The conversion efficiencies of the tested thioethers ranged from 51% to 100%, depending on the molecular structure. Application to a previously characterized reference crude oil produced thioether distributions consistent with previous reports and enabled the detection of multisulfur thioether species. The method was further validated using naphtha, atmospheric gas oil, and vacuum gas oil and was directly compared with conventional methylation. tert-Butyl derivatization selectively highlighted thioether-derived species across all three petroleum fractions, whereas methylation additionally detected abundant thiophenic sulfur compounds. Moreover, the larger mass shift introduced by the tert-butyl group improved the analytical coverage of low-molecular-weight thioethers in naphtha under the present MS acquisition conditions. These results demonstrate the selectivity and broad applicability of the method for the molecular characterization of petroleum thioethers.