Molecular-Level Structural Analysis of Hydrotreated and Untreated Atmospheric Residue Oils via Atmospheric Pressure Photoionization Cyclic Ion Mobility Mass Spectrometry and Ultrahigh-Resolution Mass Spectrometry

Changes in the structure of oil compounds subjected to hydrodesulfurization were investigated using ultrahigh-resolution mass spectrometry (UHR-MS) and cyclic ion mobility mass spectrometry (cIM-MS) techniques. The isolation of ions with a separation of less than 0.1 Da followed by collision-induced dissociation helped clarify the structure. The class and double bond equivalence (DBE) distributions of the HC class observed by UHR-MS indicated that sulfur-containing compounds were transformed to HC compounds, in accordance with the previously reported mechanisms. The transformation was further observed by examining the tandem mass spectra of HC class compounds with the DBE values. The fragility of the molecules to collision-induced dissociation increased after hydrodesulfurization as a result of the increased abundance of naphthenic rings in the processed oils. The structural change was influenced by the DBE, and the most significant structural changes corresponded to HC compounds with DBE of 7. It was considered that compounds with naphthalene in the core structure were dominant in the untreated oils; however, the number of compounds with multiple naphthenic rings increased after the treatment. Moreover, the abundance of HC compounds with short alkyl chains likely increased. The results indicate that UHR-MS and cIM-MS are effective tools to clarify molecular-level changes in processed oils.