Isolation and characterization of sulfur-containing compounds by positive-ion electrospray ionization and online HPLC/Atmospheric pressure chemical ionization coupled to Orbitrap mass spectrometry

Abstract

Extraction and fractionation of sulfur-containing compounds from vacuum gas oil were carried out by using non-reactive methods such as ligand exchange chromatography (LEC) on solid-phase extraction cartridges and a molecularly imprinted polymer (MIP). All fractions obtained by these methods were methylated, and sulfur compounds (S1) were analyzed as methyl sulfonium ions [M + CH3]+ by positive-ion ESI Orbitrap mass spectrometry. LEC on strong cation exchange sorbents modified with Ag+ (SCX-Ag+) and Cu2+ (SCX-Cu2+) cations isolate sulfides and thiophenes through complexation of the sulfur atoms with the metal cations. In contrast, LEC on the stationary silica phase modified with Ag+ cations (Si-Ag+) was selective of sulfur species (S1) with higher DBE values near the planar limit. Interestingly, LEC/SCX-Cu2+ allowed the extraction of disulfides and other sulfur compound classes such as OS and OS2 detected as [M + H]+. Among LEC separations, the Si-Ag+ provides the largest number of molecular assignments (565) of sulfur species (S1) followed by SCX-Ag+ and SCX-Cu2+ with 463 and 408 formulae, respectively. On the other hand, a polymer with dibenzothiophene molecules imprinted afforded the highest number (820) of molecular formulae (CcHhS1) with a wider DBE ~1–20 and carbon number ~30–70 in comparison with the species (S1) eluted by LEC methods. A further separation by online HPCL/APCI (+) Orbitrap MS of the fractions obtained by SCX-Ag+ allows detecting sulfur compounds with DBE values up to 23. This contribution provides alternatives for the isolation and fractionation of sulfur-containing compounds that may be extended to heavy crude oil and asphaltenes.