Molecular Characterization of Sulfur Compounds in Venezuela Crude Oil and Its SARA Fractions by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Venezuela crude oil was separated into saturates, aromatics, resins, and asphaltenes (SARA) fractions. The sulfur compounds in the crude oil and its SARA fractions were reacted with iodomethane in the presence of silver tetrafluoroborate and converted to methylsulfonium salts. The methylsulfonium salts were characterized by positive-ion electrospray ionization (ESI) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The S-1, S-2, S-3, O1S1, O1S2, O2S1, and N1S1 class species were identified in crude oil derived methylsulfonium salts. The molecular composition and mass distribution of sulfur compounds in the subfractions were distinctly different. Small amounts of SI class species (cyclic-ring sulfides) were present in the saturates fraction. Thiophenic sulfur compounds were dominant in the aromatics fraction. Complex multiheteroatoms (N1Sy, OxSy,, and NIOxSy, class species) were present in the resins and asphaltenes fractions. The relative abundance plots of double-bond equivalence (DBE) versus the carbon number of S-1, S-2, O1S1, and O2S1 class species showed that as the molecular polarity of the fraction increased, the DBE values of the abundant S-y class species increased. SARA fractionation isolates the highly aromatic sulfur species in the resins and asphaltenes fractions, which are not observed in the parent crude oil sample.