Structural Assignments of Sulfur-Containing Compounds in Crude Oil Using Ion Mobility Spectrometry-Mass Spectrometry

Through proof-of-concept experiments, a preliminary structural characterization of isomeric and isobaric sulfur containing compounds in petroleum is presented. Silver attachment to sulfur atom has been used for ionization enhancement of these compounds, which are less accessible by positive-ion electrospray ionization mass spectrometry (ESI-MS). Here, ion mobility spectrometry (IMS) is combined with the enhanced ionization approach and MS to characterize sulfur compounds in crude oil. The silver isotopes, being of nearly equal abundance and having a similar effect upon ion mobilities, provide a fingerprint for assignments of sulfur-containing ions in two-dimensional (2D) plots of mass-to-charge (m/z) vs drift time (t(D)). Fourier transform ion cyclotron resonance (FT-ICR) MS has been employed for the determination of molecular formulas for these ions. Collision-induced dissociation (CID) data are utilized to complement collision cross section (CCS) comparisons for candidate structures obtained from high mass measurements. With regard to method workflow, molecular formulas are first proposed from high mass accuracy data. Next, the structures associated with the formulas are compared to the CID data for consistency. Finally, theoretical CCS values are calculated for the remaining structures and compared with experimental tip profiles. Without the ability to exhaustively compare a large number of standards, the work provides a tentative compound assessment of petroleum samples. Although several isomeric compounds should exhibit similar CCS values, this work demonstrates how compounds may be discarded based on the complementary measurements. Future directions to improve the overall approach are discussed.