Molecular Composition of Photooxidation Products Derived from Sulfur-Containing Compounds Isolated from Petroleum Samples

Following the Deepwater Horizon (DWH) oil spill, the limited availability of the Macondo well oil prompted the use of a more widely available surrogate oil supplied by BP, collected from a nearby well. It is chemically similar but has minor compositional differences compared to the oil spilled in DWH. Similarly, the two light sweet crude oils were shown to produce abundant sulfur-containing photoproducts upon solar irradiation, despite a low sulfur content in both parent oils (<0.5 wt %). To investigate the role of chemical functionality in photooxidation of sulfur-containing hydrocarbons, sulfides and thiophenes were isolated from DWH and surrogate parent oils and subjected to laboratory-based irradiation. Their water- and oil-soluble photoproducts were analyzed by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry to access compositional differences between photoproducts of the two oils. The results demonstrate that DWH produces heavily oxidized water-/oil-soluble compounds (up to O18S1), whereas the surrogate oil yields species with only up to 14 oxygen atoms (O14S1). To understand the behavior of a geochemically different sample, a light distillation cut from an Arabian heavy oil was also subjected to the same preparatory workflow and analysis. The results point to the occurrence of photoinduced polymerization reactions during the production of high-molecular-weight/heavily oxidized water-soluble compounds. Furthermore, thiophene and sulfide fractions from all oils were shown to produce abundant water- and oil-soluble photoproducts upon laboratory-irradiation, confirming the importance of this process in oxidative, surface weathering of oil and in the production of petroleum-derived dissolved organic sulfur species. Sulfides from light oils were susceptible to photooxidation and matched previous reports of low-double bond equivalent OxS1 species from irradiated DWH and surrogate oils.