High Resolution Mass Spectrometry Advances in Oil Spill Analysis

Abstract Oil is a complex mixture of alkanes, cycloalkanes, aromatics, nitrogen/sulfur/oxygen (N/S/O) heterocycles, alcohols, ketones, carboxylic acids, porphyrins, and myriad possible combinations therein. Once introduced into the environment, some weathering processes reduce this complexity through evaporative losses (loss to atmosphere) as well as water washing of low ring number aromatics, N/S/O heterocycles, alcohols, ketones, and carboxylic acids (loss to seawater). However, Gulf of Mexico Research Initiative (GoMRI) supported research has revealed that the contributions of these mechanisms to the reduction of molecular complexity are dwarfed by that of oxidative weathering processes (photo- and bio-oxidation) that increase the compositional complexity of the transformed oil. Because these oxidative processes increase the complexity of an already analytically challenging organic mixture and the boiling point of transformed species is higher than that of the precursors, conventional analytical techniques yield little insight into the identification of oil spill transformation products. Despite the challenge, recent advances in analytical science now allow molecular-level insight into these complex systems irrespective of initial (unaltered) or transformed-product boiling point; these advances were largely made possible by GoMRI supported research efforts. They expose a continuum of oxidized transformation products that span oil-soluble, oil-soluble interfacially-active, and water-soluble species. The isolation and characterization of oil-soluble, interfacially-active species confirm a long-standing theory that photo-oxidation generates oil-soluble surfactant-like species, which limit the effectiveness of dispersants. Furthermore, photo-oxidation specific microcosms are shown to generate unique species that are also found in field samples. Bio-oxidation only microcosms were found to generate very different, oil-soluble species; thus, photo-oxidation is implicated in the formation of transformation products in the field. Finally, analyses of photooxidized distillate cuts as well as asphaltene samples confirm prior reports of photo-induced polymerization and photo-cracking of native petroleum molecules. Here, we summarize the advances in the molecular-level understanding of oil over the past 8 years, in the context of oil spill science, by high resolution mass spectrometry, and we highlight potential opportunities for future research, as well as knowledge gaps that must be addressed for future spills.