Characterization of a crude oil weathering series by ultrahigh-resolution mass spectrometry using multiple ionization modes.

Accidental crude oil releases, such as the Deepwater Horizon (DWH) accident, are always a potential threat to pristine marine ecosystems. Since the toxicity of crude oil heavily depends on its variable composition, the comprehensive characterization of crude oil compounds as a function of weathering is an important area of research. Traditional gas chromatography-based characterization presents significant limitations, and the use of ultrahigh-resolution mass spectrometric (UHRMS) techniques (that allow for the assignment of molecular formulae) has been shown to be better equipped to address the complex nature of crude oils. This study used an Orbitrap Q Exactive mass spectrometer operated at a resolving power of 140,000FWHM with both electrospray ionization (ESI) and atmospheric pressure photoionization (APPI) sources, in order to characterize a crude oil weathering series of the Macondo oil released during the DWH incident (the source oil, two differently weathered surface slicks, and a beached residue). Preliminary gas chromatography mass spectrometry (GC-MS) and gas chromatography flame ionization detection (GC-FID) results suggested that the four oils comprised a true weathering series (including biodegradation and photodegradation in addition to other well-known processes such as dissolution and evaporation). UHRMS results showed a clear increase in oxygenated compounds with weathering, and further suggested a significant gain of acidic compounds, as well as the transformation of phenols to ketonic and quinonic compounds with weathering. A complementary study on a weathered oil sample amended with selected model compounds contributed additional insight into the functional group types that are accessible in each ionization technique.