Photooxidation is well known as the dominant mechanism for the oxygenation of surface petroleum compounds in both environmental and laboratory samples. The process produces three major functionalities: alcohols, ketones/aldehydes, and carboxylic acids. The understanding of how these functionalities interact with oil and water is essential to clean-up efforts after oil spills. Although FT-ICR MS has the resolving power to assign element compositions, mass measurement alone cannot determine the chemical functionality of molecules. Such information is critical, as it affects how the weathered oil behaves in the environment. Thus, it is necessary to employ separation techniques and derivatization reactions to determine these functionalities. Here we employ several techniques for separation, which include the Wet Silica Method and modified aminopropyl silica (MAPS) solid-phase extraction. The Wet Silica Method isolates interfacially-active material (IM) from non-IM, which facilitates the characterization of those photooxidized compounds that contribute to the stabilization of emulsions. MAPS separates acids from non-acids, and thereby enables the analysis of otherwise inaccessible mid- to high-molecular weight acids due to the higher ionization efficiency of lower molecular weight acids. The combination of the Wet Silica Method followed by MAPS fractionation enables the determination of which acidic compounds contribute to emulsion stabilization. Comparison of mass spectral results from the unfractionated and fractionated samples reveals that the whole (unfractionated) sample does not account for a large extent of oxygenation observed in the fractionated samples. Specifically, the fractionated samples display a much wider level of oxygenation that extends to higher mass for all investigated irradiation periods. Each fraction also shows a higher average double bond equivalents (DBE), indicating a higher aromaticity than can be accessed for the unfractionated sample. Oxygenated sulfur species follow similar trends to oxygen species; however, oxygenated nitrogen species behave anomalously. NOx species do not display an increased DBE or carbon number with increased solar irradiation. Instead, they remain in the C20 to C40 range expected for the unirradiated oil (a vacuum gas oil) and display only a slight upward shift in DBE with increased oxygenation. A more detailed comparison of all samples analyzed, and derivatization products will be presented.

Molecular characterization of oxygenated species from photooxidized petroleum samples by 21 tesla Fourier transform ion cyclotron resonance mass spectrometry

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