Barium ion adduct mass spectrometry to identify carboxylic acid photoproducts from crude oil–water systems under solar irradiation

Abstract

Petroleum derived dissolved organic matter (DOM_HC) samples were successfully cationized with barium, revealing many [M–H + Ba]⁺ peaks in both dark and simulated sunlight treatments. The DOM_HC samples generated after light exposure exhibited a greater number of [M–H + Ba]⁺ peaks compared to the dark control. Multiple [M–H + Ba]⁺ peaks were investigated in the irradiated DOM_HC using low resolution MS/MS in order to confirm the presence of diagnostic fragment ions, m/z 139, 155 and 196 in each treatment. Due to the high complexity of the bariated DOM_HC mixture, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS/MS) was employed to obtain molecular level information for both irradiated and dark treatments. The irradiated DOM_HC treatments had more bariated oxygenated species over a wide range of H/C and O/C when compared to the dark controls. Doubly bariated species were also observed in DOM_HC, which provides evidence that photochemistry transforms DOM_HC to even more complex mixtures with multiple oxygenations per molecule. This study provides evidence that barium adduct mass spectrometry can be successfully applied to DOM_HC screening for the presence of COOHs, both in dark samples and solar irradiated samples. Furthermore, direct evidence and molecular composition of aqueous phase crude oil photoproducts is provided by this technique.