Isomeric Separation and Structural Characterization of Acids in Petroleum by Ion Mobility Mass Spectrometry

Although naphthenic acids are minor constituents in petroleum, their characterization is crucial because they are geochemically important tracers and play a key role in corrosion processes in refineries. Moreover, different isomers may exhibit different reactivity and may serve as potential biomarkers. However, determination of acid isomers in petroleum/hydrocarbon matrixes remains analytically challenging. Here, we achieve the separation and structural characterization of isomeric homologue series of naphthenic acids in petroleum samples by ion mobility time-of-flight mass spectrometry (IM-TOF MS). IM-TOF MS data processing and molecular formula assignments (for most abundant heteroatom classes), integrated with ion mobility data by PetroOrg software, expose structural differences and patterns among petroleum compounds and facilitate the identification of series of isomers. For example, a family of isomeric acids (C_cH_hO₂) of double bond equivalents (DBE) = 5 and carbon number ranging from C₂₈–C₃₄ appear to correspond to steranoic acids. As another example, C_cH_hO₂ isomers of DBE = 1 likely correspond to linear and isoprenoid acids. Ultra-high-resolution Fourier transform ion cyclotron resonance mass spectra serve to validate molecular formula assignments by IM-TOF and determine whether or not isobaric ions of different mobility are isomers. We conclude that ion mobility mass spectrometry constitutes a valuable new tool for rapid isomeric separation of polar compounds (such as naphthenic acids) in petroleum and other complex organic mixtures.