Characterization of crude oils at the molecular level by use of laser desorption ionization Fourier-transform ion cyclotron resonance mass spectrometry

In this study, laser desorption ionization (LDI) coupled to Fourier-transform ion cyclotron resonance mass spectrometry (FTICR MS) was applied to study crude oils at the molecular level. Molecular ions were the major type of ion detected by (+) mode, and deprotonated and radical anions were the major ions observed by (-) mode LDI FTICR MS. N₁ and hydrocarbon classes were dominant in the class distribution plots obtained by (+) LDI FTICR MS, but other heteroatom classes, including O_x and S₁, were abundant in plots obtained by (-) LDI FTICR MS. Detailed analysis of double-bond equivalence (DBE) vs carbon number plots revealed that LDI FTICR MS is more sensitive toward polyaromatic compounds than mono- or dicyclic-aromatic compounds. However, nonaromatic and aromatic O2 compounds could be detected simultaneously. An abundance of nonaromatic O₂ compounds (presumably naphthenic acids) are correlated with total acid numbers, but O₂ compounds with condensed structures are not. Overall, this study shows that LDI FTICR MS can be successfully used to study crude oils at the molecular level.