Structural Comparison of Asphaltenes of Different Origins Using Multi-stage Tandem Mass Spectrometry

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Abstract

In this work, six petroleum asphaltene samples of different geographical origins were studied using atmospheric pressure chemical ionization (APCI) in positive-ion mode in a linear quadrupole ion trap mass spectrometer (LQIT). APCI doped with carbon disulfide reagent was selected as the ionization method because it has been previously demonstrated to generate stable molecular ions with no fragmentation for asphaltene molecules. The mass spectra measured using this approach revealed the apparent molecular weights (MWs) of the molecules in the asphaltene samples. The results show that petroleum asphaltenes from the American continent, Europe, and China have similar apparent molecular weight distributions, ranging from 200 to 1450 Da, with slightly different apparent average MWs ranging from 570 to 700 Da. Further, molecular ions with eight randomly selected mass-to-charge ratios (m/z) ranging from m/z 500 to 808 were isolated for each asphaltene sample and subjected to collisionally activated dissociation (CAD) at the same collision energy to examine their structures. The CAD mass spectra (MS2 experiment) provided information on the maximum total number of carbons in the alkyl chains and the smallest possible size of the aromatic cores in the ionized molecules. Additionally, MS3 experiments were performed to investigate the fragmentation patterns of the fragment ions generated in the MS2 experiments. The results obtained support the island structural model for these asphaltenes. Moreover, molecules of greater MWs are shown to have more carbons in alkyl chains (ranging from 17 to 41), but the minimum core size is fairly constant.

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