Elucidation of structural information achievable for asphaltenes via collision-activated dissociation of their molecular ions in MSn experiments: A model compound study

Abstract

Despite extensive studies of the asphaltene fraction of petroleum, the molecular structures of asphaltenes remain a highly debated topic. Tandem mass spectrometry is the only technique that allows the examination of the structures of individual asphaltene molecules due to the extreme complexity of asphaltenes. Recently, atmospheric pressure chemical ionization (APCI) using CS2 as the reagent was demonstrated to produce abundant and stable molecular ions for polyaromatic hydrocarbons with long alkyl chains. Hence, coupling APCI/CS2 with tandem mass spectrometry appears to be a promising method for the examination of the structures of molecules in asphaltenes. However, the fragmentation pathways of the molecular ions of large alkyl aromatic compounds are not well understood. In order to address this issue, a detailed examination of the collision-activated dissociation reactions of the molecular ions and several of their fragment ions (MSn experiments) was carried out for several model compounds of asphaltenes. The results show that information on various structural aspects of asphaltenes can be obtained from these experiments, such as alkyl chain lengths and sizes of aromatic cores. Based on these results, MS2 experiments may provide enough information to determine approximate core sizes for molecules with archipelago structures. However, the number of ion isolation and collision-activated dissociation (CAD) experiments needed to elucidate maximum structural information for molecules with island structures depends on the number of carbon chains on the aromatic core.