Tandem Mass Spectrometric Characterization of the Molecular Radical Cations of Asphaltenes

Asphaltenes, the heaviest fraction of crude oil, are problematic for the oil industry for several reasons. For example, they have a tendency to precipitate out of oil pipelines, thus causing clogs. Further, they can foul catalysts used in refining of crude oil. However, the lack of knowledge on the exact molecular structures of asphaltenes has long prevented addressing the above problems and also hindered finding financially beneficial uses for asphaltenes. This paper focuses on recent advances on the structural characterization of the molecular radical cations of asphaltenes based on collision-activated dissociation (CAD) experiments performed using linear quadrupole ion trap mass spectrometers coupled to an Orbitrap detector. Stable molecular radical cations are readily generated from asphaltenes upon atmospheric pressure chemical ionization (APCI) when using CS2 as the solvent because this solvent cannot transfer a proton to analytes. MS2–MS5 experiments based on ion-source CAD (ISCAD), ion-trap CAD (ITCAD), and/or medium-energy CAD (MCAD) were explored for the structural characterization of these molecular radical cations by using model compounds. Energy-resolved MCAD experiments were identified as the most useful experiments because they provide substantially more structural information for the radical cations than the other CAD experiments mentioned above. In some MSn experiments, MSn–1 was first performed in the ion trap, followed by isolation of a desired ion in the ion trap and transfer into the MCAD collision cell, wherein it was subjected to energy-resolved MCAD followed by high-resolution detection of the fragment ions. The various CAD experiments were used to obtain information on compounds in asphaltene samples with different origins, such as the aliphatic carbon content, molecular weight distributions (MWDs), average molecular weight, number of aromatic carbon atoms, number of aromatic rings in the core, and relative abundances of single-core and multicore compounds.