Comparison of the Structures of Molecules in Coal and Petroleum Asphaltenes by Using Mass Spectrometry

Asphaltenes are a complex mixture of aromatic compounds that are present in petroleum and liquefied coal and have unknown molecular structures. Determination of the molecular structures of asphaltenes is greatly hindered by the extreme complexity of the mixtures. The average molecular weight of coal asphaltenes is known to be roughly half that of petroleum asphaltenes. In this study, tandem mass spectrometry experiments were used to compare the fragmentation patterns of molecular ions of the same mass-to-charge (m/z) value derived from coal and petroleum asphaltenes. The fragmentation pathways of the asphaltene ions were used to determine structural differences between the two types of asphaltenes. This type of analysis is new to the field of asphaltenes research and has not yet been utilized to compare coal and petroleum asphaltenes. Thus, this study was performed to both confirm its viability as well as further corroborate previous data gathered using different techniques. The results reported here demonstrate that the molecules in coal asphaltenes have shorter alkyl chains and larger aromatic cores than their petroleum-derived counterparts with the same molecular weight (MW). Furthermore, they have a higher aromatic-to-alkane carbon ratio, independent of the MW. However, the typical petroleum and coal asphaltenes (when considering the most abundant molecules in each) have a similarly sized aromatic core (∼8 rings), but very different total lengths of the alkyl side chains (∼4 for coal asphaltenes, ∼22 for petroleum asphaltenes).