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

Abstract

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).

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

“…25,[46][47] Based on the measured mass spectra, the apparent molecular weight distributions (MWD) were determined for six petroleum asphaltene samples originating from Bohai, Maya, Claire, Surmont, Montana and McKittrick. All of the studied asphaltenes from the American continent, Europe and China have similar apparent MWD, ranging from 200 up to 1400 Da.…”

“…This is justified as we have previously demonstrated that the fragmentation patterns and main fragment ions of ionized asphaltenes are independent of the size of the isolation window as long as it is equal or less than 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Da. 25 The data were processed by using Thermo Xcalibur software. All measurements were repeated at least four times.…”

“…[13][14][15] However, fluorescence depolarization and mass spectrometry have indicated an average molecular weight of only 450-850 Da. 1,[16][17][18][19][20][21][22][23][24][25][26][27] The disparity of these measurements has been suggested to result from the tendency of asphaltenes to aggregate. 27,28 In the past decades, two structural models have been debated for asphaltene molecules: the island model and the archipelago model.…”

“…CS 2 reagent has been demonstrated previously to generate stable molecular ions for asphaltenes. 25,[46][47] Furthermore, multi-stage tandem mass spectrometry was employed to examine the structures of asphaltene molecules by subjecting the selected molecular ions to collisionally activated dissociation (CAD). In addition to the apparent molecular weight distribution (MWD) and apparent average MW, structural information, including the approximate maximum number of carbons in alkyl chains and minimal sizes of the aromatic cores, was obtained.…”

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

“…Collision-activated dissociation 38 Asphaltenes are a subset of molecules in crude oil whose 62 structural elucidation has proven difficult [1][2][3]. They comprise 63 the fraction of petroleum that is soluble in toluene but insoluble 64 in n-alkane [4] and can correspond to up to 20% of crude oil [5].…”