Molecular Structures of Asphaltenes Based on the Dissociation Reactions of Their Ions in Mass Spectrometry
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Abstract
We report here an examination of the mass spectrometric fragmentation behavior of molecular ions generated (and excited) by electron ionization (EI) from several asphaltene model compounds simulating both the island and archipelago structural models. This behavior is compared to that of protonated molecules generated from the same compounds by atmospheric pressure chemical ionization (APCI) and excited by collision-activated dissociation (CAD). The fragmentation behavior of the protonated molecules and molecular ions is surprisingly similar. Both types of ions yielded distinct fragmentation patterns for both types of model compounds. Ions derived from the island-type model compounds fragment predominantly by losing their alkyl chains (with either all carbons or all but one), one after another, which allows for the identification of the chain lengths and counting the number of chains. Increasing the length of the alkyl chains reduces the extent of spontaneous fragmentation occurring upon EI, likely because of more efficient cooling of the fragmenting ions via emission of infrared (IR) light made possible by the reduced fragmentation rates of the longer chains. Ions derived from the archipelago model compounds with ethylene bridges connecting two or three aromatic cores (without alkyl side chains) readily undergo cleavages in these bridges. Increasing the length of the alkyl chain between the aromatic cores reduces the extent of fragmentation caused by EI. Similarly, the addition of long external alkyl chains to archipelago model compounds with an ethylene bridging two aromatic cores greatly hinders fragmentation upon EI. When these molecules are protonated and subjected to high-energy CAD, they appear to fragment almost randomly but, nevertheless, indicating some preference for cleavages of the bonds in the chain connecting the aromatic cores. A comparison of these findings to the fragmentation patterns observed for protonated asphaltenes indicates that the asphaltene molecules studied are likely composed of many isomeric and isobaric molecules. Each may contain several aromatic rings and a distribution of mostly aliphatic alkyl chains (and possibly naphthenic rings) ranging in size from 1 to at least 14 carbons, several containing methyl branching at the α carbons. The results do not allow for the unambiguous differentiation between island- and archipelago-type structures, although they are in a better agreement with the island model.
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