Laser-Based Mass Spectrometric Determination of Aggregation Numbers for Petroleum- and Coal-Derived Asphaltenes

Petroleum and coal-derived asphaltenes have been studied with three laser-based mass spectrometric techniques: laser desorption ionization-mass spectrometry (LDI-MS), in which a single laser desorbs and ionizes solid analytes; surface-assisted laser desorption ionization-mass spectrometry (SALDI-MS), in which a single laser desorbs and ionizes solid analytes from an activated surface; and laser desorption laser ionization mass spectrometry ((LMS)-M-2), in which desorption and ionization are separated spatially and temporally with independent pulsed laser sources. We find that asphaltene nanoaggregates can be detected in LDI-MS and SALDI-MS under mild conditions of relatively low laser power, whereas (LMS)-M-2 avoids aggregation and fragmentation, detecting asphaltenes as monomeric molecules. A comparison of the (LMS)-M-2 and SALDI-MS results yields an estimate of the distribution of aggregation numbers (number of molecules comprising the nanoaggregate). The most probable aggregation number observed for nanoaggregates of petroleum asphaltenes is approximately 6-8 molecules, consistent with the Yen-Mullins model of asphaltenes and predictions from the island geometry for asphaltene molecules. Additionally, the nanoaggregates are found to be relatively monodisperse, because most aggregates observed have aggregation numbers within one molecule of the most probable. In contrast, the nanoaggregates of coal asphaltenes are found to be smaller and more polydisperse, with aggregation numbers ranging from 3 to 6 molecules. Under higher powers, SALDI-MS measurements show that nanoaggregates decompose to form small multimers and monomers, suggesting that the aggregates are bound noncovalently. Coal asphaltene nanoaggregates decompose at lower laser powers than petroleum asphaltene nanoaggregates, indicating that the coal asphaltene nanoaggregates are bound less strongly than petroleum asphaltene nanoaggregates.