Two-Step Laser Desorption/Laser Ionization Mass Spectrometry of Asphaltenes (ABSTRACT)

摘要

The chemistry of asphaltenes is a controlling factor in the phase behavior of crude oil and the production of heavy oil. However, the chemical complexity of asphaltenes makes them intractable for analysis by traditional laboratory methods. After decades of research employing a diverse set of techniques, even as fundamental a quantity as the asphaltene molecular weight distribution is still under dispute, with average measured molecular weights spanning orders of magnitude. Laser desorption ionization (LDI) experiments are particularly interesting in this regard, as different groups have measured asphaltene molecular weights differing by over 100-fold using similar LDI techniques on similar asphaltene samples. The ambiguity lies in the optimization of experimental parameters such as the sample concentration and laser power. Sufficient sample must be deposited to observe an experimental signal, and sufficient laser power must be applied to desorb the heaviest fraction of asphaltenes, or erroneously low masses will be recorded. However, some experimental evidence implies that excessive sample concentration and/or laser power can lead to aggregation in the plasma produced in LDI measurements, resulting in erroneously low measured masses. Proper interpretation of LDI measurements therefore requires an understanding of the influence of these different steps that are coupled in traditional LDI experiments.