Laser-Based Mass Spectrometric Assessment of Asphaltene Molecular Weight, Molecular Architecture, and Nanoaggregate Number

Unbiased analysis of asphaltene molecules by mass spectrometry requires that the three fundamental steps in mass spectrometry-volatilization, ionization, and mass analysis-are performed reliably and with nearly equivalent efficiency for nearly all of the diverse components in asphaltenes. Here, we examine these three processes and the overall analysis mechanism in two forms of laser-based mass spectrometry: laser desorption laser ionization mass spectrometry (L 2 MS) and surface-assisted laser desorption/ionization mass spectrometry (SALDI). In L 2 MS, volatilization occurs by laser desorption, which is a thermal process not selective for any particular molecular configuration; ionization occurs by single-photon ionization, which is a soft and universal method applicable to virtually all organic molecules and aggregates; and mass analysis occurs by time-of-flight mass spectrometry, which has a nearly constant sensitivity across a broad mass range. These factors suggest that L 2 MS should analyze asphaltenes with almost no bias, and L 2 MS measurements of model compounds and asphaltenes provide direct experimental evidence that fragmentation and multiple charging are minimized, while nearly all components of asphaltenes are detected as disaggregated molecules with nearly equivalent sensitivity. These experiments make L 2 MS perhaps the most thoroughly studied mass spectrometric method for measuring asphaltene molecules, and the results indicate that L 2 MS provides essentially unbiased detection of nearly all of the components in asphaltenes. Similarly, SALDI is demonstrated to be a complementary technique, in which asphaltenes are detected in the form of nanoaggregates. These observations add credibility to conclusions from L 2 MS and SALDI that petroleum asphaltenes typically have an average molecular weight in the range of 600–700 Da with an upper mass limit near 1500 Da, asphaltenes are dominated by the island geometry, asphaltenes form stable nanoaggregates containing approximately seven molecules, and asphaltenes of different mass are not separated in a reservoir by gravitational segregation. These results provide a stringent test and confirmation of many components of the Yen–Mullins model, and they support the growing use of the Flory–Huggins–Zuo equation in the oilfield.