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

Abstract

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 (L2MS) and surface-assisted laser desorption/ionization mass spectrometry (SALDI). In L2MS, 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 L2MS should analyze asphaltenes with almost no bias, and L2MS 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 L2MS perhaps the most thoroughly studied mass spectrometric method for measuring asphaltene molecules, and the results indicate that L2MS 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 L2MS 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.

The influence of asphaltene-resin molecular interactions on the colloidal stability of crude oil

“…Therefore, to present a clear picture of intermolecular interactions in asphaltene aggregation, our choice of asphaltene model is of prime importance. Although the structure of asphaltene is not unique, there are two commonly proposed molecular architectures for asphaltenes that are consistent with the results of experiments: archipelago [54][55][56], containing two or more separate polyaromatic hydrocarbons (PAHs) connected to each other through aliphatic side chains; and island [57,31], containing a single unified PAH with 4-10 fused aromatic rings, with a centroid of the distribution around 7 [58,59]. This study used the island structure, based on the latest Yen-Mullins-modified model [60,31] proposed by Greenfield [61], including one pyrrolic nitrogen (a nitrogen atom embedded in a pyrrole ring, located in the aromatic part) [62] as shown in Fig.…”

A Geological Model for the Origin of Fluid Compositional Gradients in a Large Saudi Arabian Oilfield: An Investigation by Two-Dimensional Gas Chromatography (GC × GC) and Asphaltene Chemistry

“…Previously, the observation of homogenous asphaltene properties for sulfur chemistry, molecular weight, and nanoaggregate weight support this conclusion. 7,26,33 The stable isotope ratios of hydrogen and carbon of extracted asphaltenes measured here support this conclusion. In addition, we find that the liquid phase components are equilibrated in ratios of chemical components of similar molecular weight and density that are invariant within error.…”

“…1 for asphaltene clusters. 7,26 Additionally, L 2 MS and SALDI-MS were used to determine that there is no observed variation in molecular weight or nanoaggregate weight for the asphaltenes in this reservoir. 33 Thus, the large variation of asphaltenes with height correlates with different concentrations of asphaltenes as given by the barometric equation for asphaltene clusters, not from a large chemical change in the asphaltenes.…”

The dynamic Flory-Huggins-Zuo equation of state

“…For example, the proposed molecular weight and island molecular architecture have been strongly supported by L2MS studies [29]. Most importantly, L2MS studies have established the ability to disaggregate asphaltene nanoaggregates, [30] a known problem with some mass spectral techniques. Related mass spectral studies corroborate island molecular architecture [31,32].…”