Molecular-Weight Distributions of Coal and Petroleum Asphaltenes from Laser Desorption/Ionization Experiments

Abstract

Molecular-weight distributions (MWDs) of asphaltenes extracted from coal and petroleum have been measured in laser desorption/ionization (LDI) mass spectrometric experiments. The dried-droplet and solvent-free sample preparation methods are compared. The coal asphaltenes have a relatively narrow MWD (full width ≈ 150 amu) with an average molecular weight of ≈340 amu. The petroleum asphaltenes display a broader MWD (full width ≈ 300 amu) and are heavier on average (≈680 amu). The LDI spectra also provide evidence for the formation of noncovalent clusters of the two types of asphaltenes during the desorption process. Petroleum and coal asphaltenes exhibit aggregation as do large model polycyclic aromatic hydrocarbons (PAHs) with five or more fused rings also included in the study. Smaller PAHs (pyrene) exhibit less aggregation, especially when alkane-chain substituents are incorporated to the molecular structure. This indicates that asphaltenes possess large PAHs and, according to the relatively small molecular weights observed, that there is a preponderance of asphaltene molecules with only a single fused ring system. The coal asphaltenes present a significantly smaller propensity toward aggregation than their crude oil counterparts. This finding, coupled with the fact that (1) alkanes inhibit aggregation in LDI and (2) petroleum asphaltenes possess much more alkane carbon, indicates that coal asphaltenes have smaller PAHs on average than petroleum asphaltenes. This is further corroborated by the stronger ultraviolet absorbance of the coal asphaltenes at wavelengths shorter than 400 nm.

The dynamic Flory-Huggins-Zuo equation of state

“…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]. In conceptually simple, robust, surfacetension studies, the PAH (polycyclic aromatic hydrocarbon) size for several asphaltene types has been confirmed [33,34].…”

Combining biomarker and bulk compositional gradient analysis to assess reservoir connectivity

“…The soft ionization of petroleum molecules provided by ESI has been corroborated by good agreement of molecular weights measured by ESI with those measured by other soft ionization techniques such as two-step laser mass spectrometry (Hortal et al, 2007;Hortal et al, 2006;Pomerantz et al, 2008;Pomerantz et al, 2009;Rodgers and Marshall, 2007). ESI FT-ICR MS is useful for the analysis of complex mixtures because each elemental composition (C c H h N n O o S s ) has a unique molecular mass.…”

Petroleomics: Chemistry of the underworld

“…APPI of petroleum requires the ultrahigh resolution of FT-ICR MS for two reasons: (i) APPI ionizes a broader range of compound classes, and thus the mass spectrum contains approximately five times as many peaks as an ESI mass spectrum of the same crude oil sample; and (ii) the same analyte molecule may produce both M ϩ• and (MϩH) ϩ ions, making it necessary to resolve M ϩ• containing one 13 C from (MϩH) ϩ containing all 12 C, a mass difference of only 4.5 mDa (see below). Laser desorption/ionization mass spectrometry, although highly useful for biomolecule analysis, does not provide a reliable representation of petroleum components because of significant aggregation and fragmentation at the laser power required to generate a useful number of gas-phase ions (10). However, recent introduction of a two-color laser method (11), in which the first laser desorbs the neutrals and the second laser ionizes them, appears to overcome the disadvantages of single-color laser desorption/ ionization.…”