Molecular characterization of asphaltenes by conventional analytical techniques is a challenge because of their compositional complexity, high heteroatom content, and asphaltene aggregate formation at low concentrations. Thus, most common characterization techniques rely on bulk properties or solution-phase behavior (solubility). Proposed over 20 years ago, the Boduszynski model proposes a continuous progression in petroleum composition (molecular weight, structure, and heteroatom content) as a function of the atmospheric equivalent boiling point. Although exhaustive detailed compositional analysis of petroleum distillates validates the continuum model, the available compositional data from asphaltene fractions supports the extension of the continuum model into the nondistillables only indirectly. Asphaltenes, defined by their insolubility in alkane solvents, accumulate in high-boiling fractions and form stable aggregate structures at low parts per billion (ppb) concentrations, far below the concentration required for most mass analyzers. Here, we present direct mass spectral detection of stable asphaltene aggregates at lower concentrations than previously published and observe the onset of asphaltene nanoaggregate formation by time-of-flight mass spectrometry (TOF−MS). We conclude that a fraction of asphaltenes must be present as nanoaggregates (not monomers) in all atmospheric pressure and laser-based ionization methods. Thus, those methods access a subset of the asphaltene continuum.

Molecular characterization of sulfur-containing species in petroleum is important because sulfur-containing compounds are detrimental to the environment and the refining processes. In a recent report, the sulfur-containing compounds in a vacuum bottom residue (VBR) were methylated to enhance their detectability by electrospray ionization (ESI) mass analysis. The most abundant sulfur compounds exhibited relatively low double bond equivalents (4 < DBE < 12). Alternatively, atmospheric pressure photoionization (APPI) mass analysis can provide molecular characterization without chemical derivatization. Here, we compare the sulfur speciation of a petroleum vacuum bottom residue by ESI and APPI with a 9.4 T Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Even after methylation, ions produced by APPI extend to much higher DBE than by ESI. Moreover, analysis of the saturates and aromatics fractions of underivatized VBR by APPI shows comparable ionization efficiency across a broad DBE range. We conclude that methylation is hindered for high-DBE species (DBE > 20), so that methylation followed by ESI MS is not suitable for sulfur speciation of higher-boiling fractions from petroleum crude oil.

Here, we present a case study on a Wyoming well with known asphaltene deposition issues as a result of natural depletion. Field deposits and crude oil from the same well were collected for analysis. Compositional differences between field deposits, lab-generated capillary deposits, and C 7 -precipitated asphaltenes were determined by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and all three samples show similar trends in composition, displayed as plots of aromaticity versus carbon number. An enrichment of highly condensed aromatic molecules for the field deposit is detected with both ultrahigh-resolution mass spectrometry and thermal cracking experiments and could predict asphaltene deposition. FT-ICR mass spectral analysis of solvent-extracted fractions suggest different deposition mechanisms for field deposits (slow deposition) compared to rapid precipitation in standard asphaltene preparation protocols that contain trapped maltenes.

Silver cationization constitutes a complementary approach for analysis of petroleum components with positive-ion electrospray ionization (ESI) mass spectrometry and accesses species that lack a basic nitrogen atom and, hence, are not observed by conventional positive ESI. Four samples of different origin [Canadian bitumen, Canadian bitumen heavy vacuum gas oil (HVGO; 475–500 °C) and South American and Middle East heavy crude oils, all high in sulfur content] were used to study silver cationization by (+) ESI. Cationization with Ag+ is essentially instantaneous and accesses hydrocarbons and nonpolar sulfur-containing heteroatom classes (e.g., S s and S s O o ), providing an attractive alternative to time-consuming derivatization by S-methylation to ionize sulfur-containing species. For each sample, we compare Ag+ cationization (+) ESI to conventional (+) ESI with formic acid to promote ion formation. Other ionization methods, such as chemical ionization (CI), field desorption (FD), matrix-assisted laser desorption ionization (MALDI) chemical ionization, field desorption ionization, and MALDI, are low in throughput and/or involve thermal processes that may degrade substrate molecules from non-volatile high-boiling petroleum components. Mix-and-spray Ag+ cationization avoids tedious separation and time-consuming derivatization and results in the rapid speciation of sulfur-containing compounds in petroleum and its fractions without the need for thermal desorption.