Recent advances in instrumentation for high-field Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) have enabled access to ∼70 000 unique molecular formulas in broadband mass spectral characterization of unfractionated/whole asphaltenes. The results accumulated over a decade highlight the need for an asphaltene molecular model that acknowledges the coexistence of (1) monofunctional and polyfunctional species; (2) island and archipelago structural motifs; and (3) heteroatom-depleted/highly aromatic compounds, as well as atypical species with low aromaticity but increased heteroatom content. Collectively, results from FT-ICR MS, preparatory-scale separations (extrography/interfacial material), gel permeation chromatography, precipitation behavior in heptane:toluene, thermal decomposition, and aggregate microstructure by atomic force microscopy (among other techniques), suggest that the strong aggregation of asphaltenes results from the synergy between several intermolecular forces: π-stacking, hydrogen bonding, London forces, and acid/base interactions. This review presents general features of asphaltene molecular composition reported over the past five decades. We focus on mass spectrometry characterization and expose the reasons why early results supported the dominance of single-core motifs. Then, the discussion shifts to recent advances in instrumentation for high-field FT-ICR MS, which have enabled the detection of thousands of species in asphaltene samples, whose molecular composition and fragmentation behavior in ultrahigh vacuum agree with the coexistence of single-core and multicore structural motifs. Furthermore, evidence that highlights the limitations of commercially available/custom-built ion sources and selective ionization effects is presented. Consequently, the limitations require separations (e.g., chromatography, extrography) to gain more-comprehensive molecular-level insights into the composition of these complex organic mixtures. The final sections present evidence for the role of aggregation in selective ionization and suggest that advanced characterization by both thermal desorption/decomposition and liquid chromatography with online FT-ICR MS detection can be employed to mitigate the effects of aggregation and provide unique insights in molecular composition/structure.

Pavement sealants are of environmental concern because of their complex petroleum-based chemistry and potential toxicity. Specifically, coal tar-derived sealants contain high concentrations of toxic/carcinogenic polycyclic aromatic hydrocarbons (PAHs) that, when weathered, can be transferred into the surrounding environment. Previous studies have demonstrated the effects of coal tar sealants on PAH concentration in nearby waterways and their harmful effects in aquatic ecosystems. Here, we investigate and compare the molecular composition of two different pavement sealants, petroleum asphalt- and coal tar-derived, and their photoproducts, by positive-ion (+) atmospheric pressure photoionization (APPI) and negative-ion (−) electrospray ionization (ESI) coupled with ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry to address species (high-boiling and/or high oxygen content) that lie outside the analytical window of other techniques due to ultra-high molecular complexity. In addition, we evaluate the toxicity of the water-soluble photoproducts by use of Microtox bioassay. The results demonstrate that the coal tar sealant contains higher amounts of PAHs and produces abundant water-soluble compounds, relative to unweathered materials, with a high abundance of PAH-like molecules of high toxicity. By comparison, the asphalt sealant produces fewer toxic water-soluble species, with molecular compositions that are consistent with natural dissolved organic matter. These results capture the mass, chemical diversity, toxicity, and source/photoproduct relationship of these compositionally complex emerging contaminants from the built environment.

The ability of molecular characterization to expose the chemical and structural composition of petroleum photooxidation products can aid future optimization of oil spill remediation techniques. Previous studies have documented molecular changes induced by photochemical reactions, their compositional/structural dependence, and thus revealed that they are sample-dependent. The work herein describes the photochemical transformation of nonasphaltenic petroleum compounds (maltenes) into asphaltenes. Pentane-soluble species (maltenes) were isolated from three geologically diverse crude oils and photooxidized in a solar simulator microcosm to investigate their transformation into pentane-insoluble molecules (asphaltenes), referred to as photochemically produced asphaltenes (PPA). All oils, photoproducts, and solubility fractions were characterized by positive-ion (+) atmospheric pressure photoionization (APPI) coupled to Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and tandem-MS to access their molecular composition and structural features. The oil-soluble photooxidation products from the three deasphalted oils exhibit asphaltene contents between ∼7–19 wt % after photoirradiation, which reveals the photogeneration of asphaltenes from oils initially devoid of asphaltenes. The variation in asphaltene yield after photoirradiation suggests that the quantity of PPA is sample-dependent. PPA are shown to have lower molecular weights, much higher oxygen content, and lower aromaticity than native asphaltenes from nonoxidized oils. Compositional trends for oxygen-containing photoproducts suggest that production of new asphaltenes in the environment might occur via concurrent photooxidation, photofragmentation, and photoinduced polymerization of native petroleum compounds.