Addressing Thermal Behavior and Molecular Architecture of Asphaltenes by a Thermal-Optical Carbon Analyzer Coupled to High-Resolution Mass Spectrometry

In the past decade, extensive molecular-level research on asphaltenes, primarily based on mass spectrometric approaches, acknowledged the coexistence of two primary architecture motifs, "island" single-core- and "archipelago" (multi-core)-type structures. Nonetheless, analytical methods for a classification are still limited. In this study, the thermal desorption and pyrolysis behavior of a diverse set of asphaltenes covering island- and archipelago-enriched samples and their extrographic fractions has been investigated by a thermal-optical carbon analyzer (TOCA) hyphenated to high-resolution mass spectrometric evolved gas analysis. The capability of the TOCA for a temperature-resolved quantification of the released carbon is used together with the option of applying an inert or oxidative atmosphere. We found that the relative proportion of organic carbon emitted under an inert atmosphere and below 580 °C (OCdes/pyr) and the organic carbon released at elevated temperatures (>580 °C) and oxidative atmosphere (OCpyrogen) can be used as a classification approach for the prevalent architecture motif. This finding is likely caused by differences in the coking and charring behavior dependent on molecular structure. Hypothetically, single-core constituents will form more resistive shot-like coke due to their larger aromatic cores, whereas multi-core constituents seem to produce easier combustible sponge-like coke. Simultaneously, resonance-enhanced multiphoton ionization (REMPI), a soft ionization scheme particularly selective and sensitive for aromatic constituents, together with Orbitrap Fourier-transform mass spectrometry, allowed for time/temperature-resolved in-depth insights into the evolved chemistry. The alkylation pattern/length of the mass spectra received in OCdes/pyr (480/580 °C) fractions has been identified as a classification measure with lower and more narrow patterns for the asphaltenes dominated by single-core species. However, grouping based on the quantified TOCA results has been significantly more striking. Conclusively, TOCA of asphaltenes and their extrographic fractions can be used for structural classification as well as insights into coprecipitated maltenes, presumably also successfully applicable in future studies on residues from renewable oil sources.