Combination of Different Thermal Analysis Methods Coupled to Mass Spectrometry for the Analysis of Asphaltenes and Their Parent Crude Oils: Comprehensive Characterization of the Molecular Pyrolysis Pattern

摘要

In this study, the asphaltene and corresponding crude oil, distributed within the Asphaltene Characterization Interlaboratory Study for PetroPhase 2017, were characterized on the molecular level. For this purpose, three different thermal analysis mass spectrometry hyphenations with five diverse ionization techniques varying in selectivity were deployed: (1) thermal desorption/pyrolysis gas chromatography electron ionization (TD/Pyr-GC-EI-QMS), (2/3) thermogravimetry singlephoton/resonance-enhanced multiphoton ionization time-of-flight (TG SPI/REMPI TOF-MS), and (4/5) thermogravimetry atmospheric pressure photo-/chemical ionization ultrahigh-resolution mass spectrometry (TG APPI/APCI FT-ICR MS). For the investigated C-7 asphaltene, no mass loss was detected at 300 degrees C and the pyrolysis phase was dominant, whereas the parent crude oil exhibits a high abundant desorption phase. At roughly 330 degrees C, pyrolysis begins and mass loss as well as complex mass spectrometric patterns were recorded. The resulting information on the effluent gained by the different soft ionization mass spectrometric approaches was combined with the GC-EI-MS data for structural cross-evaluation. We showed that the combination of the applied techniques leads to a more comprehensive chemical characterization. For the asphaltene, TG SPI TOF-MS shows high abundances of alkanes, alkenes, and hydrogen sulfide during pyrolysis. TG REMPI TOF-MS is selective toward aromatics and reveals clear patterns of polyaromatic hydrocarbons (PAHs) and minor amounts of nitrogen-containing aromatics tentatively identified as acridine-or carbazol-like structures. GC-EI-MS provides information on the average chain length of alkanes, alkenes, and PA(S)H. Both atmospheric pressure ionization techniques (APPI and APCI) hyphenated to FT-MS showed CHS (in particular, benzothiophenes) and CH as dominant compound classes, with an average number of condensed aromatic rings of 2-4. Combining the information of all techniques, including the average asphaltene mass obtained by field desorption experiments and aromatic core size received by collision-induced dissociation, the archipelago-type molecular structure seems to be dominant for the investigated asphaltene.