Profiling of Petrochemical Samples by Thermal Analysis with mass spectrometric detection: Evolved gas analysis using photoionsation-mass spectrometry, high resolution mass spectrometry and ultra-fast gas chromatography-MS
Mass Spectrometry (MS) with soft photoionisation (PI for fragmentation-free ionisation of organics, including saturates) enables the direct detection of intact molecules from complex organic vapours. Soft photoionisation mass spectrometry (PIMS) is particularly well suited for Evolved Gas Analysis (EGA) in Thermal Analysis (TA, techniques such as thermo gravimetry, TG or programmed vaporisation via a direct insertion probe, DIP). In particular PIMS allows the determination of the molecular organic signatures (i.e. intact organic molecules) of desorption-, pyrolysis- and combustion-processes. State of the art TA-PIMS systems were applied for detection of the evolved signatures of intact organic molecules from thermal analyses of petrochemical samples. In addition to the soft MS separation also a newly developed ultra-fast cycling gas chromatography technique was implemented. The fast cycling GC technique is based on a rapidly IR-radiation heat-able (and quickly cool-able) gas chromatograph which is inserted between the TA and PIMS device, allowing the separation of evolving isobaric compounds. The TA-PIMS approach is used for the detailed analysis of the thermochemical behaviour of petrochemical samples. In the case of crude oils, the yield and chemical composition of the different distillation fractions was determined (50-400°C). At higher temperatures, cracking products of the non-volatile residue (resins, asphaltenes etc.) are fingerprinted. From very heavy matrices and source rock samples the amount and composition of the thermally extractable volatile organic compounds were determined. Another approach is the used high mass resolution mass spectrometers for TA-EGA. For example, a high resolution TOFMS (Pegasus HRT, LECO, USA), equipped with a direct insertion probe (DIP) and soft photoionization capability can be used for fast determination of crude oil- and bitumen-samples. The achieved resolution (R ˜ 30.000) allows already a Kendrick mass defect-basd analysis of compound classes. On the high-end side also ultra-high mass resolution mass spectrometry (7T FT-ICR, Bruker, Germany, (R > 200.000) can be applied for TA-EGA. Crude oils etc. were placed a TA device (TG system, Netzsch, Germany). Atmospheric pressure chemical and photo ionisation sources (APCI and APPI) were employed for soft gas phase ionisation of the evolving vaporized or pyrolyzed compounds. For a deeper structural insight, collision induced dissociation (CID) for tandem mass analysis (MS/MS) was applied. CID-MS/MS with moderate dissociation energies allows the analysis of aromatic core structures e.g. from asphaltenes. The value of this concept was demonstrated exemplary by the comprehensive chemical characterisation of heavy petroleum and its fractions. The TA-PIMS-, DIP-PI-high-resolution-TOFMS-, TA-fast-GC-PIMS- and TA-FTICR-results suggest a broad applicability of the TA-MS approach for analysis of crudes and petrochemical fractions as well as the simulation and optimization of industrial petrochemical processes.
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Dr Ralf Zimmermann (University of Rostock)
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