|Thermal Analysis in hyphenation with mass spectrometry as a versatile tool for the analysis of complex and high boiling petroleum products
|Energy & the Environment
|Mr Thomas Gröger
|Helmholtz Zentrum München GmbH
Abstract Information :
Thermal analysis (TA) enables the analysis of high boiling and residual matrices derived from petroleum. TA is primarily of interest if the matrix cannot be analysed by methods like gas-chromatography and direct-injection mass spectrometry due limits in temperature range or ionisation effects. TA techniques can generally be divided into setups working under a defined atmosphere at pressures ranging from sub-ambient to increased pressure like thermogravimetry (TG) and methods working under highly reduced pressure such as direct insertion probe (DIP). However, TA only provides temperature resolved fundamental physical data like weight loss or heat consumption. For a more detailed chemical analysis, an evolved gas analysis (EGA) could be utilized by hyphenation to, e.g. mass spectrometry.
In this respect, an appropriate ionisation technique is essential for an in-depth chemical description. Electron impact (EIMS) and Photoionization mass spectrometry (PIMS) are particularly well suited for Evolved Gas Analysis (EGA). Both techniques could be used for a wide range of compound classes ("universal ionization") and are not very prone to produce ionisation artefacts. In particular, PIMS allows the determination of the molecular signatures (i.e. intact organic molecules) derived from desorption-, pyrolysis- and combustion-processes. If EI is applied, a pre-separation step is indicated to reduce the complexity of the spectral information. Therefore, a newly developed, ultra-fast cycling gas chromatography technique was implemented. This 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 EI/PIMS device. Various petrochemical samples have been analysed in detail with this approach. Exemplarily, in the case of crude oils, the yield and chemical composition of the different distillation fractions were specified (50-400°C). At higher temperatures, cracking products of the non-volatile residue (e.g. resins, asphaltenes) are fingerprinted. From very heavy matrices and source rock samples the amount and composition of the thermally extractable volatile organic compounds were determined.
Aside from fast chromatographic pre-separation high-resolution mass analyzes is beneficial for TA based petrochemical description. For this reason, DIP could be directly hyphenated to high-resolution TOFMS (Pegasus HRT, LECO, USA) equipped with EI as well as PI for rapid screening and comparison of crude oil- and bitumen-samples. The achieved resolution (R ~ 30.000 - > 50.000) allows a partly temperature resolved Kendrick mass defect-based analysis of compound classes of the highly complex evolved gas mixtures. On the high-end side ultra-high mass resolution mass spectrometry (7T FT-ICR, Bruker, Germany, (R > 200.000) can be applied for TA-EGA with atmospheric pressure chemical and photo ionisation sources (APCI and APPI). For a more in-depth 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. Summarizing, 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 the analysis of crudes and petrochemical fractions as well as the simulation and optimization of industrial petrochemical processes.