Application of Thermal Analysis: Single Photon Ionization Time-of-Flight Mass Spectrometry for Studies of Inorganic Clusters and Ionic Liquids

In this study, Thermal Analysis – Skimmer – Single-Photon Ionization Time-of-Flight Mass Spectrometry technology (TA–Skimmer–SPI-TOFMS) was used for the elucidation of the gas phase behavior of inorganic elemental clusters and ionic liquids. Thermal analysis coupled with mass spectrometry is an often-used tool in material sciences. However, it is seldom used for the characterization of inorganic vapors with less volatility nor the investigation of clusters. The skimmer coupling enables the online transfer of compounds even with low volatility from the thermal analyzer into the mass spectrometer. The ionization energies of sulfur and selenium clusters are between 8 and 10 eV. For characterization, the fragmentation of the individual species must be avoided. Therefore, conventional electron ionization (EI), with kinetic energies of 70 eV, is not suitable. Single photon ionization (SPI) is a soft ionization method where VUV (vacuum ultraviolet) photons are generated by a deuterium lamp (Hamamatsu Photonics, Hamamatsu City, Japan). The non-fragmenting character of the ionization was tested and verified by gas chromatography (GC) separation of the GC-measurable species with subsequent mass spectrometric investigation, using the same deuterium photon source for the ionization as in the aforementioned TA–Skimmer–SPI-TOFMS. We detected different molecular species with 2 to 8 atoms in temperature-dependent equilibria in the vapor phase. The relative concentrations of the individual species could be calculated from their individual mass traces. Ionic liquids are often referred to as green alternatives of volatile organic solvents. Their thermal behavior is relevant due to the emerging number of high-temperature, larger-scale applications. Also, knowledge regarding the decomposition products is necessary for the treatment and recycling of used ionic liquids. The objective of this study was the stability of several 1-alkyl-3-methylimidazolium halides, the determination of the degradation products, and the elucidation of their decomposition patterns and structure-stability relations. The applied technology provided real-time monitoring of the evolving species. Therein, the almost fragment-free soft ionization with VUV photons, generated by a deuterium lamp (Hamamatsu Photonics), played a crucial role. The main decomposition products were alkylimidazoles, alkyl halides, alkenes, and hydrogen halides. We detected unfragmented molecules whose formation was previously only assumed by electron ionization mass spectrometry. From the decomposition products, we deduced the fragmentation patterns and discussed their dependence from the alkyl chain length and the halide anion. Generally, the decomposition occurred via the reversed Menshutkin reaction, elimination, and bond cleavages. Our results did not suggest the formation of clusters from the investigated ionic liquids nor the evaporation prior to their decomposition