Investigation of Sulfur and Selenium Vapors with Thermal Analysis - Single Photon Ionization Mass Spectrometry

摘要

The investigation of sulfur and selenium vapors is still challenging today, as they exhibit a complex molecular composition. In this study, we tested the applicability of an innovative thermal-analysis - single-photon-ionization time-of-flight mass spectrometry system (Netzsch-Geraetebau GmbH, Selb, Germany; Tofwerk, Thun, Switzerland) with a Skimmer interface (TA-Skimmer-SPI-TOFMS) for the elucidation of these vapors. 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 the sulfur and selenium clusters are between 8 and 10 eV. For the characterization, the fragmentation of the individual species has to be avoided. Therefore, the often used electron ionization (EI) with kinetic energies of 70 eV is not suitable. Single photon ionization (SPI) is a soft ionization method where VUV photons were generated by a deuterium lamp (Hamamatsu Photonics, Hamamatsu City, Japan). The non-fragmenting character of the ionization was tested and verified by GC-separation of the GC-measurable species with subsequent mass spectrometric investigation, using the same deuterium photon source for the ionization as in the 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 can be calculated from their individual mass traces. For these calculations, mass spectrometric data were corrected with the photoionization cross sections. They were determined from quantum chemical simulations using time-dependent density functional approach and Dyson orbital formalism.