Thesis defense of Luisa Speicher

For analytical methods such as mass spectrometry, ionisation sources are among the core components. Particularly soft ionisation types are preferred here, as they deliver intact molecules. The noble gas used by plasma-based ionisation sources is usually helium, although in recent years more and more plasmas have been operated with argon and show similar ionisation efficiencies. While the underlying ionisation mechanism in helium plasmas is largely understood and is considered to be similar to chemical ionisation at atmospheric pressure, it remains unclear how soft ionisation takes place in other noble gases. This question is the starting point for several explanations, which are investigated in this dissertation. A self-developed plasma source that can be operated with various noble gases without changing the geometry is used for the investigation. In addition to helium and argon, neon, krypton and xenon are also used to investigate the role of collision processes for gases with different energy levels. Photoionisation is also examined. The use of a diagnostic gas offers the possibility of replacing ambient air in a targeted manner and thus offers insights into possible alternative mechanisms outside the plasma capillary. This was carried out both with and without a barrier between the plasma and diagnostic gas. The results show that the classical helium mechanism is not transferable to other noble gases and suggests a possible alternative process, and based on this, gas-saving plasma sources. Those are subsequently evaluated via mass spectrometry.