For gas chromatographic (GC), liquid chromatographic (LC) or capillary electrophoretic (CE) analytical methods coupled to a mass spectrometer, the ionization of the separated analytes must be as quantitative as possible. Therefore, different ionization methods are used for analytes with different molar masses and polarities.

For polar substances, electrospray ionisation (ESI) is the gold standard for small molecules and - due to multiple charges - also for large biomolecules. In contrast, analytes with up to 2000 Da and with a medium polarity are preferably ionized by chemical ionization at atmospheric pressure (APCI), whereas photoionization at atmospheric pressure (APPI) or dopant-assisted (DA)-APPI are normally used for nonpolar substances. The laser ionization at atmospheric pressure (APLI) developed by us in cooperation with the research group of Prof. Thorsten Benter (University of Wuppertal) shows an outstanding sensitivity for medium polar to non-polar aromatic substances.

Gas chromatography coupled to mass spectrometry (GC/MS) is a well-established and powerful method for the analysis of volatile and thermally stable analytes. Normally, the effluent from the GC is introduced into the vacuum system of mass spectrometer. The separated analytes are traditionally ionized in a vacuum system prior to detection by MS. Electron ionization (EI) is the most adopted ionization technique for GC/MS because of its capability to ionize a broad range of organic compounds and availability to search against an EI-MS spectral library. Typically, the molecule is extensively fragmented during ionization process. For many compounds, characteristic mass spectra are obtained. But in other cases, fragment ions are less specific, or fragmentation is too extensive, which reduces the sensitivity. Furthermore, the highly diagnostic molecular ion is often absent, which hinders the identification of co-eluting low abundance molecules. Chemical ionization (CI) are also commonly used in GC/MS, particularly in determination of molecular weights. CI has considerably less fragmentation but, unfortunately, with reduced sensitivity in comparison with EI. Therefore, other soft ionization techniques for GC are necessary. Although the coupling of GC/MS with a "soft" atmospheric pressure ionization (API) has been reported in 1970s, the interest in coupling GC with more optimized atmospheric pressure ion sources was expanded in the last years. Ion sources such as atmospheric pressure photo ionization (APPI), atmospheric pressure laser ionization (APLI), low temperature plasma (LTP) and atmospheric pressure chemical ionization (APCI) are interesting "soft" ion sources for GC and one of our research topics is the optimization of these kinds of ion sources.