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) is 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 "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.