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The coupling of high performance liquid chromatography and mass spectrometry (LC/MS) has become one of the most powerful tools for the analytical chemist since the mid 1980s. Electrospray ionization (ESI) und atmospheric pressure chemical ionization (APCI) allow for both the transfer of the HPLC eluent into the gas phase and the ionization of the analytes. Protonation (in the positive mode) and deprotonation (in the negative mode) are most frequently observed as ionization mechanisms. In many cases, adducts of the analytes with ammonium, sodium or chloride ions are observed as well. Owing to these "soft" ionization mechanisms, ESI is exceptionally well-suited for the analysis of highly polar compounds, e.g. metabolites, and APCI also for less polar compounds. ESI conditions are even gentler than the APCI conditions. In ESI-MS, a high voltage is applied to a capillary, which leads the HPLC eluent into the interface. This high voltage is responsible for both the evaporation of the eluent and the ionization of the analytes. In the APCI interface, the eluent is first evaporated in a heated capillary and while ionization is subsequently performed applying a coronar discharge at a metal needle. ESI and APCI therefore complement GC/MS with electron impact ionization in a favourable way.