Radical Ion Generation Processes of Organic Compounds in Electrospray Ionization Mass Spectrometry

In electrospray ionization mass spectrometry (ESI-MS), charge formation is proposed to occur as a result of acid-base reactions or a coordination with metal cations. However, the increasing number of papers have reported the formation of radical ions (molecular ions) as a result of loss of one or two electrons. In this paper, we present an overview of the current literature on the balance between the formation of molecular ions and of protonated forms of organic compounds in ESI-MS. Correlations between the radical formation process and the ionization energy, calculated by molecular orbital methods or obtained by eletrochemical techniques, were applied to explain this apparently unexpected behavior. The use of high level theoretical methods especially designed to produce accurate thermochemical data is also reported.