Atmospheric Pressure Ionization Mass Spectrometry

Abstract The atmospheric pressure ionization (API) techniques serve to transform neutral analytes into ionic species that may then be analyzed by mass spectrometry (MS). Ionization takes place under ambient pressure (∼1 atm), and formed ions are subsequently directed into the low‐pressure regions of the mass spectrometer for mass analysis. The API techniques have offered notable successes in enabling ionization of nonvolatile compounds contained in solution that are not amenable to “classical” mass spectrometric ionization techniques such as electron ionization and conventional chemical ionization. Under favorable API conditions, neutral analytes may be converted to ionic forms in extremely high efficiencies approaching 100%. Fundamental aspects and applications of the two most widely used API techniques, atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI), are discussed at length in this article.