Atmospheric pressure ionization

Any ionization process in which ions are formed in the gas phase at atmospheric pressure.

atmospheric pressure ionization (obsolete) in mass spectrometry

An ambiguous term; in essence, it is used to describe chemical ionization at atmospheric pressure. It is recommended that use of the term be discontinued.

Source: PAC, 1991, 63, 1541 (Recommendations for nomenclature and symbolism for mass spectroscopy (including an appendix of terms used in vacuum technology). (Recommendations 1991)) on page 1547

Page 11: "...when used as an abbreviation for atmospheric pressure chemical ionization..."

Ionization technique(s) that occur at atmospheric pressure. Specific API ionization techniques include electrospray, pneumatically assisted electrospray and atmospheric pressure chemical ionization and is often used to couple LC to MS.

A reader’s comment: Atmospheric pressure ionization (API) should remain a general term, for any form of ionization at atmosphere. The definition given is specific for chemical ionization at atmosphere. The definition given should be for a separate term, atmospheric pressure chemical ionization (APCI). This was merely the first atmospheric pressure ionization means that was commercialized. API would then properly include, electrospray (ESI), APCI, Ionspray (a coined term, and more generally this is pneumatically-assisted electrospray), and flame ionization (in some of the early papers on API this was the mode of ionization). Lots of others have been tried including microwave, etc.