Gas-phase ion–molecule reactions between analyte molecules and a reagent-gas plasma constitute chemical ionization (CI) mass spectrometry. In the classical form, a reagent gas such as methane, isobutane, or ammonia is ionized by electrons; the resulting reagent ions transfer a proton, hydride, or charge to the analyte, producing even-electron ions with less fragmentation than electron ionization [1]. Atmospheric-pressure chemical ionization (APCI) and desorption APCI extend the same ion–molecule chemistry to an atmospheric corona or related discharge, so that the ionization step remains chemical rather than purely photonic or electrospray-based [2]. Negative-ion CI uses electron-capture or proton-abstraction reagent ions and is a polarity variant of the same concept [3]. Conceptual boundaries exclude electron ionization, electrospray ionization, and photoionization, each of which generates ions by a different primary process. Essential features are a reagent-ion population, a pressure regime that permits ion–molecule collisions, and a mass analyzer that records the resulting analyte ions, often after chromatographic introduction [4].
- chemical ionization
- ion–molecule reaction
- APCI
- reagent gas
Metabolomics and Mass Spectrometry Studies • Molecular Biology • Biochemistry, Genetics and Molecular Biology • Life Sciences
References
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