Illustrated Glossary of Organic Chemistry - Chemical ionization (CI; CI-MS)

Chemical ionization (CI; CI-MS): In mass spectrometry, a method for analyte ionization in which another molecule (called the reagent gas) is ionized by electron impact, and then these reagent gas molecular ions are impacted on the analyte causing the analyte to be ionized. The reagent gas often methane or ammonia. Chemical ionization imparts less kinetic energy to the analyte, so compared to electron impact ionization, the relative amount of fragmentation is reduced.


Methane		Methane molecular ion		Analyte molecular ion

In this general scheme for chemical ionization mass spectrometry, methane (the reagent gas) is subjected to electron impact ionization to form [CH₄]^.+, the methane molecular ion. The analyte is then bombarded with the methane molecular ions, resulting in ionization of the analyte, and subsequent fragmentation.