Gas chromatography–mass spectrometry
In chemical ionization (CI), a reagent gas, typically methane or ammonia is put into the mass spectrometer. There are two types of CI: positive CI or negative CI.  Either way, the reagent gas will interact with the electrons and analyte and cause a 'soft' ionization of the molecule of interest.  A softer ionization fragments the molecule to a lower degree than the hard ionization of EI.  Chemists prefer CI over EI. This is because CI produces at least one mass fragment with a weight, that is nearly the same as the molecular weight of the analyte of interest.