Electron ionization (EI) and chemical ionization (CI)
In electron ionization (EI), a beam of energetic electron is created inside a chamber by heating a metal filament to a sufficiently high temperature (approx. 2000 K) the electrons are attracted toward a plate (away from the filament), called the trap, by maintaining the trap at a relatively positive potential in a gas phase. A relatively volatile analyte is directed into this high-energy beam travelling from filament to the trap, and an interaction with electron beam causes loss of electrons from the analyte molecules, this generating charged molecular ions. Depending on the compound and the ionization energy, the molecular ions may then fragment. The spectra, usually containing many fragment-ion peaks, are useful for structural characterization and identification. Small impurities in the sample are easy to detect.

Electron-capture ionization
Electron-capture, sometimes called negative ion chemical ionization (NICI), is used for molecules containing halogens, NO₂, CN, etc. It usually requires that the analyte be Derivatized to contain highly electron-capturing moieties (e.g. fluorine atoms or Nitrobenzyl groups). Such moieties are generally inserted into the target analyte after isolation and before mass spectrometric analysis. The sensitiveity of NICI analysis is generally than that of positive ion chemical ionization (PCI) or EI analysis. LittleFragmentation occurs during NICI, and the technique can be used for quantitative analysis of trace amounts of compounds of known structure in conjunction with the use of heavy isotope-labelled internal standards.