Illustrated Glossary of Organic Chemistry

Electron impact ionization (electron impact; electron ionization; EI; EI-MS): In mass spectrometry, a method for analyte ionization by impact with a beam of high-energy electrons. This ionization method tends to enhance the formation of fragment ions.

The electron impact mass spectrum of methane is obtained by bombarding methane with a beam of high-energy electrons. This produces molecular ions (M, M+1, M+2, etc.) which may then produce fragment ions before reaching the mass spectrometer's detector.