Electron Ionization (EI)
An ionization method in which energetic electrons interact with gas phase atoms or molecules to produce ions. EI was one of the first ionization techniques developed for mass spectrometry and is still a popular ionization technique. This technique is considered a hard (high fragmentation) ionization method, since it uses high energetic electrons to produce ions. This leads to extensive fragmentation, which is helpful for structure determination of unknown compounds. EI is the most useful for organic compounds which have a molecular weight below 600 Da.