How does an electron impact ionize a molecule

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Electron impact ionization

Electron impact ionization (EI) is the classic ionization method in mass spectrometry. Energy is supplied to the analyte by an electron beam. This energy causes ionization and fragmentation of the molecule. Ideally, the mass spectra obtained can be used to determine the molecular weight (from the molecular ion) and the structure (from the fragment ions) of the molecule.

Electron impact ionization is still of great importance because of its advantages. Problems arise, however, when thermally sensitive samples are to be examined, because electron impact ion sources are gas phase sources. The sample must therefore be undecomposed in the gaseous state before ionization. This means that the application is restricted to samples that are volatile in a high vacuum. This limitation can be reduced by using direct electron impact ionization (desorption electron ionization, DEI).

Tab. 1
Advantages and disadvantages of electron impact ionization
advantagesdisadvantage
▪ easy to use ▪ high ion currents and thus high sensitivity; hohe high reproducibility; fragmentation enables identification of the analyte; detailed interpretation instructions are available in the literature and extensive spectrum collections are available▪ no molecular ion if fragmentation predominates; therefore no information about the molecular weight of the analyte; restriction to volatile analytes; risk of thermal decomposition