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Electron ionization

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Title: Electron ionization  
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Electron ionization

Electron ionization (EI, formerly known as electron impact) is an organic molecules.


  • Principle of operation 1
  • Notes 2
  • References 3
  • External links 4

Principle of operation

Diagram representing an electron ionization ion source

The following gas phase reaction describes the electron ionization process:[2]

M + e^- \to M^{+\bullet} + 2e^-,

where M is the analyte molecule being ionized, e is the electron and M+• is the resulting ion.

In an EI ion source, electrons are produced through thermionic emission by heating a wire filament that has electric current running through it. The electrons are accelerated to 70 eV in the region between the filament and the entrance to the ion source block. The accelerated electrons are then concentrated into a beam by being attracted to the trap electrode. The sample under investigation which contains the neutral molecules is introduced to the ion source in a perpendicular direction to the electron beam. Close passage of highly energetic electrons, referred to as a hard ionization source, causes large fluctuations in the electric field around the neutral molecules and induces ionization and fragmentation.[3] The radical cation products are then directed towards the mass analyzer by a repeller electrode. The ionization process often follows predictable cleavage reactions that give rise to fragment ions which, following detection and signal processing, convey structural information about the analyte.

Diagram of Electron Ionization. The proportion of products depend on the energy of the electrons.



  1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "electron ionization".
  2. ^ R. Davis, M. Frearson, (1987). Mass Spectrometry – Analytical Chemistry by Open Learning, John Wiley & Sons, London.
  3. ^ K. Robinson et al. Undergraduate Instrumental Analysis, 6th ed. Marcel Drekker, New York, 2005.


  • Edmond de Hoffman; Vincent Stroobant (2001). Mass Spectrometry: Principles and Applications (2nd ed. ed.). John Wiley and Sons.  
  • Stephen J. Schrader (2001). Interpretation of Electron Ionization Data: The Odd Book. Not Avail.  
  • Peterkops, Raimonds (1977). Theory of ionization of atoms by electron impact. Boulder, Colo: Colorado Associated University Press.  
  • Electron impact ionization. Berlin: Springer-Verlag. 1985.  

External links

  • NIST Chemistry WebBook
  • Mass Spectrometry. Michigan State University.
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