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Electron Ionization Mass Spectrometry

Mass Spectrometry

  1. J. Throck Watson

Published Online: 15 SEP 2006

DOI: 10.1002/9780470027318.a6006

Encyclopedia of Analytical Chemistry

Encyclopedia of Analytical Chemistry

How to Cite

Watson, J. T. 2006. Electron Ionization Mass Spectrometry. Encyclopedia of Analytical Chemistry. .

Author Information

  1. Michigan State University, East Lansing, USA

Publication History

  1. Published Online: 15 SEP 2006


Electron ionization (EI) is the basis for one of the most effective mass spectrometric means for identification of a given organic compound. The identification is based on an indication of the molecular weight (MW) coupled with a fragmentation pattern that serves as a chemical fingerprint for the compound. Of the many ionization techniques available in mass spectrometry, EI is one of the most harsh in terms of promoting fragmentation of the molecular ion. The analyst can take advantage of this extensive fragmentation by using some of the fundamental principles of organic mechanistic chemistry and physical chemistry for the purposes of rationalizing the fragmentation pathways that characterize various structural features in an organic molecule. Four basic mechanisms of fragmentation are considered in this chapter; most of them operate on an odd-electron ion, although some pathways will be applicable to even-electron ions as well. The multiplicity of peaks that represent a given ion allows one to estimate the elemental composition of an ion. This information on composition, coupled with a rationalization of a given fragmentation pathway, often enables the structure of a given organic compound to be deduced.1–3