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Electron impact ionization

The electron impact ionization [EI] is the classical ionization technique in the mass spectroscopy; at 10-6 - 10-7 torr and up to 200°C the sample is vaporized and bombarded with 70 eV electrons. Molecule radical cations M+. are formed which have on account of the fast electron impacts an excess of inner energy (thermal energy) (M+.*) and tend to disintegrate into neutral particles and daughter ions. The neutral particles as well as the fragment ions can have a radical character and if the daughter ions still have enough excess energy, they can disintegrate into further smaller fragment ions. This can have the result that compounds like e.g. alcohols, esters, etc. fragment so easily, that there can be found no molecule ion M+ in the spectrum, but only by corresponding characteristic disintegrations formed quasi molecule ions, e.g. (M - H2O)+, (M - CH3COOH)+, and the like. Since the temperature of the ion source and the electron energy is kept constant, the number and relative size of the fragment ions and so the electron impact [EI]-spectra are generally comparable and characteristic for each substance. The application of the EI-mass spectrometry is limited to volatile compounds of low molecular weight (up to ca. 1.500 Da) and is also usually used for the identification of unknown samples and for the structure elucidation as it gives us information concerning the chemical structure of the analytes. The classical EI spectra allow for an automatized literature search and interpretation. Air- and moisture-sensitive substances can be analyzed with this technique, too.