Electron Ionization and Fragmentation Rates

Using conventional electron ionization mass spectrometry, everything occurs in a vacuum such that any collision is highly unlikely. The molecule receives energy from an electron beam and is ionized into a radical cation. The ion that is thus formed is subjected to an electric field that directs it towards the analyser. For instance, suppose a singly charged 100 u mass ion is accelerated by a 1000 V potential difference in the source. This ion has a mass of 

Thus the time spent in the source ranges from 10 6 to 10-7 s, and in the analyser about one or two orders of magnitude longer. 

Let us remember also that the ionization is usually carried out with a 70 eV electron beam, and that 1 eV corresponds to 96.48 kJ mol 1. The ion excess energy can be several electronvolts. In the condensed phase, the cooling of an excited ion or molecule results from the collisions or from the emission of photons. Under high vacuum, only the latter possibility remains. The time delays necessary for cooling by radiation can be measured 

Mass Spectrometry Principles and Applications, Third Edition Edmond de Hoffmann and Vincent Stroobant Copyright 2007, John Wiley Sons Ltd 

In electron ionization, the ionization occurs in a very short time. The energy is redistributed through the vibrations in the ion. After 10 8 s (one-hundredth of a microsecond), the ion deactivates to the ground state if it did not fragment. This occurs well before the ion leaves the source. This explains why electron ionization is reproducible from one spectrometer to another. 

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