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that optimum fragmentation takes place that will provide the maximum information with respect to the
structure of the compound being examined. An example of the effect of electron energy on the
fragmentation pattern of a compound is shown in Figure 5. 5. It is seen from the pattern obtained at low
energies (ca 14 eV) that a large parent ion is produced but relatively few fragments. This means that the
molecular weight of the material could be fairly easily identified but its structure would not be easy to
determine as there were very few fragments to work with.

Figure 5.5
Fragmentation Patterns for a Molecule Ionized
with Electrons Having Different Energies
In contrast, at high electron energies (ca 60 eV) there are a large number of fragments, particularly at
low molecular weight, but the parent ion is hardly discernible. This means that although some of the
secondary structure of the molecule may be revealed, the lack of a definite parent ion would again make
the total molecular structure difficult to identify. However, at mid-electron energies (ca 20 eV)
numerous fragments together with an unambiguous parent ion are produced, providing ample
information for structural identification. It is seen that the electron energy in electron impact ionization
is an important parameter on which to