Fragmentation Patterns

Fragmentation Patterns

Electron Impact sources introduce a vapor into a beam of electrons. The electron transfers energy to the vaporized molecule; this energy can often result in fragmentation of the molecule. As a result, less (and sometimes none) of the molecular ion is seen on the mass spectrum. The energy added to the molecule can be controlled by the ionization energy setting, though in many instruments this is fixed at 70 eV. The fragmentation pattern which results is reproducible, allowing these patterns to be used as an identifying 'fingerprint'. For small numbers of unknown compounds, these patterns can be visually matched, though libraries of patterns can be searched by computer for more complex problems, with the best matches being returned by the software. Automatic searching is never better than the library used, however, and chemists must use caution in interpreting the results of library-matches.