Electronic Impact (EI)

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Pechar

The ionisation chamber is kept at low pressure to minimise ion-molecule collisions. It is a so-called ‘hard’ ionisation technique, as gas-phase samples require high voids and high temperatures at the ionisation source, receiving a high energy that makes it fragment. Thanks to this fragmentation, some characteristics of the molecule can be deduced from the fragmentation pattern of the molecular ion.

One factor to take into account in this technique is the energy of the electrons, which is measured in electron volts (eV). One eV is the energy gained by an electron (equivalent to 23 kcal/mol) in the course of an electric field maintained by a potential difference. Most of the reference spectra in this technique are obtained at this eV value, because at this value the perturbations in electron energy have negligible effects on ion production, and also because the fragmentation patterns are reproducible.

This technique is especially used for apolar compounds with high volatility, low molecular weight and high thermal stability.