Electron impact is the traditional and possibly the most common method of ionisation.
The sample for analysis is introduced into the ion source, either through a solids inlet or through a gas chromatography column. It is essential that the sample enters the ion source in the gaseous state and the ability to heat the source and solids probe are important for successful sample analysis. A beam of electrons produced by a heated filament of either Tungsten or Rhenium collides with the sample gas molecules, removes an electron and produces a positively charged ion corresponding to the relative molecular mass of the sample being analysed.
Electron impact is an energetic ionisation technique and also produces fragment ions which are smaller parts of the original molecule. The ions are accelerated out of the source and pass through a series of slits to produce a focussed beam. The beam of ions pass through a flight tube which is located between the poles of an electromagnet, and are seperated according to their mass/charge ratio by scanning the magnetic field. The separated ions are detected by electron or photomultipliers to produce a plot of intensity versus mass/charge ratio respresentative of the sample being analysed .ie the mass spectrum.