Gas Chromatography - Tandem Techniques - Gas Chromatography Mass Spectroscopy (GC-MS) Systems > Ion Generation > Electron Impact Ionization > Page 65


Figure 53. The Electron Impact Ion Source

Electron Impact Ionization


Electron impact ionization source has already been briefly discussed but will now be dealt with in more detail. Electron impact ionization is a relatively hard ionizing process, and, as a consequence, most of the parent molecule is often broken up producing a variety of fragments with a relatively small amount of the parent ion. In some circumstances, if the molecule is sufficiently labile, no parent ion may be produced at all.


Figure 53. The Electron Impact Ion Source



A diagram of an electron impact ion source is shown in figure 53. Electrons, formed by thermal emission from a heated tungsten or rhenium filament are accelerated by an appropriate potential (e.g.,5-100 v) to an anode trap. The magnitude of the filament current can be controlled automatically to maintain constant ionizing conditions. The sample is introduced at the center of the electron beam. The ions formed are repelled through a hole in the wall of the ion source enclosure by a suitable potential and then enter the accelerating field of the mass spectrometer.


The electrons are usually confined to a narrow helical path by a magnetic field of a few hundred gauss directed along the axis of the electron beam. About 0.1% of the molecules entering the ion source are usually ionized. The electron energy that will provide optimum ionization varies between different compounds, but an average value appears to fall within the range of 50 and 100 eV. The approximate relationship between ion current and electron energy takes the form shown in figure 54.