Page 186 - Tandem Techniques

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Electrons are formed by thermal emission from a heated tungsten or rhenium filament and accelerated
by an appropriate potential to the anode trap. The magnitude of the potential may range from 5 to 100 V
depending on the electrode geometry and the ionization potential of the substances being ionized. The
filament current is sometimes automatically controlled by the magnitude of the trap anode current to
maintain constant ionizing conditions. The sample is introduced into the gas stream at the center of the
electron beam. The ions formed are repelled by a suitable potential, through a hole in the wall of the ion
source enclosure and thus pass into the accelerating field of the mass spectrometer.

A magnetic field of a few hundred gauss is often maintained along the axis of the electron beam, to
confine the electrons to a narrow helical path. In general only about 0.1% of the molecules entering the
ion source are ionized. The optimum ionization energy of the electron varies with different compounds,
but an average value appears to lie between 50 and 100 eV. The approximate relationship between ion
current and electron energy takes the form shown in Figure 5.4.

Figure 5.4
The Approximate Relationship Between
Ion Current and Electron Energy
It is seen that the ionizing energy of the electrons can be easily controlled by adjusting the accelerating
potential of the electrons in the ion source. This capability is important, because it allows conditions to
be adjusted so