A potential difference is applied between the filament and the ionizing chamber to attract the electrons leaving the filament
so that they pass through the ionizing region. This is the so-called electron energy, since it is responsible for imparting
sufficient energy to the electrons for them to be able to promote ionization. The magnitude of the electron energy dictates
the number and types of positive ions formed. A typical chemical bond has an energy of 450 kJ/mol, corresponding to about
5 eV, and this corresponds to the ionization potential. At electron energies slightly above the ionization potential of the
target molecule (soft ionization), little molecular fragmentation occurs, but at higher electron energies (hard ionization),
all of the possible molecular fragments are produced. Inspection of the ionization efficiency curves for a number of gases
indicate that the maximal positive ion currents for the molecular ion and its fragments occur at very much the same electron
energy, which is in the range 50–100 V.
If it is decided to reduce the electron energy to create 'softer' ionization conditions, then the draw of the electrons from
the filament through the ionization chamber will be reduced, and the filament current needed to support a given trap current
will increase, as will the source current–to–trap current ratio.
Repeller Voltage
This is the potential difference between the repeller plate and the ionization chamber. As noted previously, the voltage applied
is usually in the range –20 to +20 V, and is set to optimize performance. The term ion repeller is therefore a misnomer, because when it is operated at a negative potential with respect to the ionization chamber it clearly
acts in the opposite manner. As previously mentioned, ions are extracted from the ionization chamber by a weak field due to
a combination of field interpenetration from the half-plates, which lie just outside of it and the repeller plate. Somewhat
counterintuitively, it is often found that a negative potential applied to the repeller plate optimizes the ion yield. However,
the term ion repeller is still in current usage.
When measuring isotope ratios of hydrogen, a relatively large positive voltage is usually chosen to minimize the residence
time of the ionized hydrogen molecules in the source chamber, to lessen the formation of H3+ by molecular collisions.
Ion Energy
Figure 3: An ion beam in a magnetic field: (a) the mass separation increases with increasing mass; that is, the radius of
flight increases with increasing mass; (b) a divergent ion beam is refocused in a magnetic sector.
This is the potential of the ionization chamber with respect to ground (the vacuum system). This voltage (in combination with
the magnetic field) determines the radius of flight of the ions through the magnetic sector. The voltage can be scanned to
detect ions of different m/z, with higher m/z being selected by lower ion energy. Many of the other ion source parameters must be optimized for specific ion energies, and this somewhat limits
the mass range, which can be covered. To achieve the best possible quantitative analysis with a low-resolution sector instrument,
the ion energy should be stable to about 10 ppm.
