7.9.6 Configuring "V2 Cathode" ion source parameter
NOTICE
Overloading of the filament
At a reduced ionization energy setting ("
Cathode
" e.g. 40 eV), the filament temperature required for
the emission increases. The filament burns out.
► In this case, reduces the emission to 0.1 mA, for example.
► Calibrate "
Protection Current
" (filament protection).
The cathode voltage determines the acceleration voltage of the electrons and thus the nominal ioniza-
tion energy. The effective ionization energy deviates slightly from that value, for instance, due to the ex-
traction field. Calibration measurements are required for applications for which the exact ionization en-
ergy has to be known. The reference data in spectra libraries are usually referenced to 70 eV. At lower
electron energies (e.g. 40 V), fewer double charged ions are formed by the ion source. This prevents for
instance contribution of 36Ar++ to mass 18, which would complicate the detection of water vapor traces
in argon.
7.9.7 Configuring "V3 Focus" ion source parameter
Procedure
1. Adjust "
Focus
" to maximum peak height.
2. If several maximums occur, select the one with the lowest voltage value, but observe the section
applicable for your ion source.
3. Also optimize the "
Extraction
" parameter (if present) when changing the "
Focus
".
7.9.8 Configuring "V4 Field Axis" ion source parameter
The field axis voltage is the potential difference between the ionization area (Ion Reference) and the
quadrupole mass filter. The field axis voltage therefore defines the kinetic energy (velocity) of the ions in
the rod system. The optimum value for "
Field Axis
" depends, among other factors, on the frequency
(QMH type) and the QMA type. Lower frequencies (higher mass ranges) or shorter rod systems require
lower ion energies because the ions must stay in the mass filter longer to be resolved. The higher the
value, the greater the peak height. However, the resolution is lower and the peak shape potentially de-
teriorated. Excessively high "Field Axis" values result in "frayed" peaks.
Fig. 21:
Frayed peaks
Fig. 22:
Reducing "Field Axis"
Reducing "Field Axis"
► Reduce the "
Field Axis
" value if, instead of improving the resolution, adjusting the "
Resolution
"
parameter only causes the peak height to decrease.
The aim of the mutual optimization of "
Field Axis
" and "
Resolution
" is to achieve the maximum peak
level with the desired resolution and a sufficiently good peak shape. If a bad peak shape cannot be im-
Operation
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