7.9.2 Optimization with test gas
Procedure
1. Let a suitable gas with a pressure of
5 × 10
-6
hPa
flow in (for special ion sources).
– To optimize for higher masses, corresponding components have to be included, else air is ad-
equate.
2. If you cannot let in gas, perform the optimization with an appropriate residual gas peak.
– Pay attention to outgassing changes that occur as a result of parameter changes, e.g. regard-
ing H
2
O.
Frequently, to improve the sensitivity, inlet procedures are used in which the pressure in the ion source
is higher than in the environment. In these cases, determining the sensitivity in A/hPa does not make
sense.
7.9.3 Configuring "Emission" ion source parameter
"
Current
": The typical emission current is 1 mA. For yttriated cathodes, this is the maximum value. In
some cases (e.g. grid ion source), the sensitivity is higher with 2 mA. However, sometimes, the maxi-
mum sensitivity is reached at lower emission settings, e.g. for crossbeam ion sources with electron colli-
mation magnets. This is caused by space charge effects. Observe the detailed information for your ion
source type. Recommendation: If you change the "
Emission Current
" value, also optimize the "
Protec-
tion Current
" value.
Working with low electron energy and high pressures
► At a low electron energy setting ("
Cathode
" e.g. <50 V) set "
Emission Current
" to 0.1 – 0.2 mA
or lower, to avoid filament overload.
► At pressures >10
-5
hPa, for example, reduce the "
Emission Current
" value to 0.2 mA, to improve
the linearity of the measurement (ion current versus partial pressure).
7.9.4 Configuring "Protection Current" ion source parameter
If the pressure rises in the ion source, the heating current of the filament increases. This effect is used
to turn the cathode off when the pressure rises. "
Protection Current
" determines the switch-off thresh-
old. If it is not possible to turn on the emission, the reason is usually that the "
Protection Current
" set-
ting is too low.
Procedure
► To achieve optimum protection, set the switch-off threshold as low as possible.
– The setting is optimal if you are just barely able to switch on the emission without triggering
the protection circuit (filament protection).
7.9.5 Configuring "V1 Ion Reference" ion source parameter
"
Ion Reference
" is the nominal potential on which the ion source forms the ions. The effective potential
is somewhat lower because of the penetration coefficient of the extraction field and the electron space
charge. "
Ion Reference
" is the reference potential for all other potentials (see the technical data). "
Ion
Reference
" is the highest positive voltage. This causes virtually all negative particles (mainly electrons)
to travel to the ionization area. Desorption induced by electron impact can thus only take place there, if
at all. Interference due to nearby ionization gauges is also effectively prevented. In general, the "
Ion
Reference
" value should be set slightly higher (approx. 20 V) than the electron energy ("
V2 Cathode
").
The cathode is thus on a positive potential with regard to ground, so that the ion source does not emit
any electrons to the environment. This prevents interference with the Faraday cup of the system and
nearby measurement instruments (e.g. ionization gauges). Moreover, gases adsorbed in the environ-
ment could be emitted through electron impact, which could influence the measurement.
The following effects of the "
Ion Reference
" setting are also influenced by the mechanic tolerances,
e.g. by the exact cathode position:
● At low values (25 – 40 V), the sensitivity for lower masses is higher. For higher masses however,
the maximum sensitivity is at higher values.
● The higher the "
Ion Reference
" setting, the lower the mass discrimination, i.e. the sensitivity de-
creases with as the mass numbers increases.
● These relationships become even clearer with higher mass ranges and smaller filter dimensions.
● If you would like to minimize the mass discrimination, select a peak with the highest possible mass
for optimizing the ion source parameters.
Operation
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