41 300781172_002_C0 - 01/2019 - © Leybold
Cautions in operation
Fig. 9.2 Mass number calibration screen
9.5.2 SEM sensitivity calibration
The sensitivity of the mass spectrometer is determined factors such as the intrinsic
sensitivity of the ion source, transmittance of the quadrupole, and applied voltage to
the secondary electron multiplier (SEM). But the sensitivity of the SEM may decrease
due to the usage history of the analyzer tube. In order to maintain sensitivity, you can
calibrate by adjusting the voltage applied to the SEM.
You will need:
A pumping system with exhaust performance that can be used with the
analyzer
A vacuum gauge for measuring pressure
Gas for maintaining a constant pressure (N
2
and general gas such as Argon)
A pressure adjustment mechanism (e.g. variable leak valve, mass flow
controller)
In the dedicated gas analysis software, use the "calibration mode for sensitivity
calibration mode" recipe.
For example, a recipe which tunes the SEM voltage to measure an ion current of 2.5
x 10
-6
A when N
2
gas is introduced up to 5.0 x 10
-4
Pa would give a sensitivity of
5.0 x 10
-3
A/Pa (=2.5 x 10
-6
A / 5 x 10
-4
Pa).
The Residual Gas Analyzer/process gas monitor measure ion currents in the order
of 0.1
μA to 0.1 pA. If the insulation resistance between the grid electrode (150 V)
and the ion collector electrode (0 V) is 200 M
Ω, the insulation may be held
acceptable in normal sense, but in a micro-current measurement such as by an
ionization gauge, a leakage current of 0.75
μA would flow from the grid electrode.
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