6. Push the screening sleeves over the feedthroughs and tighten their screws.
– The screening sleeves have to slide into the countersink in the flange.
7. Fit the protective pipe again.
7.4 Baking out analyzer
Prerequisites
● Connector plate removed for bakeout temperatures >200 °C.
● EP 422, CP 400 and HF generator removed from analyzer prior to bakeout
● EP 422 and HF generator outside bakeout zone
Procedure
► Bake out the analyzer up to max. 400 °C.
► Operate the SEM only with 1000 V at a bakeout temperature of >150 °C.
► Let the flange cool to <50 °C before you mount the EP 422, CP 400 and HF generator again.
7.5 Assessing sensitivity
Pfeiffer Vacuum determines the sensitivity with and without SEM in the factory. The test log included in
the delivery of the product contains the values that were determined. The sensitivity is specified in
A/hPa for a reference gas. It is recommended to use N
2
or air to check the sensitivity.
Procedure
1. For air, use 80% of the total pressure as N
2
pressure and add the ion currents of masses 14 and
28.
2. Subtract the corresponding residual gas peak heights, if they are not negligible.
3. Make sure the you have measured the reference pressure correctly, e.g. with an ionization gauge
installed at an appropriate place (please note the gas type dependence).
7.6 Secondary electron multiplier (SEM)
Setting amplification
► With the "
SEM Voltage
" parameter for the SEM high voltage, set the amplification, and thus the
sensitivity, using very large thresholds.
► Avoid both values below 1 kV and ion currents above >1 μA for a longer period (> several mi-
nutes), because the amplification is not stable in these ranges.
Avoiding contamination
► When working with an unfavorable gas composition (hydrocarbons and other organic vapors), use
as low a current as possible.
► Work in Faraday mode if this is expedient.
Low partial pressures
At very low partial pressures (very small peaks), the ion current consists of individual impulses. At a
very high SEM amplification, these pulses can overmodulate the input stage of the electrometer pre-
amplifier and thus cause measurement errors (e.g. non-linearity) which are not obvious. Considerable
deviations (>10%) of the values measured within various electrometer ranges, discontinuities of the
measured value curves in autorange mode, flattened peaks, incorrect isotope ratios, etc. may be due to
this effect. In such cases, reduce the "
SEM Voltage
", select a less sensitive "
Detector Range
" or use
"
AUTO DOWN
" um to lock the most sensitive measuring ranges. With the ion counter, you can avoid
this problem.
Amplification factor
Determining amplification factor
► Record the range of interest of the mass spectrum in SEM operation and in Faraday operation.
The ratio of the currents of two corresponding peaks is the amplification that applies for the operating
conditions entered. This method incorporates the influence of the 90° ion deflection.
Operation
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