Thermo Finnigan LTQ - Replacing the Electron Multiplier

MS Detector Maintenance
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____________________________________________________Replacing the Electron Multiplier
_______________________ Finnigan LTQ Hardware Manual _____________________
4-39
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4.7 Replacing the Electron Multiplier
The electron multiplier of the ion detection system includes an anode and a
cathode. The anode and cathode have finite lifetimes. The anode loses
sensitivity over time due to contamination of its surface. Things that decrease
the lifetime of the cathode are: heat; electron flow (which produces internal
heat); air (which causes oxidation and arcing); and water (which causes
arcing).
The following symptoms suggest that the electron multiplier may need
replacing:
• Excessive noise in the mass spectrum
• Inability of the multiplier gain calibration procedure to achieve a gain of
3
× 10
5
electrons per ion with an electron multiplier voltage less than or
equal to 2.5 kV
You can read the current value of the electron multiplier voltage in the Ion
Detection System dialog box, which can be reached from the Tune Plus
window by choosing Setup > Ion Detection System.
If you are having problems with the ion detection system, you need to replace
the electron multiplier assembly.
To replace the electron multiplier assembly, proceed as follows:
1. Shut down and vent the system as described in the topic Shutting the
System Down Completely in the System Shutdown, Startup, and
Reset chapter.
2. Remove the top cover of the MS detector as described in the topic
Removing the Top Cover of the MS Detector on page 4-28.
3. Remove the top cover plate of the vacuum manifold as described in the
topic Removing the Top Cover Plate of the Vacuum Manifold on
page 4-29.
4. With an Allen wrench, remove the two socket-head screws that hold the
electron multiplier support to the top cover plate of the vacuum manifold.
See Figure 4-17.
5. With one hand hold the high voltage tube and with the other hand hold the
electron multiplier support. Then, detach the high voltage tube from the
CAUTION. Make sure that the LTQ power cord is unplugged before you
proceed.
Note. Wear clean, lint-free, nylon or cotton gloves when you handle the
electron multiplier components.