MCP gain signal does not plateau during MCP gain setup on a Xevo QTof - WKB27507

SYMPTOMS

• Micro Channel Plate (MCP) gain signal does not plateau during the MCP gain setup procedure; the signal increases up to the detector maximum output of 2500 V
• Beam is weak and unresolved on the m/z 556.28 ion (leucine-enkephalin solution infused)

ENVIRONMENT

• Xevo QTof
• MassLynx 4.1

CAUSE

The signal threshold was not optimized.

FIX or WORKAROUND

• During the MCP gain setup, extract only the BPI profile on mainly the m/z 556 isotope (do not include other isotopes), as noted on page 9 of tech ref TOF Optics. Check again if the MCP gain plateaus or not.
• There is no "TDC setup" tech ref document, but the following procedure can be followed before proceeding to the MCP gain setup.
  According to the installation manual (rev F, page 34), the signal threshold should be between 35 and 120 mV (usually around 70 mV).

*********  Signal Threshold setup  *********
The aim is to find a minimum signal threshold value:

1. Open the acquisition settings and set Signal Threshold to 35 mV.
2. Set Detector voltage to 1800 V.
3. Set the capillary to 0 and switch off all gases.
4. Start a TOF MS acquisition in continuum mode from mass 100 to 1000 Da, 1 sec scan speed, and 0.1 sec ISD.
5. Open the chromatogram.
6. If necessary, reduce the Signal Threshold until significant noise appears (TIC >1e3 counts).
7. Increase the Signal Threshold in 10-mV steps until the noise becomes grass (TIC <200 counts).
8. Increase the Detector voltage to 1900 V and ensure that the noise has a TIC of <200 counts (minimum Signal Threshold value).
9. Using this minimum Signal Threshold value, run the MCP Gain test.
10. If the noise is high, increase the Signal Threshold in steps of 10 mV until the noise reaches an acceptable level.
11. Ensure that the MCP Gain test is still successful with the new Signal Threshold value.

MCP gain setup can be found in the following documents:

• Xevo QTof installation manual (715001949rev F), page 34
• Tof Optics technical reference document, page 16
   

ADDITIONAL INFORMATION

Readjusting the signal threshold value and rerunning the MCP gain setup as described fixed the issue.