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3 Troubleshooting the MSD
Mass spectral symptoms
Mass assignments are incorrect
Small shape changes at the top of the mass peaks can cause 0.1 amu shifts in mass
assignments. Shifts greater than 0.2 amu indicate a possible malfunction.
• MSD has not had enough time to reach thermal equilibrium
• Large variations in the temperature of the laboratory
• MSD has not been tuned recently, or at the temperature at which it is operating
• Incorrect tune file (inappropriate parameters)
Peaks have precursors
The tune report lists the size of the precursors for the tune masses. Small precur-
sors are not unusual. There are no specifications for acceptable or unacceptable
precursor size. If the precursors are unacceptably large for your application, one
of the following may be responsible:
• Repeller voltage is too high
• Peaks are too wide
• Incorrect dc polarity on the quadrupole mass filter
• Dirty quadrupole mass filter
Peak widths are inconsistent
• MSD has not had enough time to reach thermal equilibrium
• Large variations in the temperature of the laboratory
• Incorrect tuning
• Calibration vial is empty or almost empty
• Calibration valve is not working correctly
• Dirty ion source
• Electron multiplier is nearing the end of it useful lifetime
• Ground loop
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