Why did the multiplier autotune voltage suddenly jump by several hundred volts after several months of stable operation?
This is the typical symptom of a vacuum accident, where the multiplier voltage was left on during venting or where high pressure in the chamber has caused pump oil to backstream. This can also be caused by a power failure or instrument malfunction. Labs where there are many different users, such as teaching labs, are prone to this problem (due to a less-experienced operator). If the problem resulted from pump oil backstreaming onto the multiplier while no voltage was applied to the multiplier (and it was not used afterwards), then the multiplier can be cleaned in an Ultrasonic bath using a non-polar solvent (such as heptane – see care and handling brochure BR-0061-A for details). This may restore performance, otherwise a new multiplier must be purchased. This situation is the same for all types of multipliers and is a fault in the mass spectrometer and not in the electron multiplier.