Introduction 1
Troubleshooting and Maintenance Manual 17
GC off A GC can be shut down deliberately. It can also be
shut down accidentally by an internal or external
failure. Different GCs react in different ways. If a
7890A GC equipped with Electronic Pressure
Control (EPC) is shut off, the EPC stops the flow of
carrier gas. If the carrier flow is not under EPC
control, the flow increases to its maximum. This
flow may be more than some mass spectrometers
can pump away, resulting in the accumulation of
hydrogen in the mass spectrometer. If the mass
spectrometer is shut off at the same time, the
accumulation can be fairly rapid.
Power failure If the power fails, both the GC and mass
spectrometer shut down. The flow of carrier gas,
however, is not necessarily shut down. As
described previously, in some GCs a power failure
may cause the carrier gas flow to be set to
maximum. As a result, hydrogen may accumulate in
the mass spectrometer.
Table 2 Hydrogen accumulation mechanisms (continued)
Mechanism Results
Once hydrogen has accumulated in a mass spectrometer, extreme caution must be
used when removing it. Incorrect startup of a mass spectrometer filled with
hydrogen can cause an explosion.
After a power failure, the mass spectrometer may start up and begin the pumpdown
process by itself. This does not guarantee that all hydrogen has been removed from
the system or that the explosion hazard has been removed.
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