Installation manual

Introduction 1
5975 Series MSD Operation Manual 21
Mass spectrometer automated shutoff
valves closed
Some mass spectrometers are equipped with
automated diffusion pump shutoff valves. In these
instruments, deliberate operator action or various
failures can cause the shutoff valves to close.
Shutoff valve closure does not shut off the flow of
carrier gas. As a result, hydrogen may slowly
accumulate in the mass spectrometer.
Mass spectrometer manual shutoff
valves closed
Some mass spectrometers are equipped with
manual diffusion pump shutoff valves. In these
instruments, the operator can close the shutoff
valves. Closing the shutoff valves does not shut off
the flow of carrier gas. As a result, hydrogen may
slowly accumulate in the mass spectrometer.
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
6890 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 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 4 Hydrogen accumulation mechanisms (continued)
Mechanism Results