Agilent Technologies 5977B Series Operating Manual page 25

Table 4 Hydrogen accumulation mechanisms
Mechanism
Mass spectrometer turned off
MS automated shutoff valves
closed
Mass spectrometer automated
shutoff valves closed
Mass spectrometer manual
shutoff valves closed
GC off
Power failure
Once hydrogen has accumulated in a mass spectrometer, extreme caution must be
WA R N I N G
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
WA R N I N G
process by itself. This does not guarantee that all hydrogen has been removed from
the system, or that the explosion hazard has been removed.
5977B Series MSD Operating Manual
Results
A mass spectrometer can be shut down deliberately. It can also be shut down
accidentally by an internal or external failure. There is a safety feature that will shut
down the flow of carrier gas in the event of an MSD foreline pump shut down. However,
if this feature fails, hydrogen may slowly accumulate in the mass spectrometer.
The mass spectrometers are equipped with automated shutoff valves for the calibration
vial, optional JetClean system, and the reagent gases. 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 MS.
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.
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.
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 7890 Series GC equipped
with Electronic Pressure Control (EPC) is shut off, the EPC stops the flow of carrier gas.
If a GC's 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.
If the power fails, both the GC and mass spectrometer shut down. The carrier gas,
however, is not necessarily shut down. 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.
Introduction 1
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