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Operating the MSD
2
MSD temperatures are controlled through the data system
The MSD has independent heaters and temperature sensors for the ion
source and quadrupole mass filter. You can adjust the setpoints and view
these temperatures from the data system.
The GC/MSD interface heater is powered and controlled by the Thermal
Aux #2 heated zone of the HP 6890 Series GC. The GC/MSD interface
temperature can be set and monitored from the data system or from the
GC keypad.
Column flow is controlled through the data system
Carrier gas flow through the GC column is controlled by head pressure in
the GC. For a given head pressure, the column flow will decrease as the GC
oven temperature increases. With electronic pneumatic control (EPC) set
to
(constant flow), the same column flow is be maintained regard-
Const Flow
less of oven temperature.
The MSD can be used to measure actual column flow. You inject a
small
amount of air or other unretained chemical, and time how long it takes to
reach the MSD. With this time, you can calculate the column flow.
The data system aids in venting
A program in the data system guides you through the venting process. It
switches off the GC and MSD heaters and the diffusion pump heater or turbo
pump at the correct time. It also lets you monitor temperatures in the MSD
and indicates when to vent the MSD.
The MSD
be damaged by incorrect venting. A diffusion pump will
will
backstream vaporized pump fluid onto the analyzer if the MSD is vented
before the diffusion pump has fully cooled. A turbo pump will be damaged
if it is vented while spinning at more than 50% of its normal operating speed.
34
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