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Chemical Ionization Options
Checking the Reagent Gas Plumbing for Leaks
To check for air leaks in the reagent gas line connections and the presence of
water vapor in the gas line, follow the procedure using a Leak Detection Gas to
Troubleshoot for Air Leaks in the Troubleshooting Section. Depending upon the
results you obtain, you may need to modify the procedure as follows:
If a large air leak exists, check the CI GAS fitting on the rear of the instrument
and the fitting on the pressure regulator for tightness. Then recheck the air/water
spectrum; or
If excess water vapor is indicated by a high 19/18 ratio, there may be water in the
gas line and/or an atmospheric air leak in the reagent gas plumbing. Proceed as
follows:
1. Shut off the flow of reagent gas into the manifold by closing the CI solenoid
valves. If necessary, click on the CI icon in the Control and Status field of the
Manual Control tab dialog in System Control. When the valves are closed,
the CI button is black or red - not green.
2. Recheck the air/water spectrum. If the peak at mass 19 (for water)
decreases, then water is present in the gas line. In this case, go to step 3. If
the peak at mass 19 does not decrease significantly, little water is present in
the gas line. In this case, the MS system probably has an air leak. You will
need to fix the leak as described in the Troubleshooting Section. Be sure to
check for leaks around:
•
The CI GAS port on the rear of the mass spectrometer
•
The fitting that connects the reagent gas line to the pressure regulator
3. To flush excess water from the gas line proceed as follows:
a. Ensure that the electron multiplier, filament, and RF voltage are off.
b. Open the main valve on the lecture bottle. (The secondary valve on the
pressure regulator is already open.)
c. Turn the CI needle valve fully counterclockwise.
d. Open the CI gas solenoid valves and allow the system to pump down for
about 1 hour.
e. Close the main valve on the gas cylinder but keep the CI GAS solenoid
valves open. Allow the system to pump down for about 15 minutes.
f. Recheck the air/water spectrum. If excess water is not present, go to
paragraph: Setting Delivery Pressure of the CI Reagent Gas.
Setting Flows of CI Reagents
After any leaks have been located and fixed, set the delivery pressure of the CI
reagent gas as follows:
1. Ensure that the CI gas solenoid valves are closed. If necessary, click on the
CI icon in the Control and Status field of the Manual Control tab dialog in
System Control. When the valves are closed, the CI button is black or red -
not green.
2. Open the main valve on the lecture bottle. Using the pressure adjustment
valve on the regulator set the head pressure to about 5 psi (34 kPa).
