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Chemical Ionization Options
for more than 20 seconds. After a few seconds, return the valve setting to ½
open or their previous setting (approximately 7 turns).
14. Do not turn on the filament or multiplier for about 2-3 minutes after opening the
CI valves. A convenient way to verify that air and water have been removed
sufficiently is to check the ion gauge pressure with the CI valves open. Verify
that the pressure has returned to less than 35 x 10
-6
Torr before turning on the
filament and multiplier.
Leak Checking
If a leak in the CI plumbing is suspected, use the general leak-checking
procedures found in the Saturn Maintenance and Troubleshooting manual under
the heading
How to check for leaks
. To isolate suspected leaks within the MCI
module, replace the liquid vials or gas lines with empty CI reagent vials as
outlined in this manual titled
Adding Liquid CI Reagents to the MCI Module
.
After carefully opening the needle valves and relays according to the procedure,
allow the air to be pumped from the vials and tubing. Now each Channel can be
tested individually by selecting it using the GC relays and with the CI valve open,
carefully spraying compressed argon around the fittings while monitoring the
spectrum for m/z 40, or you may use tetra-fluoroethane (Dust-Off
®
) and monitor
the spectrum for m/z 69. A leak at one of the O-ring seals may be fixed by
cleaning the tubing and O-ring, but usually a new O-ring is required. Extras are
provided with the MCI module and also in the accessory kit.
Cross-Contamination Effects
Vapor from liquid CI reagents is absorbed to some extent in the O-ring seals of
each Channel used for liquid CI. If the liquid or gas chosen for the Channel is
changed, the residual vapor from the previous reagent may create artifact CI
reagent ions observable when adjusting the CI gas flow. If these artifact ions
comprise more than 10-20% of the total reagent ions observed during CI flow
adjustment, they may affect the CI spectra of the analytes. For example, residual
soft reagent ions from acetonitrile at m/z 42 and 54 may be observed if one
switches the reagent to methanol in a given Channel. These softer reagent ions
may lead to an increased proportion of M+1 ions in the methanol CI spectrum
compared to the ratios observed without the artifact acetonitrile reagent ions
present.
Absorbed solvent in Channel seals may be reduced by prolonged vacuum
pumping of the Channel:
1. Replace the liquid CI vial or gas line in the Channel with an empty CI reagent
vial.
2. Open the Channel from the GC keyboard. See the instructions in
Selecting
Reagent Channels from the 3800 GC Keyboard
.
3. Do not turn on the CI icon in the Saturn software.
4. Open the Needle Valve for the Channel full counterclockwise so that all flow from
the Channel is directed to the Foreline Pump.
5. Leave the Channel open overnight or over a weekend.
6. Replace the desired reagent in the Channel position and observe reagent ions
during CI gas adjustment.
