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3
CI Troubleshooting
Special NCI notes
Since NCI is so extremely sensitive, air leaks that are not detectable in EI or PCI
can cause sensitivity problems in NCI. To check for this kind of air leak in NCI,
inject OFN. The base peak should be at m/z 272. If the abundance of m/z 238 is
much greater than that of m/z 272, you have an air leak.
How do I find the air leak?
1 See
2 Look for the last seal that was disturbed.
3 Check for tightness of seals at GC inlet and GC/MS interface column nuts.
4 If any of the fittings inside the flow module (VCR fittings) were loosened and
Do not loosen the nuts on any VCR fittings unless you intend to replace the
C AU T I O N
gaskets. Otherwise, you will create an air leak.
5 Remember that most small air leaks visible in CI mode are located in either
6 Half-split the system.
7250 Accurate-Mass Q-TOF Troubleshooting and Maintenance Manual
Figure
6, "Schematic of CI flow control," on page 54 and
module valve state diagram," on page 54.
•
If you just pumped down the MS, press on the side plate to check for
proper seal. Poor alignment between the front analyzer and the GC/MS
interface seal can prevent the side plate from sealing.
•
If you just replaced the reagent gas bottle or gas purifier, check the fittings
you just opened and refastened.
Ferrules for capillary columns often loosen after several heat cycles. Do not
overtighten the interface nut.
then retightened, the gasket must be replaced. These gaskets are good for
one use only.
the carrier gas or reagent gas flow paths. Leaks into the analyzer chamber
are not likely to be seen in CI because of the higher pressure inside the
ionization chamber.
•
Close valves starting at the gas select valves (Reagent gas and Carrier gas
purge), then close the shutoff valve. See
control," on page 54 and
page 54.
•
Cool and vent the MS, remove the GC column, and cap off the interface.
Figure
Table
4, "Flow module valve state diagram," on
Table
4, "Flow
6, "Schematic of CI flow
53
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Troubleshooting
