Operating in Chemical Ionization (CI) Mode 4
Other Reagent Gases
Operation Manual 79
Because many compounds of interest have insufficient proton
affinities, ammonia chemical-ionization spectra often result
from the addition of NH
4
+
and then, in some cases, from the
subsequent loss of water. Ammonia reagent ion spectra have
principal ions at m/z 18, 35, and 52, corresponding to NH
4
+
,
NH
4
(NH
3
)
+
, and NH
4
(NH
3
)
2
+
.
Ammonia tends to break down vacuum pump fluids and seals.
Ammonia CI makes more frequent vacuum system maintenance
necessary. (See the Agilent 7200 Q-TOF GC/MS Troubleshooting
and Maintenance Manual.)
Frequently, a mixture of 5% ammonia and 95% helium or 5%
ammonia and 95% methane is used as a CI reagent gas. This is
enough ammonia to achieve good chemical ionization while
minimizing its negative effects.
Carbon dioxide CI
Carbon dioxide is often used as a reagent gas for CI. It has
obvious advantages of availability and safety.
CAUTION
Use of ammonia affects the maintenance requirements of the MS. See
Chapter 6, “General Maintenance”” for more information.
CAUTION
The pressure of the ammonia supply must be less than 5 psig. Higher
pressures can result in ammonia condensing from a gas to a liquid.
Always keep the ammonia tank in an upright position, below the level
of the flow module. Coil the ammonia supply tubing into several
vertical loops by wrapping the tubing around a can or bottle. This will
help keep any liquid ammonia out of the flow module.
CAUTION
When running ammonia for 5 or more hours a day, the foreline pump
must be ballasted (flushed with air) for at least 1 hour a day to
minimize damage to pump seals. Always purge the MS with methane
after flowing ammonia.
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