6
Operating in CI Mode
Carbon Dioxide CI
134
7000/7010 Series TQ GC/MS Operating Manual
Ammonia CI
Ammonia (NH
3
) is commonly used for CI when less fragmentation is desired in
the CI spectrum. This is because the proton affinity of ammonia is higher than
that of methane; hence less energy is transferred in the ionization reaction.
Because many compounds of interest have insufficient proton affinities,
ammonia CI 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. Refer to the
7000/7010 Series Triple Quad 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
CI 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
, “General Maintenance,” starting on page 147
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.