120 5975 Series MSD Operation Manual for MassHunter
4 Operating in Chemical Ionization (CI) Mode
Ammonia tends to break down vacuum pump fluids and seals. Ammonia CI
makes more frequent vacuum system maintenance necessary. (See the 5975
Series MSD 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 MSD. See “CI
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 MSD with methane after flowing ammonia.
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