HP 5973 MSD Hardware Manual page 66

 CI operation
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C A U T I O N Use of ammonia affects the maintenance requirements of the MSD. See the maintenance
chapter for more information.
C A U T I O N 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. 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.
Ammonia tends to break down vacuum pump fluids and seals. Ammonia CI makes
more frequent vacuum system maintenance necessary.
C A U T I O N When running ammonia for five or more hours a day, the foreline pump must be ballasted
for at least one hour a day to minimize damage to pump seals. See page 96. Always purge
the MSD with methane after flowing ammonia.
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 NCI
Carbon dioxide is often used as a buffer gas for negative CI. It has obvious
advantages of availability and safety.
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