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Functional Description
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___________________________________________________________________ MS Detector
_______________________ Finnigan LTQ Hardware Manual _____________________
2-31
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Damping Gas Inlet Assembly
The damping gas inlet assembly controls the flow of helium into the mass
analyzer cavity. Helium (40
±
10 psig [275
±
70 kPa], 99.999% [ultra-high]
purity) enters the MS detector through a 1/8-in. port on the rear of the MS
detector. LTQ regulates the flow of helium by use of a capillary restrictor and
a pressure regulator on the helium line. The helium enters the mass analyzer
through a nipple on the exit endcap electrode.
Helium in the mass analyzer cavity dampens ionic motion and improves the
performance of the MS detector. Refer to the topic Helium Damping Gas in
the Mass Analyzer Cavity on page 2-19.
Sheath Gas, Auxiliary Gas, and Sweep Gas Valves
The sheath gas, auxiliary gas, and sweep gas valves control the flow of
nitrogen into the API source. The sheath gas is the inner coaxial nitrogen gas
of the API probe that sprays (nebulizes) the sample solution into a fine mist as
it exits the sample tube. The auxiliary gas is the outer coaxial nitrogen gas
that assists the sheath gas in the nebulization and evaporation of sample
solutions. The Sweep gas flows out from behind the sweep cone in the ion
source interface. Sweep gas aids in solvent declustering and adduct reduction.
Dry nitrogen (100
±
20 psig [690
±
140 kPa], 99% purity) enters the MS
detector through a 1/4-in. port in the rear of the MS detector. The nitrogen
pressure is regulated by valves that are controlled by the data system. You can
set the flow rates from the Tune Plus window. Sheath gas is not used with an
NSI source. The sheath gas, auxilary gas, and sweep gas enter the API source
through 1/8-in. ID tubing.
Cooling Fans
Five fans provide cooling for the MS detector. One 21 ft.
3
/min. fan cools the
RF voltage coil. One 21 ft.
3
/min. fan cools the turbomolecular pump. Three
100 ft.
3
/min. fans cool the electronics in the tower. Air is drawn in from the
rear of the MS detector. The exhaust air is expelled from the vent slots on the
sides of the MS detector.
Caution. To ensure proper cooling, the MS detector must always be
operated with its covers in place.