PC with Data Acquisition Board Using LabWindows 0 - 10 Torr Baratron
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Quadr ole
Trace Gas Through a 10 sccm Mass Flowme __
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SMass Spec I
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SI1500
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Rotary Pump L/min
e Through a *ss Flowmeter Figure. 11.2. Schematic of complete setup. Computer monitors gas inputs through mass flowmeters and controls the quadupole mass spectrometer. Partial pressures of all species are recorded and data are presented and stored in useful form.
H.2.4 Mass Spectrometer Construction The differentially pumped mass spectrometer system is illustrated in Figure 1.3. The flowtube enters the gap before the first chamber. A 1200 liter/min vacuum pump line perpendicular to the flowtube removes the majority of the flow. The gas enters the first vacuum chamber by a pin hole of 0.1 mm in diameter. The first chamber is evacuated by a Varian diffusion pump capable of pumping enough to maintain pressures of 1.0 x 103 torr or less when the flowtube pressure is one torr. The expanding gas stream hits a skimmer cone at the entrance to the second chamber. The second chamber is pumped by a turbo molecular pump which keeps pressures below 1.0 x 10-6 torr. Thus, the narrow beam of gas that passes into the second chamber forms a molecular beam. In this chamber, the gas is ionized by electron impact and is filtered by the quadrupole mass spectrometer. To reduce any background signal, a chopper chops the molecular beam and the mass spectrometer signal is demodulated with a lock-in amplifier. This reduces the background signal by at least a factor of ten.