TABLE OF CONTENTS An important component of all mass spectrometers, not often considered during operation, are the vacuum pumps. Many instruments use at least two pumps; one to evacuate the instrument chamber of air after assembly or venting and a second to reduce the pressure to that required to operate. Pressures vary within different mass spectrometers, and within each component, but it is typical for instruments to operate over a range of 10 ?3 to 10 ?6 Pa (1 Pa = 133.3226 torr or millimetres of mercury, mmHg). As described in Section 3.1, mass spectrometers operate under vacuum to prevent the collision and reaction of ions with residual gas molecules during their flight from the ion source to the detector. Low pressures also prevent condensation from building up inside the instrument that would coat critical lens and surfaces and create electrical discharges. The detector, too, must be protected from moisture and oxidation for it to operate properly.
The most efficient way in which to reduce the pressure in a mass spectrometer to the 1 Pa level is with a rotary or roughing pump. In this pump, a rotating inner barrel draws the gas in from one side of a chamber and compresses it on the other (Figure 3.28). The compressed gas passes through a hydrocarbon oil reservoir, where the vapour pressure of the oil influences the lowest pressure that can be obtained. Rotary pumps have the capacity to dispel approximately 100...
