6.3.4 Vacuum systems
The ions must be able to pass through the quadrupole filter without colliding with neutral gas particles. Mean path lengths which are attained at pressures $p$ < 10-4 hPa are required for the operation of quadrupole mass spectrometers. This necessitates an appropriate pumping station with pressure monitoring. In order to perform gas analysis with optimal sensitivity, not only a low base pressure is necessary but the residual gas should contain only unavoidable partial pressures stemming from desorption from the walls of the equipment. Residual gas spectra of this type and low base pressures are best attained with turbo drag pumping stations (Figure 4.27). An additional total pressure gauge protects the mass spectrometer against being energized at excessively high pressures. When setting up such a system, attention must be paid to an appropriate arrangement of gas inlet, valves, pumps and measurement instruments in order to avoid falsification stemming from unfavorable flow conditions. A separate pumping station that evacuates the measurement system is often required during the course of vacuum processes that run at high pressure. Small pumping stations with turbo drag pumps and diaphragm pumps are used for this purpose.