Quadrupole Mass Spectrometers are one of the most widely used methods of Residual Gas Analysis used in vacuum technology. There is a very good description of how RGAs work and their anatomies the book, Understanding Modern Vacuum Technology. This animation is helpful to round out one’s understanding of how the gases are ionized and passed through the analyzer.

Gas is ionized, extracted from the ionization chamber and directed down the mass filter to a detector. The QMS consists of four parallel rods arranged in the form of a square. Each opposing rod pair is connected together electrically and a radio frequency (RF) voltage with a DC offset voltage is applied between one pair of rods and the other. Ions travel down the quadrupole between the rods. Only ions of a certain mass-to-charge ratio will reach the detector for a given ratio of voltages:

Other ions have unstable trajectories and will collide with the rods. This permits selection of an ion with a particular M/z or allows the operator to scan for a range of M/z-values by continuously varying the applied voltage.

The Pfeiffer Quadrupole Mass Spectrometer video shows the two common ion detection schemes used, the Faraday cup and the electron multiplier. The Faraday cup is more robust and can operate at higher pressures while the electron multiplier is used for more UHV work as they amplify the ion signal in the device.

Understanding Modern Vacuum Technology, 2nd Ed discusses pressure measurement in detail and covers the history of partial pressure measurement and alternative methods used for residual gas analysis.