For rest gas analysis and leak testing of a UHV system, a so-called quadrupole mass spectrometer is frequently used. Its operating principle is shown below

In the first section of the spectrometer, the rest gas is ionised, just as in an ion gauge. The ions are then accelerated and focused into the second section, the actual mass filter. It consists of four bars which set up an electrical quadrupole field, driven by the superposition of a DC and a radio frequency AC voltage. The filter works such that only ions with the same q/m ratio can pass it. The last element of the spectrometer is the ion detector which contains an electron multiplier or a Faraday cup. The lower limit of partial pressures which can be detected with such an instrument is about 10-14 mbar.

A typical mass spectrum is shown in the figure below. The main residual gases in this vacuum system are hydrogen, water, carbon monoxide and some carbon dioxide. It is important to note that the criterion for some rest gas molecules to pass the mass spectrometer is the right q/m and not simply m!  This means means that doubly charged ions appear as particles with half the mass in the spectrum. CO does for example not only give a peak at m=28 but also one at m=14 due to double ionization. The gases are also dissociated in the spectrometer such that one does not only find one peak for single-ionized water at m=18 but also peaks at m=16, 17 and 2 for the fragments.

Without any bakeout, the pressure of the system will be dominated by a high partial pressure of water. After the bakeout of the system, this will be strongly reduced and the total pressure will be determined by CO and  hydrogen. If there is an air leak in the system this would show up as peaks of 28 (N2) and 32 (O2). The trouble is that a peak at 28 is always present due to CO and a small peak of 32 might even be present in a leak-tight chamber. However, more conclusions can be drawn from the cracked fragments: if the peak at 14 (N) is bigger than the peak at 12 (C), this is usually an indication of an air leak.