Residual Gas Analysis and Partial Pressure Measurement

Measurement of total pressure is insufficient for the characterisation of vacuum systems. Detailed information (system cleanliness, presence of leaks, etc.) can only be obtained by the identification of components present in the gas phase by means of residual gas analysis. Having established gas composition, however, the determination of partial pressures is relatively straightforward. 

An ion beam extracted from the ion source passes to the mass separation system where continuous changes in the voltages applied to the electrodes allow ions of the appropriate mass/charge (M/Z) ratio to achieve stability in the mass filter and pass to the detector. A spectrum is therefore generated giving the variation of the amplitude of the 

As with the hot cathode ionisation gauge, there is a relationship between I+ and gas pressure (see Equation 5.11) so the RGA sensitivity (Srga) can be expressed as  

The ionisation process does not reproduce, in the output of ions, the relative amount of gas at the source. By the use of reference gases, such as N2, the gauge sensitivity can be established and, by the use of appropriate correction factors, differing extents of ionisation can be accounted for. 

Explain the presentation of the residual gas analysis data in Table 5.2. Calculate the fragmentation factors (FF) associated with the given peaks. 

---