1.3.5 Residual gas composition
When working in ultra-high vacuum, it can be important to know the composition of the residual gas before starting vacuum processes or in order to monitor and control processes. The percentages of water (m/e = 18) and its fragment OH (m/e = 17) will be large in the case of vacuum chambers that are not clean or well baked. Leaks can be identified by the peaks of nitrogen (m/e = 28) and oxygen (m/e = 32) in the ratio N2/O2 of approx. 4 to 1.
Hydrogen (m/e = 2), water (m/e = 17 and 18), carbon monoxide (m/e = 28) and carbon dioxide (m/e = 44) will be found in well-baked chambers. No hydrocarbons will be found when using turbomolecular pumps. They are very effectively kept out of the chamber due to the high molecular masses and the resulting high compression ratios. A typical residual gas spectrum for a clean vessel evacuated by a turbomolecular pump is shown in Figure 1.10.
Figure 1.10: Typical residual gas spectrum of a vessel evacuated by a turbomolecular pump
Further information on working with mass spectrometers is given in Chapter 6, Mass spectrometers and residual gas analysis.