1.3.5 Residual gas spectrum
When working in ultra high vacuum, it can be important to know the composition of the residual gas. The percentages of water (M = 18) and its fragment HO (M = 17) will be large in the case of vacuum chambers that are not clean or well heated. Leaks can be identified by the peaks of nitrogen (M = 28) and oxygen (M = 32) in the ratio of N2/O2 = 4/1.
Hydrogen (M = 2), water (M = 17 and 18), carbon monoxide (M = 28) and carbon dioxide (M = 44) will be found in well-heated 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 2.22.
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