1. The problem statement, all variables and given/known data What is the likely main residual gas constituent of a high vacuum system (HV) (P~1x10-4 Pa) and where does it come from? Explain how the vacuum may be improved to give ultra-high vacuum (UHV) (P~1x10-8 Pa). What is the likely main residual gas constituent of ultra-high vacuum? 3. The attempt at a solution My first guess was hydrocarbons for HV, but apparently that's something that occurs in rotary (rough) pumps which goes to about 10^-1 Pa. I've searching google with no answer to this particular question. Would anyone like to share their thoughts or a link with some answers? Thanks a lot.