Residual Gas in High Vacuum Systems

[Ryomega]

Homework Statement

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?

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.

 

[haruspex]

Some clues here: http://en.wikipedia.org/wiki/Ultra-high_vacuum#Achieving_ultra-high_vacuum