Analysis was made of residual hydrocarbons in an ultra-high vacuum system with an oil diffusion pump as a roughing means. The system was intermittently pumped by an oil diffusion pump and variation in hydrocarbon mass peaks was measured using a quadrupole mass filter. The most characteristic groups of mass peaks which suggest oil contamination were the C
3H
m, C
4H
m and C
5H
m groups. In the present system, partial pressures of hydrocarbons larger than the C
3H
m groups increased with total pumping time until the pressures reached some saturated values. The rate of contamination of a surface exposed in the system was estimated on the assumption that the condensation coefficient of all hydrocarbon molecules was unity.
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