Measurement of Vapor Pressures and Backstreaming with a Mass Spectrometer Calibrated against a Quartz Crystal Thickness Monitor

Abstract

Oil backstreaming is directly measured by detection of the free molecular flow coming from the baffle to be examined. A quadrupolemass spectrometer with open-type ion source is used. For absolute calculation it is first exposed to a panel covered with pump fluid at defined temperature. Simultaneously the evaporation rate is measured by a quartz crystal thin film monitor. The mass spectrometer gives the relative contribution of different constituents to the total vapor pressure as measured by the quartz monitor. The detection limit of the quartz monitor corresponds to vapor pressures in the 10 −9 Torr range. For the correct extrapolation of the mass spectrometer reading to far lower levels, the dependence of the molecular beam density on the particle velocity has to be considered. Interference by a background of vapor coming from any other source is excluded by a shutter between the detector and the vapor source to be investigated.