The use of perfluoropolyether fluids in vapour stream pumps

Abstract The performance of a 5 in. diameter diffusion pump (Edwards EO4) has been studied using a distillation cut of the perfluoropolyether fluid with an average mass of 3000 amu. A pumping speed for air of 700 l/s was measured which is similar to the speed obtained when the pump is charged with Silicone 704 fluid. A higher critical backing pressure was obtained, however, for the new fluid viz 1.2 torr compared with 0.8 torr for the conventional fluid. Fragmentation spectra of the vapour molecule obtained by direct injection into the ion source and from backstreaming fluid showed prominent peaks at mass to charge ratios 31 and 69 corresponding to the fragments CF and CF 3 . The similarity of the spectra obtained by the two methods suggests that the fragmentation takes place in the ion source and that the fluis is not degraded by use in the pump. An analysis of the pumping mechanism in vapour stream pumps is given.