Patent Number: 
Section: claims

1. A lithographic projection apparatus comprising: a radiation system to supply a projection beam of electromagnetic radiation having a wavelength of 250 nm or less;  a support structure adapted to support patterning structure which can be used to pattern the projection beam according to a desired pattern;  a substrate table to hold a substrate;  a projection system to project the patterned beam onto a target portion of the substrate; and  a gas supply to supply a purge gas to a space in said apparatus, said space containing an optical component positioned to interact with the projection beam, wherein said purge gas comprises molecular oxygen at a total partial pressure of from 1xc3x9710 xe2x88x924  Pa to 1 Pa. 2. An apparatus according to  claim 1 , wherein said purge gas further comprises an inert gas selected from the group comprising helium, argon, nitrogen and mixtures thereof, and wherein the total amount of molecular oxygen present in said purge gas is from 1 ppb to 10 ppm by volume. claim 1 3. An apparatus according to  claim 1 , wherein said space is substantially evacuated. claim 1 4. An apparatus according to  claim 1 , which apparatus further comprises a further supply of electromagnetic radiation having a wavelength of 250 nm or less and arranged to supply such radiation onto said optical component. claim 1