the domed surface 102 is coated with a coating material 106 such as a high temperature silicone grease which helps capture particles at surface 102 rather than allowing them to rebound into the nitrogen gas jet stream an example of a suitable coating material is a valve and lubricant sealant marketed by dow corporation under the trade designation 34 dow sealant 111 34 gas flowing in the path illustrated by arrows 104 impinges against a pair of helical blades 108 109 see fig4 which are interposed in the annular space 110 between the inner housing 56 and outer housing 16 the blades 108 109 are spaced axially from each other along the housing 16 and each extends about 180 u00b0 about the inner housing member 56 the axial spacing is illustrated in fig1 and 2 with blade 108 being shown in phantom lines the blades are also circumferentially spaced about 180 u00b0 relative to each other thus about one half of the gas flow engages each blade and is directed into a helical path through which the gas swirls about the inner housing member 56 this causes the stream of gas to adopt a helical path which results in further inertial separation of.