a gas sampler includes an impact disc for removing larger particles from a gas flow drawn by a pump and a filter for capturing smaller particles , also positioned in the flow path . the sampler is conveniently dissembled and reassembled to remove and replace the impact disc and the filter . particles removed , especially on the filter , may be observed or analyzed . the impact disc may be coated with an oil or other viscous substance to enhance its ability to retain larger particles . the flow path may be entirely annular after striking the impact disc — that is , around the outside of the impact disc but a portion of the flow path may proceed directly through a central aperture in the impact disc after striking it .