Abstract: An apparatus for separating particles from a fluid comprising a housing, means for flowing a fluid through said housing, a plurality of parallel perforated plates supported in said housing spaced from one another in the direction of fluid flow, the plates being effective to accelerate the fluid as it passes through each plate, and a sample-holder immediately downstream of each perforated plate, the fluid flowing through each said plate onto its associated sample-holder and thence around the sample-holder to the next downstream plate, the inner wall of the housing being spaced from the path of fluid flow at least sufficient to substantially avoid contact with fluid flowing along the path.