Abstract: A cartridge is disclosed which comprises a volume of particulate packing material through which a fluid is to flow, a piece of solid porous material adjacent to the surface of the packing material though which the fluid is to enter the volume and a second piece of solid porous material adjacent to the surface of the packing material through which the fluid is to exit from the volume, and a heat-shrinkable tubing which triaxially compress the packing material into a tightly packed bed predominantly in the directions which are normal to and parallel to the direction of fluid flow through the packed bed and which extends beyond the pieces of solid porous material to form an inlet opening to and an outlet opening from the packed bed.