column the base assembly 263 comprises a distributor 266 and a bed support 268 and optionally a backing plate 264 fastened to each other by releasable fixing means 269 the bed support 268 comprises a screen or mesh or filter and optionally a sealing ring e g 274 the bed support may be made of an inert plastic or metal material such as stainless steel releasable fixing means 269 are for example a screw a nut a bolt or a clamp it will be appreciated that other releasable fixing means are also possible as can be seen from the figure the fixing means 269 secures bed support 268 and the distributor 266 through a central hole in each component the fixing means 269 is only accessible and may therefore only be released from the exterior face of the distributor 266 in fig3 nozzle 272 must first be removed to provide access to release fixing means 269 additional releasable fixing means accessible from the exterior face of the backing plate may optionally be employed to fasten the backing plate distributor and bed support together these fixing means can take the form of bolts inserted through corresponding holes around the perimeter of the components access from the exterior face of the backing plate 264 avoids operator exposure beneath a suspended load were access only to be available from within the interior of the column it will be understood that separation of chemical or biological materials on the column when the tube 212 is full of chromatographic media can be carried out in either a downward or upward flow thus in a downward flow liquid.