a method for casting - in - place composite and / or non - filled structures which are useful as sorptive or reactive media or for size - based separations . any particular housing size or configuration can be used , and the inclusion of a large amount of adsorptive particles in polymer is achieved while still maintaining the membrane three dimensional structure . in a first preferred embodiment , the composite structures comprise particles entrapped within a porous polymeric substrate , and are cast in - place into a housing such as a pipette tip , thereby providing an effective platform for micromass handling . with the appropriate selection of particle chemistry , virtually any separation or purification operation can be conducted , including selective bind / elute chromatography operations , on sample mass loads less than 1 microgram in volumes of a few microliters , as well as larger mass loads &# 39 ; and volumes . the present invention also encompasses the composite structures as well as sample preparation devices containing the same . in a second preferred embodiment , self - retaining , self - supporting structures are cast in situ in a suitable housing and can be used for size - based separations wherein the cast structure acts as a semi - permeable barrier . the present invention also encompasses these structures as well as housings containing these structures .