A method for casting-in-place composite and/or nonfilled structures (11) which are useful as sorptive or reactive media or for size-based separations. Any particular housing size or configurationcan 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 apropriate 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. 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 (12) and can be used for size-based separations wherein the cast structure (11) acts as a semipermeable membrane barrier. The present invention also encompasses these structures as well as housings containing the structures.