Organic polymer monoliths as stationary phases for capillary HPLC

Modern rigid porous polymer monoliths were conceived as a new class of stationary phases in classical columns in the early 1990s and later extended to the capillary format. These monolithic materials are typically prepared using a simple molding process carried out within the confines of the capillary. Polymerization of a mixture comprising monomers, initiator, and porogenic solvent affords macroporous materials with large through-pores that enable applications in a rapid flow-through mode. Since all the mobile phase must flow through the monolith, convection considerably accelerates mass transport within the monolithic separation medium and improves the separations. As a result, monolithic columns perform well even at very high flow rates. Various mechanisms including thermally and UV initiated free radical polymerization as well as ring opening metathesis copolymerizations were demonstrated for the preparation of monolithic capillary columns. The versatility of these preparation techniques was demonstrated by their use with hydrophobic (styrene, divinylbenzene, butyl methacrylate, ethylene dimethacrylate), hydrophilic (2-hydroxyethyl methacrylate, methacrylamide, methylenebisacrylamide), ionizable (vinylsulfonic acid, 2-acrylamido-2-methyl-propanesulfonic acid), and tailor-made (norborn-2-ene, 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,endo-dimethanonaphthalene) monomers. Variation of polymerization conditions enables control of the porous properties of the monolith over a broad range and mediates the hydrodynamic properties of the monolithic columns. The applications of polymer-based monolithic capillary columns are demonstrated for numerous separations in the microHPLC mode.