Magnetically Immobilized Beds for Capillary Electrochromatography Yucong Wang, Zhichao Zhang, Lei Zhang, Feng Li, Lei Chen, and Qian-Hong Wan* School of Pharmaceutical Science and Technology, Tianjin University, Tianjin 300072, China Fritless packed beds comprised of magnetically respon- sive octadecylsilane bonded silica particles have been constructed for reversed-phase electrochromatography. The magnetic particles were immobilized in the capillary by applying an external magnetic field transverse to the direction of electroosmotic flow. Being subjected to the interplay of fluid dragging and magnetic forces, the initial loosely packed particle assembly was compacted into a uniform packing structure. The magnetically immobilized beds obtained were used as stationary phases for separa- tion of neutral compounds, with retention behavior and column efficiency similar to those of slurry-packed col- umns. The results suggest that the magnetic attraction approach to fritless column packing may be used for construction of advanced chip-based chromatography, especially in complex architectures comprising curved and intersecting channels. Capillary electrochromatography (CEC) is a hybrid separation technique that couples the high separation efficiency of capillary electrophoresis with the superior selectivity of high-performance liquid chromatography. 1-3 Despite the fact that CEC is recognized as of tremendous potential in the pharmaceutical and biomedical fields, several technical problems have to be tackled before the technique enjoys general acceptance and finds widespread ap- plications. One such problem is associated with column prepara- tion. 4 Conventional methods for preparation of particle-packed capillary columns include (i) slurry packing of reversed-phase silica into a fused-silica capillary with frits formed by thermal sintering to hold the packing material inside the capillary; 5-8 (ii) pulling of a glass tube filled with packing material followed by in situ derivatization; 9 (iii) in situ formation of a monolithic or continuous bed in a capillary. 10-14 Each of these methods has advantages and limitations. For instance, with the slurry packing technique, the stationary phase can be chosen from a wide variety of commercial HPLC packing materials and previous experience gathered in HPLC can be readily transferred to CEC. Neverthe- less, it suffers from problems associated with frit fabrication. The frits produced by thermal sintering cause the nonuniformity of the packed bed that is liable for bubble formation during the operation. Pulling the glass tube at high temperature produces packed capillaries with high permeability. Without polymeric coatings, however, the glass columns are fragile and difficult to use. The monolithic columns hold great potential for CEC as they are free from the problems associated with particle packing and frit fabrication. Unlike the slurry packing method, however, the monolithic approach lacks the flexibility with regard to the selection of stationary phases. Clearly, there is still a need for developing alternative column preparation methods that are complementary to those currently available. We wish to report a magnetic field-assisted immobilization approach to fritless packed columns for CEC. The use of magnetic fields for confinement of particles has a long history of applications in chemical and biological engineering. 15-18 A notable example in this area is the magnetically stabilized bed. 19 A fluidized bed is formed by applying fluid drag forces on a mass of solid particles whereby the behavior of the mass of the fluidized bed corresponds to that of a liquid. Fluidized beds possess many desirable attributes in temperature control, heat transfer, and chemical reaction; they therefore have found widespread applications in numerous tech- nological processes. One of the major problems associated with fluidized beds is that of bubble formation, frequently resulting in slugging, channeling, spouting, attrition, and pneumatic transport. The formation of bubbles and slugs in a fluidized bed may be eliminated by an external magnetic field due to the interaction of this field with fluidized ferromagnetic particles. 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Chem. 2007, 79, 5082-5086 5082 Analytical Chemistry, Vol. 79, No. 13, July 1, 2007 10.1021/ac070288b CCC: $37.00 © 2007 American Chemical Society Published on Web 06/01/2007