Single-Particle Fritting Technology for Capillary Electrochromatography Bo Zhang, † Edmund T. Bergstro 1 m, † David M. Goodall,* ,† and Peter Myers †,‡ Department of Chemistry, University of York, York, YO10 5DD, UK, and Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD, UK Large perfusive silica beads (particle size 110 μm, through- pore ∼2 μm) held in place by the keystone effect were used as single-particle frits for the manufacture of par- ticulate packed capillary columns. High-quality capillary electrochromatographic separations of a standard test mixture of alkylbenzenes were obtained over the full voltage range of 5-30 kV, with no requirement for pressurization. Excellent robustness was demonstrated by the reproducibility of migration times, peak efficien- cies, and resolution during 100 consecutive runs at the highest voltage (30 kV) without thermostating and pres- surization. Superior performance relative to traditional sinter-fritted columns is ascribed to the heat-free fritting process and short frit length of ∼110 μm. Capillary electrochromatography (CEC) is a relatively new miniaturized high-performance liquid chromatographic technique driven by electroosmotic flow (EOF). 1-4 CEC combines the advantages of high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE), having the properties of high selectivity and high efficiency. 5-9 Despite the excellent perfor- mance reported, the retaining frits are the Achilles heels of the particulate packed columns used in this technique. 10,11 The frits are porous plugs created at both ends of the packed bed to retain the particulate packing material inside the capillary tubing. A good frit should have the following features: (1) high mechanical strength, to sustain a high packing pressure; (2) high permeability, to allow a high packing speed; (3) short length, to diminish the nonuniformity of the packed bed; (4) good reproducibility, to enable good column-to-column reproducible separations and ideally; (5) be simple and fast to fabricate. Over the years, many fritting strategies have been adopted and evaluated. 12,13 The silica-based sintered frit 14-16 is widely used due to its simplicity, although the reproducibility is poor. In this method, the mechanical strength, permeability, and length of the frit are sensitive to the heat applied, which consequently influences the CEC performance. The sintering process may change the surface chemistry of the fritted portion and lead to the nonuni- formity of solvent flow and electrical field distribution at the packed bed/open tube interface, where bubbles are prone to form. 17-19 Pressurization of the column is normally necessary to prevent bubble formation and ensure a stable CEC separation. 4,12,14 In addition, removal of polyimide coating of the fused-silica tubing during sintering makes the column fragile. Among alternative fritting methods reported, Chen et al. 20 improved the quality of the silica-based frit via careful choice and control of the condition for silicate polymerization and obtained robust CEC columns. Based on the keystone effect to hold particles inside the tubing, taper end columns are another choice as there is no need for a frit segment. 21-25 Although the fragility of the tapered end should be taken into account, it provides a good interface for mass spectrometric detection. Since the late 1990s, organic polymer- 26-29 and silica- 30 based monolithic columns have been successfully applied in CEC. Chemically bonded onto the capillary wall, the single-piece * Corresponding author. E-mail: dmg1@york.ac.uk. Phone: 44(0)1904 432574. Fax: 44(0)1904 432516. † University of York. ‡ University of Liverpool. (1) Pretorius, V.; Hopkins, B. J.; Schieke, J. D. J. Chromatogr. 1974, 99, 23- 30. (2) Jorgenson, J. W.; Lukacs, K. D. J. Chromatogr. 1981, 218, 209-216. (3) Knox, J. H.; Grant, I. H. Chromatographia 1987, 24, 135-143. (4) Knox, J. H.; Grant, I. H. Chromatographia 1991, 32, 317-328. (5) Colo ´ n, L. A.; Guo, Y.; Fermier, A. M. Anal. Chem. 1997, 69, 461A-467A. (6) Unger, K. K.; Huber, M.; Walhagen, K.; Hennessy, T. P.; Hearn, M. T. W. Anal. Chem. 2002, 74, 200A-207A. (7) Krull, I. S.; Stevenson, R. L.; Mistry, K.; Swartz, M. E. Capillary Electro- chromatography and Pressurized Flow Capillary Electrochromatography; HNB Publishing: New York, 2000. 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