This review article is intended to provide the reader with the recent advances made in the fabrication of silica-based monolithic capillary columns for use in capillary electrochromatography (CEC). The silica-based monoliths can be produced by three different approaches, namely (i) fusion of silica particles by thermal sintering, (ii) cross-linking/entrapping silica particles in a packed bed using the sol-gel process, and (iii) polymerization of silicon alkoxide precursors using the sol-gel process. Thus far, approach (iii) is the most widely used for fabricating silica monoliths. After providing a thorough description of each of the three approaches used for the production of silica-based monolithic capillary columns, the analytical separations performed by CEC on each kind of monolith are discussed.
Monolithic silica columns with surface-bound octadecyl (C18) moieties have been prepared by a sol-gel process in 100 microm ID fused-silica capillaries for reversed-phase capillary electrochromatography of neutral and charged species. The reaction conditions for the preparation of the C18-silica monoliths were optimized for maximum surface coverage with octadecyl moieties in order to maximize retention and selectivity toward neutral and charged solutes with a sufficiently strong electroosmotic flow (> 2 mm/s) to yield rapid analysis time. Furthermore, the effect of the pore-tailoring process on the silica monoliths was performed over a wide range of treatment time with 0.010 M ammonium hydroxide solution in order to determine the optimum time and conditions that yield mesopores of narrow pore size distribution that result in high separation efficiency. Under optimum column fabrication conditions and optimum mobile phase composition and flow velocity, the average separation efficiency reached 160 000 plates/m, a value comparable to that obtained on columns packed with 3 microm C18-silica particles with the advantages of high permeability and virtually no bubble formation. The optimized monolithic C18-silica columns were evaluated for their retention properties toward neutral and charged analytes over a wide range of mobile phase compositions. A series of dimensionless retention parameters were evaluated and correlated to solute polarity and electromigration property. A dimensionless mobility modulus was introduced to describe charged solute migration and interaction behavior with the monolithic C18-silica in a counterflow regime during capillary electrochromatography (CEC )separations. The mobility moduli correlated well with the solute hydrophobic character and its charge-to-mass ratio.
Three different synthetic routes have been introduced and evaluated for the preparation of amphiphilic silica-based monoliths possessing surface-bound octadecyl ligands and positively charged groups. The amphiphilic silica monoliths (designated as cationic C18-monoliths) have been designed for use in reversed-phase capillary electrochromatography (RP-CEC) with hydro-organic mobile phases. These amphiphilic stationary phases yielded anodic electroosmotic flow (EOF) over a wide range of mobile phase pH. The magnitude of EOF remained constant up to pH 4.0 and then decreased at pH > 4.0 due to the ionization of silanol groups and the subsequent decrease in the net positive surface charge density of the amphiphilic monoliths. The cationic C18-monoliths exhibited reversed-phase chromatography (RPC) behavior toward non-polar solutes (e.g., alkyl benzenes), which parallels that observed with octadecyl-silica (ODS) monoliths. On the other hand, the amphiphilic stationary phases exhibited both non-polar and polar interactions toward slightly polar solutes such as anilines and PTH-amino acids. CEC retention factor k* and velocity factor k*e, which reflects the contribution of the electrophoretic mobility, were evaluated for charged solutes such as anilines and proteins.
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