Capillary electrochromatography with monolithic silica column: I. Preparation of silica monoliths having surface‐bound octadecyl moieties and their chromatographic characterization and applications to the separation of neutral and charged species

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Abstract
Monolithic silica columns with surface‐bound octadecyl (C18) moieties have been prepared by a sol‐gel process in 100 μm 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 μm 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.