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

Abstract

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.

Hybrid organic-inorganic phenyl monolithic column for capillary electrochromatography

“…According to Fig. 7a in [10], the calculated value of a CH2 is 1.45 for the monolithic C18-silica column under the same experimental conditions. This result suggests that the hydrophobicity of the phenyl hybrid monolithic column is lower than that of the C18-silica rod column, which may be attributed to the difference in the length of alkyl chain.…”

“…Up till now, both organic polymer [3][4][5] and silica-based [6][7][8][9][10] monoliths have been developed. Polymeric monolithic stationary phases have the advantage of simple polymerization procedure and easy tuning of porosity and surface chemistry, but they suffer from shrinking or swelling when exposed to different organic mobile phases, leading to lack of mechanical stability.…”

“…Polymeric monolithic stationary phases have the advantage of simple polymerization procedure and easy tuning of porosity and surface chemistry, but they suffer from shrinking or swelling when exposed to different organic mobile phases, leading to lack of mechanical stability. Monolithic silica capillary columns can be prepared by sintering silica particles [6], immobilizing silica particles in a silica xerogel matrix [7], or in situ hydrolytic polycondensation of alkoxysilanes by a sol-gel process [8][9][10]. The latter one is the most widely used.…”

Preparation of monolithic silica column with strong cation‐exchange stationary phase for capillary electrochromatography

“…Nakanishi et al [6 -7] via a sol-gel process based on the hydrolytic polymerization of alkoxysilanes. Tanaka et al [8 -10] and Allen et al [11] prepared reversed-phase silica monoliths and evaluated their chromatographic performance in the separation of neutral and charged species under pressuredriven and electro-driven conditions. Hydrophilic silica monoliths were also prepared by Allen et al [12] for separation of carbohydrates, nucleosides, and other neutral polar species.…”

Dynamically coated silica monolith with ionic liquids for capillary electrochromatography

“…However, as the potential applications of unmodified silica monoliths are generally limited, it is necessary to introduce chromatographic functionality onto the silica matrix to exert selectivities. Thus far, two methods, namely post-modification and hybrid organic-inorganic functionalization, are the most commonly used ones [9][10][11][12][13][14]; nevertheless, they need additional surface modification steps or reoptimization for each sol-gel precursor [15]. A much simpler and more convenient method, called dynamic coating, has also captured researcher's attentions, in which a running buffer containing the coating agent is used [16].…”