Monolithic silica columns with various skeleton sizes and through-pore sizes for capillary liquid chromatography

Reduction of through-pore size and skeleton size of a monolithic silica column was attempted to provide high separation efficiency in a short time. Monolithic silica columns were prepared to have various sizes of skeletons (∼1-2 μm) and through-pores (∼2-8 μm) in a fused-silica capillary (50-200 μm I.D.). The columns were evaluated in HPLC after derivatization to C₁₈ phase. It was possible to prepare monolithic silica structures in capillaries of up to 200 μm I.D. from a mixture of tetramethoxysilane and methyltrimethoxysilane. As expected, a monolithic silica column with smaller domain size showed higher column efficiency and higher pressure drop. High external porosity (>80%) and large through-pores resulted in high permeability (K=8·10^-14-1.3·10^-12 m²) that was 2-30 times higher than that of a column packed with 5-μm silica particles. The monolithic silica columns prepared in capillaries produced a plate height of about 8-12 μm with an 80% aqueous acetonitrile mobile phase at a linear velocity of 1 mm/s. Separation impedance, E, was found to be as low as 100 under optimum conditions, a value about an order of magnitude lower than reported for conventional columns packed with 5-μm particles. Although a column with smaller domain size generally resulted in higher separation impedance and the lower total performance, the monolithic silica columns showed performance beyond the limit of conventional particle-packed columns under pressure-driven conditions.