Fabrication and Chromatographic Performance of Porous-Shell Pillar-Array Columns

We report on a new approach to obtain highly homogeneous silica-monolithic columns, applying a sol−gel fabrication process inside a rectangular pillar-array column (1 mm in width, 29 μm in height and 33.75 mm in length) having a cross-sectional area comparable to that of a 200 μm diameter circular capillary. Starting from a silicon-based pillar array and working under high phase-separation-tendency conditions (low poly(ethylene glycol) (PEG)-concentration), highly regular silica-based chromatographic systems with an external porosity in the order of 66−68% were obtained. The pillars, 2.4 μm in diameter, were typically clad with a 0.5 μm shell layer of silica, thus creating a 3.4 μm total outer pillar diameter and leaving a minimal through-pore size of 2.2 μm. After mesopore creation by hydrothermal treatment and column derivatization with octyldimethylchlorosilane, the monolithic column was used for chip-based liquid-chromatographic separations of coumarin dyes. Minimal plate heights ranging between 3.9 μm (nonretaining conditions) and 6 μm (for a retention factor of 6.5) were obtained, corresponding to domain-size-reduced plate heights ranging between 0.7 and 1.2. The column permeability was in the order of 1.3 × 1013 m2, lower than theoretically expected, but this is probably due to obstructions induced by the sol−gel process in the supply channels.