Monolithic columns containing sol–gel bonded octadecylsilica for capillary electrochromatography

Abstract

A new method for preparing monolithic capillary columns is reported. A fused-silica capillary packed with porous octadecylsilica (ODS) particles using a CO 2 slurry was partially filled with a siliceous sol formed by hydrolysis and polycondensation of tetramethoxysilane and ethyltrimethoxysilane. After gelling and aging of the siliceous sol at room temperature, the column was dried with supercritical CO 2. The scanning electron micrograph of a cross-section of the capillary column revealed that the ODS particles were bonded to each other and to the column inner wall by the sol–gel, forming a monolith. The performance of the monolithic column was evaluated for capillary electrochromatography using small aromatic compounds and polycyclic aromatic hydrocarbons with Tris buffer in aqueous acetonitrile mobile phase. The electroosmotic flow velocity increased with a decrease in Tris buffer concentration from 10 to 5 m M. The electroosmotic flow velocity increased significantly with an increase in Tris buffer pH from 5 to 7.5, and then remained nearly constant with Tris buffer pH between 7.5 to 9.0. The electroosmotic flow velocity increased with an increase in acetonitrile content from 20 to 50%, and then remained nearly constant with acetonitrile content between 50 to 90%. No bubble formation was observed during CEC operation, and all of the test compounds eluted as symmetrical peaks. The logarithms of the retention factors of the aromatic compounds and polycyclic aromatic hydrocarbons decreased linearly with an increase in acetonitrile content. For a 21/29 cm×75 μm I.D. monolithic column containing 9% sol–gel bonded 5 μm ODS, approximately 2.7×10 4 theoretical plates (plate height 7.8 μm; 1.3×10 5 theoretical plates per meter) were measured. It was found that the resistance to mass transfer of unretained thiourea was small, which was attributed to partial filling of the small pores in the particulate packing and partial filling of the interstitial space by the porous sol–gel network. The procedure for column fabrication was simple, and three columns were prepared without failure.