Study on preparation and characterization of monolithic silica capillary columns for high separation efficiency in high performance liquid chromatography

Abstract

This thesis addresses the preparation and characterization of monolithic silica capillary columns to examine column performance in high performance liquid chromatography. The monolithic silica capillaries and monolithic silica rods, prepared under similar preparation conditions, were evaluated. The most important findings in this thesis are as follows: First, it was successful to control retention ability and retention selectivity of solutes in reversed-phase liquid chromatography by changing the composition of the preparation feed solution. The hybrid columns prepared with tetramethoxysilane and methyltrimethoxysilane under the present preparation conditions were able to show higher column efficiency than the hybrid columns reported previously, while maintaining the retention factors in a similar range by reducing the concentration of methyltrimethoxysilane and increasing the total silane concentration in the feed solution. Second, the effects of hydrothermal treatments at 80 °C and 120 °C on mesoporosity of monolithic silica were investigated. The results of pore characterization of the capillary columns by inverse size exclusion chromatography were compatible with those of the nitrogen physisorption measurements performed on the silica rods regarding pore size distribution. The effect of hydrothermal treatment or silica precursor on mesopore size and surface area could be detected. In reversed-phase liquid chromatography, the hybrid column treated at 80 °C showed low column efficiency for large molecules (e.g. insulin), but not for small molecules (alkylbenzenes), because of the absence of the additional hydrothermal treatment at 120 °C. In contrast, for pure silica columns produced with tetramethoxysilane only, no significant difference in column efficiency was observable. The aforementioned difference supported the results of the examination by inverse size exclusion chromatography and the nitrogen physisorption measurement performed on the hybrid silica, treated at 80 °C only, showing the presence of a large volume of small pores below 60 Å. Consequently, it was suggested that the hydrothermal treatment at 120 °C has a stronger influence on the hybrid column in comparison with the pure silica column, to provide higher column efficiency with an increase in molecular size of solute.