Recent developments in the evaluation of chemically bonded silica packings for liquid chromatography

Abstract

The basic variables and conditions in the synthesis of chemically bonded silica packings (CBSPs), obtainable by means of a surface reaction between porous silica and an organochlorosilane or organoethoxysilane as a modifier, yielding a dense monolayer of bonded functional groups, are discussed. The CBSPs can also be prepared by a novel procedure termed bulk modification, which involves a co-hydrolysis and co-condensation of tetraethoxysilane or poly(ethoxysiloxane) and a corresponding organotriethoxysilane. In this way, the organic functional groups become a constituent of the bulk phase as well as of the surface. The retention behaviour of solutes on columns packed with both types of packings was examined in normal and reversed-phase liquid-solid chromatography. Corresponding to the high surface concentration of functional groups, the capacity factors of solutes normalized to unit surface area of the adsorbent, k′/ S BET, were found to be relatively larger on bulk- than on surface-modified silica packings for a given bonded group. For n-alkyl bonded phases in reversed-phase chromatography, the plot of log k′/ S BET versus the number of carbon atoms in the n-alkyl chain was linear for each solute. Further, it is shown that the selectivity of solutes on polar modified packings is largely controlled by the type of polar functional groups substituting the silanol groups.