Effect of calcium-modified silica on retention and selectivity in normal-phase liquid chromatography

Abstract

Modification of a silica surface by calcium ions was studied by high-performance liquid chromatography. After treatment at pH 7.0 and 9.0 with calcium hydroxide, the calcium-modified silicas were subjected to physical and chemical analysis. From calcium measurement with a flame atomic absorption spectrometer, the extent of calcium adsorption was determined as 650 ppm on calcium-modified silica at pH 7.0 and 4800 ppm on calcium-modified silica at pH 9.0. The amount of calcium on the original silica was 58 ppm. The separation factor (α) for N-methylaniline versus N,N-dimethylaniline was measured to be 5.1 on calcium-modified silica at pH 9.0, but could not measured on calcium-modified silica at pH 7.0 or the original silica, using methanol- n-hexane (1:99, v/v) as the eluent. By comparison of calcium and amino-modified silica, the separation factors were measured under the same HPLC conditions and was found to be 9.0 on amino-modified silica. The results show that calcium-modified silicas are able to separate some basic model compounds as well as amino-modified silica.