Classification of Octadecyl-Bonded Liquid Chromatography Columns

Abstract

Abstract The differences in performance characteristics exhibited by columns available under the same octadecyl (C-18) label cause difficulties for the authors and users of official methods. This paper presents a simple scheme for classifying C-18 columns on the basis of the 2 predominant reverse phase retention mechanisms—hydrophobic and silanophilic interactions, which are dependent on the hydrocarbon coverage and the unreacted silanol sites, respectively. Hydrophobic interactions are represented by the ratio of anthracene k : benzene k in an acetonitrile-water (65 + 35) mobile phase. The unreacted silanols are represented by the /V,/V-diethyltoluamide k : anthracene k ratio with acetonitrile as the mobile phase. Column efficiency, which is largely dependent on particle size and packing technique, is included as a third classification criterion. Twelve brands of C-18 packings were subjected to this test scheme and qualitatively classified into 3 major groups on the basis of 3 criteria: hydrophobicity index, free silanol index, and efficiency. This scheme is intended as an aid in the selection of similar or equivalent columns and in defining the type of column suitable for an analytical method.