Comparison of conventional microparticulate and a monolithic reversed-phase column for high-efficiency fast liquid chromatography of basic compounds

Abstract

The effect of mobile phase flow on column efficiency for a neutral compound together with weak and strong bases was compared for conventional microparticulate (3/3.5 μm and 5 μm) silica RP columns and a monolithic silica RP. For benzene, the minimum plate height (Hmin) at optimum flow-rate (uopt) for weak bases was similar for the 5 μm and the monolith phases. However, the monolith generated much flatter Van Deemter curves, such that at high flow-rate (5 ml min−1) the plate height was nearly 3.5 times lower on the monolith. For weak bases analysed in unbuffered mobile phases, and stronger bases with acid phosphate buffer, increased tailing was obtained on the monolith compared with the conventional phases. Nevertheless, Van Deemter plots on the monolith still showed some advantages over particulate phases, even when asymmetry factor was included in the calculation of the plate height. However, at pH 7 considerable tailing of strong bases was found using the monolith; it is not clear whether this results from unique features of the monolith structure, or whether it is due merely to usual problems of silica activity. Van Deemter plots for conventional phases may be improved considerably by operating the column at elevated temperatures. At pH 3, these improvements are influenced to a considerable extent by increases in Dm, as shown by measurements of Dm using the Taylor–Aris procedure. However, at pH 7.0, improvements are much too substantial to be explainable wholly on this basis.