THE EFFECTS OF TYPE-B SILICA AND TRIETHYLAMINE ON THE RETENTION OF DRUGS IN SILICA-BASED REVERSE-PHASE HIGH-PERFORMANCE CHROMATOGRAPHY

The effect of type B silica and the presence of triethylamine (TEA) in an acidic mobile phase on the retention time of drugs containing neut ral, acidic and basic functional groups was investigated. At the pH of the mobile phase (2.2) used in the study, none of the test drugs that contained acidic functional groups were ionized, whereas most of the drugs containing basic functional groups were ionized. The retention t ime of the non-ionized compounds showed little change when analyzed on reverse phase columns prepared from type A (Zorbax C-8) Or type B sil ica (Zorbax RX). Without TEA in the mobile phase the retention times o f the ionized basic compounds were different on the two types of phase s. When TEA was used in the mobile phase, the retention times of the i onized bases were similar on the two phases. On Zorbax RX the between column standard deviation of the retention times of all the test drugs on four different batches of the column material, with and without TE A in the mobile phase, was < 0.21 min. When TEA was used in the mobile phase, the between column standard deviation of the ionized bases was significantly (p < 0.05) less than that of the non-ionized drugs. The data suggested that the best system for obtaining reliable retention data for reverse phase HPLC analysis of drug mixtures containing ioniz ed basic compounds was a reverse phase column prepared from type B sil ica and the use of an acidic mobile phase containing TEA.