Peak-splitting in reversed-phase ion-pair HPLC of sympathomimetic drugs and its probable mechanism

In the determination of ephedrine using reversed-phase, ion-pair liquid chromatography, a chromatographically pure sample was observed to give three peaks under certain mobile phase conditions. The mobile phases which produced maximum peak splitting were determined for ephedrine and a number of other sympathomimetic drugs. A proposal that peak splitting was the result of the composite interplay of two discrete chromatographic mechanisms, was investigated. The results of analysis by GC/MS confirmed that each peak was due to ephedrine, however, only one of the three split peaks was found to contain ion pairs. It is postulated that peak splitting is a physical phenomenon on reversed-phase columns and the separation of these drugs by ion-pair HPLC is based on a mixed rather than a single mechanism. This study has also shown that errors can arise in ion-pair HPLC when multiple peaks are assumed to indicate heterogeneity. Interconvertible species of the same solute can give rise to these peaks.