Reversed-Phase Ion-Pair Liquid Chromatography of a Pharmaceutical Compound and Its Photolytically Transformed Isomer

Abstract

Reversed-phase HPLC is usually the method of choice for compounds that dissolve in water/organic mixtures, even for samples that contain ionizable compounds. However, reversed-phase ion-pair chromatography (RP-IPC) is often a useful alternative for such samples, particularly if the components are highly basic or acidic. With this technique an aqueous/organic mobile phase is used along with a buffer and an ion-pairing reagent to provide more retention and higher selectivity than are afforded by the column and organic solvents alone. The retention effects of ammonium acetate, sodium acetate, potassium phosphate, and triethylamine (TEA) on a pharmaceutical compound (SK&F 108566) and its stereoisomer (SB 206328) were studied. Evidence indicated that SK&F 108566 and SB 206328 exhibited ion-pair formation with simple buffer systems and the ion-pair occurred in the mobile phase prior to the complex binding rapidly to the stationary phase. As a result, buffer systems such as sodium acetate, ammonium acetate, and potassium phosphate could be used as ion-pairing reagents. Thus, a simple HPLC system which not only provided better buffer capability but also improved selectivity through ion-pair interactions could be utilized for ionic compounds separation. One important result to emerge from this study is the potential for isocratic, multidimensional HPLC separations when reversed-phase columns are coupled and used with the appropriate buffer solutions.