Direct separation of captopril diastereoisomers including their rotational isomers by RP-LC using a teicoplanin column

A direct reversed-phase liquid chromatography (LC) method has been developed for the separation and analysis of captopril and its 2R,2S diastereoisomer using a teicoplanin stationary phase. The proline containing diastereoisomers, which are known to form conformers in aqueous solution, were also separated from their rotational isomers. The influence of temperature, different organic modifiers and buffer type, concentration and pH were optimised to obtain a working resolution between the two diastereoisomers and their respective rotational isomers. The diastereoisomeric purity of several commercial captopril batches was subsequently evaluated using a 0.05% triethylammonium acetate (TEAA) buffer (pH 3.8) run at 1.0 ml/min with mobile phase reservoir and column temperature controlled at 0 degrees C. Throughout the study online UV diode array and mass spectrometry detection was carried out simultaneously to confirm that peaks eluting from the teicoplanin column were in fact captopril and not its readily converted disulphide dimer. Additionally, as a result of the greater detection sensitivity of mass spectrometry, it also facilitated a more accurate optimisation study where trace amounts of the rotational isomers were found to be present in the baseline at temperatures higher than optimum.