Reversed-phase liquid chromatography of metal chelates of 4-(2-pyridylazo) resorcinol: retention model and its verification Original Research Article

Based on the concept of limiting capacity factors, a semi-quantitative retention model was proposed to describe the chromatographic behaviour of metal chelates with 4-(2-pyridylazo) resorcinol in reversed-phase and ion-pair reversed-phase systems. Separations were carried out on octadecyl-bonded stationary phase with mobile phase consisted of an aqueous buffer — methanol mixture; tetrabutylammonium hydroxide was used as ion-pairing agent in ion-pair reversed-phase chromatography. Experimental dependencies of the capacity factors on the mobile phase characteristics (pH, methanol concentration, concentration of ion-pairing agent) were measured for nickel, iron, copper and cobalt chelates, and confronted with the retention model. A good agreement was found between experimental data and the dependencies predicted from the retention model for the chelates of Ni2+, Fe2+ and Cu2+, whereas the cobalt chelate exhibited anomalous behaviour, which can not be fully explained with the aid of the proposed model. The chelate retention is strongly affected by the presence of methanol in mobile phase in a similar way as was commonly observed in the reversed-phase chromatography of organic substances. The buffer concentration (ionic strength of mobile phase), on the other hand, has almost negligible effect on the chelate retention. Side equilibria, such as acid–base or ion-pairing, can be used to manipulate effectively the chelate retention and separation; these equilibria are involved in the retention model.