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

Based on the concept of limiting capacity factors, a semi-quantitative rete ntion model was proposed to describe the chromatographic behaviour of metal chelates with 4-(2-pyridylazo) resorcinol in reversed-phase and ion-pair r eversed-phase systems. Separations were carried out on octadecyl-bonded sta tionary phase with mobile phase consisted of an aqueous buffer - methanol m ixture, tetrabutylammonium hydroxide was used as ion-pairing agent in ion-p air reversed-phase chromatography. Experimental dependencies of the capacit y 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 agreem ent was found between experimental data and the dependencies predicted from the retention model for the chelates of Ni2+, Fe2+ and CU2+, whereas the c obalt chelate exhibited anomalous behaviour, which can not be fully explain ed with the aid of the proposed model. The chelate retention is strongly af fected by the presence of methanol in mobile phase in a similar way as was commonly observed in the reversed-phase chromatography of organic substance s. The buffer concentration (ionic strength of mobile phase), on the other hand, has almost negligible effect on the chelate retention. Side equilibri a, 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. (C) 2000 Elsevier Science B.V. All rights reserved.