EXPLANATION OF THE SELECTIVITY DIFFERENCES BETWEEN REVERSED-PHASE ION-PAIR CHROMATOGRAPHIC SYSTEMS CONTAINING TRIFLUOROACETATE OR HEPTAFLUOROBUTYRATE AS PAIRING ION

The well documented selectivity differences found between reversed-phase ion-pair chromatographic systems containing trifluoroacetate or heptafluorobutyrate as pairing ion were explained after determining sorption isotherms for trifluoroacetate and heptafluorobutyrate on Nucleosil 100-5 C-18 from a solution similar to the eluents used for the separation of transmitteramines and peptides. Based on the isotherms and retention data obtained with reversed-phase, ion-exchange and reversed-phase ion-pair chromatographic systems, it is proposed that the selectivity differences between the systems studied are caused by the fact that trifluoroacetate and heptafluorobutyrate are not interchangeable in terms of their surface concentrations at the practical eluent concentrations of the pairing ions concerned.