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-pha se ion-pair chromatographic systems containing trifluoroacetate or hep tafluorobutyrate as pairing ion were explained after determining sorpt ion isotherms for trifluoroacetate and heptafluorobutyrate on Nucleosi l 100-5 C-18 from a solution similar to the eluents used for the separ ation of transmitteramines and peptides. Based on the isotherms and re tention data obtained with reversed-phase, ion-exchange and reversed-p hase ion-pair chromatographic systems, it is proposed that the selecti vity differences between the systems studied are caused by the fact th at trifluoroacetate and heptafluorobutyrate are not interchangeable in terms of their surface concentrations at the practical eluent concent rations of the pairing ions concerned.