Studies on System Peaks in Ion-Pair Adsorption Chromatography: I. Evaluation of Ion-Pair Adsorption Constants and Effects by the Injection of Finite Analyte Amounts

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Abstract
Adsorption of a secondary amine, protriptyline, as phosphate ion-pair, to Nucleosil C18, was studied. Two different techniques were compared; the traditional break-through and the system peak technique. Both techniques indicated that the adsorption of the ion-pair was best fitted to a 2-site adsorption model of the Langmuir type, indicating a heterogenous solid phase surface. The reliability of the system peak technique was tested by comparing the retention volumes of the system peaks for different kind of equilibrium disturbances introduced into the system. When injecting finite amounts of analytes the system peak retention was affected by the analyte concentration, charge and hydrophobicity, keeping the eluent composition constant. The effects could qualitatively be explained in light of the developed model for the retention.