Ion-interaction chromatography of zwitterions. The fractional charge approach to model the influence of the mobile phase concentration of the ion-interaction reagent

The chromatographic behavior of zwitterions in ion-interaction chromatography (IIC) was investigated theoretically, on the basis of the following picture of the retention mechanism. The zwitterionic analyte is considered to interact with the charged stationary phase via an effective fractional charge, opposite to the surface one. Adsorption competitions between analytes and the ion interaction reagent (H) concur to explain the retention factor of zwitterions. The present model is quantitatively able to explain the retention behavior of zwitterions in IIC and also to account for the influence of the zwitterion ionization degree, according to the mobile phase pH, on the course of retention as a function of the H concentration. The approach we used is simple and epistemologically interesting because the retention equation for zwitterions may also be obtained from the general retention equation of our extended thermodynamic approach to IIC. Estimated magnitudes of the effective charges are very reasonable and show the same trend as that of the molecular dipole moments, as expected; the total ligand concentration compares well with the bonded phase coverage of the two columns used. For the homologous series from 4-aminobutyric-, to 8-aminocaprylic acid, the estimated effective charge always increases with increasing chain length and this results in parallel growth of the analyte retention increase upon H addition, so that the retention increase for the highest member of the series compares with that of a positively charged analyte. The estimated dipole for this analyte compares excellently to the estimate obtained via a quantum mechanics calculation. The influence of the mobile phase pH on retention was taken into account for the very first time. The good predictive abilities of the retention equations, and the reliability of the estimated constants that make sense physically confirm that retention modeling is able, at the thermodynamic level, to disclose the complexity of the IIC system.