Mechanism of ion-pair chromatography and chromatographic behavior of liophilic ions

The main driving force in classical reversed-phase chromatography is hydrophobic molecular interactions, or weak dispersive forces. However, often the simple hydrophobic interactions do not provide sufficient selectivity for the separation of complex mixtures. One of the solutions to this problem is the addition of amphiphilic ions in the mobile phase. This chromatographic mode, usually called ion-pair chromatography, is a powerful tool, but often applied as a last resort if all other attempts to achieve separation have failed, because this method involves dynamic modification of adsorbent surface and it is usually irreversible.; Recently, the application of liophilic ions as moderators of the retention of basic analytes was introduced. Application of these ions allows fast equilibration of the chromatographic system, and the reversed-phase column can usually be recovered after the application of these ions. The mechanism of the analyte retention on the presence of these ions currently is a subject for intensive discussion in the literature.; We studied the chromatographic behavior of the strongest representative of inorganic liophilic ions---hexaflurophosphate (HFP or PF 6-). We measured its excess adsorption isotherms from acetonitrile/water and methanol/water mixtures. Correlation of the PF 6- isotherms with the adsorption behavior of the organic eluent components allowed us to conclude that acetonitrile, with its four pi-electrons, shows significant interactions with electron-rich PF6- ions, which have very high degree of electron delocalization. At the same time the adsorption of PF6- on the same surfaces from methanol/water mixtures was very weak.; Based on these studies we conclude that a higher effect of the addition of liophilic ions in the mobile phase is expected if acetonitrile as opposed to methanol is used as organic modifier. Direct HPLC experiments with a set of probe analytes confirm that conclusion. The retention of charged analytes increases in the presence of PF6- anions in the mobile phase, and this increase is correlated with the adsorption isotherm of acetonitrile from water.; Some increase of the probe analyte retention was observed when methanol was used as organic modifier, but this effect was significantly lower then in case of acetonitrile and there was no correlation with its adsorption isotherm.; Comparison of the chromatographic behavior of several adsorbents was made using st retention factors and surface specific adjusted retention. A special procedure for the assessment of the total surface area of the adsorbent in the column has been developed. The correlation of two different methods of retention representation shows that surface specific values properly reflect the chromatographic behavior of studied stationary phases.