Extended thermodynamic approach to ion interaction chromatography. Influence of the chain length of the solute ion: a chromatographic method for the determination of ion-pairing constants

The influence of the chain length in Ion Interaction Chromatography (IIC), previously named Ion Pair Chromatography, was investigated. The system examined is a C18 stationary phase and a variable, salt-controlled, mixture of a phosphate buffer pH 2.1 in methanol containing tetrabutylammonium bromide as Ion Interaction Reagent. IIC proves to be a good alternative to the well established spectrophotometric and conductometric methods of obtaining thermodynamic ion-pairing constants. The latter increase with increasing analyte chain length and decrease with increasing methanol concentration in the eluent: both results support a physical chemical description of the hydrophobic ion-pairing process at variance with the classical, Bjerrum-type modelling of the purely electrostatic interaction between inorganic ions. The calculated deltaG(o) are reliable because they compare to non-chromatographic estimates of deltaG(o) for similar systems. The present extended thermodynamic approach is able to predict and explain why the purely electrostatic approach works better for lower analyte lipophilicity or higher organic modifier concentration.

Extended thermodynamic approach to ion interaction chromatography: a thorough comparison with the electrostatic approach, and further quantitative validation

The most reliable literature experimental results, concerning retention behavior of charged molecules, in the presence of an ion-interaction reagent (IIR), were used to obtain a further quantitative validation of a new theory. The present work emphasizes the fact that the extent to which electrostatic interactions, ion pair formation in the adsorbed and the mobile phases, and adsorption competitions are one more important than the other depends on experimental conditions. Further insight into the meaning of the linearity of the log k vs. log [IIR] plot, which is common to many theoretical models, is given. The experimental conditions under which the linearity of this plot can be expected not only practically, but also theoretically, are elucidated. The dependence of the ratio of retention factors with and without IIR in the eluent on the analyte nature, which cannot be predicted by the electrostatic approach, was explained and tracked. The difference between the actual surface potential and that predicted by the electrostatic approach is also rationalized. The model is also theoretically shown to be able to elucidate the enantioselective retention mechanism, in the presence of chiral counter ions.