Retention as a Function of Analyte Nature

The first effort to use LSERs in IPC relied on a retention equation based on a mixture of stoichiometric and electrostatic models. Several approximations were made [1-3]. First, ion-pairing in the eluent was neglected, but this is at variance with clear qualitative and quantitative experimental results [4-13]. In Chapter 3 (Section 3.1.1), the detrimental consequences of this assumption were clarified and danonstiated that extensive experimental evidence cannot be rationalized if pairing interactions in the eluent are not taken into account. Furthermore, in the modeling of A as a function of the analyte nature, the presence of the IPR in the eluent was assumed not to influence the retention of neutral analytes. This assumption is only occasionally true [14,15] and the extended thermodynamic retention model of IPC suggests the quantitative relationship between neutral analytes retention and IPC concentration in the eluent [16]. 

Moreover, the only solute descriptor for ion-pair effect was the analyte charge, but it was shown that the analyte charge status did not explain (1) different experimental curves when k is plotted as a function of the stationary phase concentration of the IPR for various IPRs (2) the dependence of the ratio of the retention of two different analytes on IPR concentration (3) the dependence of the k/ ko ratio on the analyte nature if experimental conditions are the same [16,17] and (4) ion-pairing of peptides [12]. The model makers realized that the charge may have been a too-simple solute descriptor for ion-pairing because it did not exhibit the hydrophobic effect, but they did not devise a better descriptor. Section 

3 (Chapter 2) details the peculiarities of the ion-pairing process of hydro-phobic ions and it is clear that interactions other than electrostatic are essential to quantitatively describe the duplex formation that is not even conceivable in 

The authors presumed that the estimate must have been zero when phosphoric acid was used as the IPR but they did not test their guess. A theoretical analysis of the meaning of this parameter (Equation 9 in Reference 1) confirms that the value cannot be zero. A phosphate ion was used as an IPR [18] and was demonstrated to be effective [11,19-23], Unfortunately, the estimates of this variable obtained for different IPRs are not in agreement with the theoretical values. If one keeps in mind that enough processing of large quantities of data with a sufficient number of fitting parameters may lead to any deduction [24], it can be concluded that the use of QSRRs in IPC is still in its infancy. 

In a stndy of retention of aromatic carboxylic acids under IPC conditions, linear free energy relationships were observed between the capacity factors and the extraction eqnilibrinm constants of benzoic acid and naphthalene carboxylic acid. The capacity factor of benzene polycarboxylic acids was directly related to then-association constants and qnatemary ammonium ions calculated on the basis of an electrostatic interaction model [27,28], 

---