Modelling of solute retention in the reversed-phase high-performance liquid chromatography system with the chemically bonded 3-cyanopropyl stationary phase

We investigated the applicability of three models of solute retention (devised by Snyder et al., Schoenmakers et al., and Kowalska) for reversed-phase high-performance liquid chromatography (RP-HPLC) for description of the chromatographic behaviour of 25 test solutes in the RP-HPLC systems with the 3-cyanopropyl stationary phase and the methanol+water eluent. The employed test solutes considerably differed with respect to molecular structure, as they belonged to chemical classes of (i) benzene and alkylbenzenes, (ii) hydroxyaromatics, and (iii) crown ethers. In the case of the chemically bonded stationary phases, the 3-cyanopropyl phase included, one encounters the mixed mechanism of solute retention with the contributory effects of partition and adsorption. From our investigations, it comes out that the retention model proposed by Schoenmakers et al. (i.e., model II) is the best suited for description of the chromatographic behaviour of solutes on such packings, while the remaining two models (i.e., models I and III) perform less accurately. An additional investigation was performed with aid of the relationship between the retention parameters (i.e., log k) of the employed test solutes and the logarithms of their partition coefficients (log P) in the octanol/water system (as calculated from the hydrophobic fragmental constants after Rekker), depending on the quantitative composition of the binary eluent. The greater is the deviation of the log k vs. log P relationship from linearity (as monitored by the decreasing values of the respective linear correlation coefficients, r), the less pronounced becomes the contribution of partition and hence the importance of adsorption for the overall retention increases. Thus, the log k vs. log P study was performed in order to follow the partition↔adsorption balance with the changing volume proportions of methanol in the mixed eluent, and ultimately to link an extent of contamination of the nominal reversed-phase partition mechanism by adsorption with the predictive power of models I–III. Finally, an effort was undertaken aimed at demonstration of a considerably enhanced performance of model III, when applied to the experimental conditions that corresponds better with the theoretical assumptions of this approach.