Modelling of solute retention in normal-phase HPLC with the chemically bonded 3-cyanopropyl stationary phase

We have investigated the applicability of two models of solute retention (i.e. Models I and II, devised by Kowalska for liquid chromato-graphy with alcohol-hydrocarbon mobile phases) for description of the chromatographic behavior of 25 test solutes in normal-phase (NP) HPLC systems with the 3-cyanopropyl stationary phase and 2-propanol-n-hexane as mobile phase. The test solutes employed were from five chemical classes, (i) an alkylbenzene, (ii) crown ethers, (iii) hydroxyaromatic and methylhydroxyaromatic compounds, (iv) naphthylamines and (v) quino-line and its derivatives, and thus differed considerably in molecular structure, and hence in their ability to compete with 2-propanol molecules for active sites (i.e. the -CN and ≡Si-OH groups) of the stationary phase, which is the quintessence of the so-called displacement mechanism of solute retention. The performance of Models I and II was evaluated in terms of the numerical values of the root-mean-squared error (RMSt), calculated for each employed test solute and our investigations showed that Models I and II are very well suited for description of the chromato-graphic behavior of solutes for which there is a non-linear empirical relationship between retention parameter (RF or lnk) and the composition of a binary mobile phase.