Abstract
The solvation equation [Eq. (1)] can be applied to reversed-phase HPLC capacity factors (i.e. logSP=logk).(1)logSP=c+rR2+sπH2+aΣαH2+bΣβH2+vVxS.SP is a solute property (e.g., solubility or partition coefficient) and the explanatory variables are solute descriptors as follows: R2 is an excess molar refraction, π2H is the solute dipolarity/dipolarisability, Σα2H and Σβ2H are the solute overall or effective hydrogen-bond acidity and basicity, and Vx is the McGowan characteristic volume and c, r, s, a, b and v are constants that are characteristic of the particular mobile/stationary phase combination. Only a limited hydrophobicity range can be covered using logk values obtained by isocratic chromatography at a fixed mobile phase composition. Gradient elution is more versatile and 29 compounds were examined under 20 different reversed-phase HPLC conditions using automated fast gradient methods. Using Eq. (1)with gradient retention time (tRg) in place of logSP an excellent correlation was obtained with the solute descriptors. This provides an experimental demonstration that gradient retention times (tRg) can be treated as linear free-energy related parameters just like logk or logP values. Because gradient retention times cannot be used for inter-laboratory comparisons, they were scaled by conversion to chromatographic hydrophobicity index (CHI) values by correlation with data from a calibration set of compounds with known CHI values. By comparing the coefficients r, s, a, b and v of Eq. (1)as determined from different chromatographic systems, the relative effects of different solute properties can be revealed and thus the selectivity of HPLC columns can be predicted. It is also possible to compare chromatographic behaviour with that of other partition systems, such as octanol–water, cyclohexane–water and blood–brain barrier distributions.
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