pH-gradient reversed-phase liquid chromatography of ionogenic analytes revisited

The work carried out on the linear pH-gradient is critically reviewed in combination with the development of new expressions for the retention time of ionizable analytes. It is shown that under ideal linear pH-gradient conditions the better a model describes the 1/k versus pH plot, where k is the retention factor of the analyte, the better performance it exhibits. However, under real experimental conditions the fitting and prediction performance of a model depend upon its ability to compensate via the regression procedure the various nonidealities, like deviations from the linearity of the pH-gradient profile and the effect of the organic modifier on pK. In general, all models examined exhibit good fitting and prediction performance, and the best model is based on a simple algebraic sigmoid function for 1/k versus pH. However, the main drawback of this model, as well as of all models that are based on the solution of the fundamental equation of gradient elution, is the rather complicated expressions of the retention time. This weakness is overcome by using simple linear models, which give very satisfactory results especially in cases where the retention time varies linearly with the programmed gradient time. The extension of these models so that they are not subject to this restriction is also considered.