Modeling the retention of neutral compounds in gradient elution RP-HPLC by means of polarity parameter models.

A three-parameter expression, which was already employed before for the prediction of the retention time in gradient mode in reversed-phase high-performance liquid chromatography (RP-HPLC) with satisfactory results, has been tested here under a variety of gradient patterns, using methanol and acetonitrile as the organic modifiers. A wide variety of compounds have been employed as test solutes, including some complex ones used as drugs, such as hydrocortisone and estriol. Simplifications of this expression have been made by considering two- and one-parameter expressions based on the p polarity parameter model, which was successfully employed before in isocratic mode to perform predictions of retention time. The advantages that this model gives in isocratic conditions, namely simplicity and less previous experimental work, have been applied with profit in its application to gradient mode. Good correlations between the experimental retention times and the predicted ones have been obtained with both equations in most cases.