Prediction of the chromatographic retention of acid–base compounds in pH buffered methanol–water mobile phases in gradient mode by a simplified model

• Gradient retention model for acid–base analytes with methanol as organic modifier.
• The model embodies equations for the analyte pKa evolution along the gradient.
• The model is validated for some drugs, buffered mobile phases and gradient patterns.
• Good prediction ability is achieved despite little experimental effort is required.

Retention of ionizable analytes under gradient elution depends on the pH of the mobile phase, the pKa of the analyte and their evolution along the programmed gradient. In previous work, a model depending on two fitting parameters was recommended because of its very favorable relationship between accuracy and required experimental work. It was developed using acetonitrile as the organic modifier and involves pKa modeling by means of equations that take into account the acidic functional group of the compound (carboxylic acid, protonated amine, etc.). In this work, the two-parameter predicting model is tested and validated using methanol as the organic modifier of the mobile phase and several compounds of higher pharmaceutical relevance and structural complexity as testing analytes. The results have been quite good overall, showing that the predicting model is applicable to a wide variety of acid–base compounds using mobile phases prepared with acetonitrile or methanol.