Retention mechanism of weak polar solutes in reversed phase liquid chromatography

The retention mechanism of a weak polar solute, a series of 10 benzodiazepines in reversed phase liquid chromatography, was investigated over a wide range of mobile phase compositions. The values of enthalpy (Delta H degrees) and entropy (Delta S degrees) of transfer from the mobile to the stationary phases were determined. The method studied each factor (water fraction Phi in the acetonitrile (ACN)/water mixture and column temperature) controlling the retention mechanism. The changes in Delta H degrees and Delta S degrees as a function of the water fraction Phi in the ACN/water mixture were examined. These variations are explained using the organization of organic modifier (ACN) in clusters in the ACN/water mixture. A change in the retention mechanism thus indicated when the ACN/water mixture was used instead of the hydrogen-bonded mobile phase such as CH3OH/water. Enthalpy-entropy compensation revealed that the retention mechanism was independent of the water fraction Phi but showed that differences between the molecular structures of the benzodiazepines contributed more significantly to changes in the retention process in the CH3-OH/water mixture than in the ACN/water mixture.