Mechanism of retention of benzodiazepines in affinity, reversed-phase and adsorption high-performance liquid chromatography in view of quantitative structure retention relationships

Abstract

Quantitative structure-retention relationships (QSRRS) were compared for a set of 1,4-benzodiazepine (BDZ) test solutes analyzed by high-performance liquid affinity chromatography (HPLAC), reversed-phase (RP) and adsorption (normal-phase, NP) high-performance liquid chromatography (HPLC). The HPLAC data reflected the enantioselective retention on a human serum albumin-based chiral stationary phase (HSA-CSP); RP-HPLC data were determined on a specially deactivated hydrocarbon-bonded silica material; NP-HPLC was performed on a graphitized carbon stationary phase. Molecular descriptors reflecting additive-constitutive properties of the solutes as well as their geometry and electron charge distribution were generated using molecular modelling software. The QSRR equations derived for each chromatographic mode involved different sets of molecular descriptors. Analysis of the physical meaning of the individual descriptors allowed for interpretation of separation mechanisms at molecular (submolecular) level and rationalization of the observed separation patterns. For HPLAC the contributions by hydrophobic, steric and electrostatic factors were quantified and accounted for the differential retention of enantiomers. Retention in RP-HPLC was demonstrated to be a net effect of both non-specific (dispersive) and directional (electrostatic) intermolecular interactions between solute and molecules of both stationary and mobile phases. QSRR equations derived for NP-HPLC proved the predominance of specific dipolar and charge-transfer attractive interactions. The QSRR-based models obtained appear a reliable and convincing proof for involvement of distinctive mechanisms in different HPLC systems.