The combination of partition, size exclusion, and hydrodynamic models in chromatography, and application to bonded phases on porous supports

Liquid-liquid partition chromatography has been used for many years as a model and teaching introduction to column chromatography. However, the partition model does not describe separations on bonded phases with porous supports particularly well, especially regarding the thermodynamics controlling solute distribution. Further difficulties arise when more than one mechanism is involved in solute retention. Nomenclature is not perfectly aligned with the underlying thermodynamic descriptors and is inconsistently applied over various chromatographic techniques. Presented here is a general description of retention that spans partition, size exclusion, and hydrodynamic separation processes, and is then applied to bonded-phase separations on porous supports. The model provides a general description applicable to adsorption, reversed-phase, hydrophilic interaction, size-exclusion, hydrodynamic chromatography, and any combination of these techniques including liquid chromatography at the critical condition. Further expansion to include retention by ion-exchange and field-flow fractionation appears to be possible. Recommendations on retention factor definition and evaluation are given. (C) 2020 Elsevier B.V. All rights reserved.