The implicit role of the hold-up volume in defining the adsorbed layer thickness

The retention factor is a fundamental parameter for interpreting adsorption behavior in chromatography. However, its calculation requires a hold-up volume—often taken as the column's true void volume—without explicitly considering the thickness of the adsorbed layer. This study reveals that such an approach implicitly adopts an excess-adsorption perspective. Different definitions for the adsorbed-layer thickness affect whether a resulting isotherm is interpreted as excess or total adsorption. The results highlight the necessity of carefully aligning the choice of hold-up volume with the intended theoretical framework—such as Langmuir or BET models—to avoid misinterpretations in thermodynamic analyses. By emphasizing the subtle but critical link between the adsorbed-layer definition and adsorption data, this study provides a clearer foundation for understanding adsorption and retention mechanisms.