HUNG-WEI TSUIWen-Lan ZhouCheng-Da Wu2025-05-222025-05-222025-06-21https://www.scopus.com/record/display.uri?eid=2-s2.0-105002493434&origin=resultslisthttps://scholars.lib.ntu.edu.tw/handle/123456789/729654The 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.Adsorbed-layer thicknessAdsorption isothermHold-up volumeRetention factor[SDGs]SDG13journal article10.1016/j.chroma.2025.465952