Retention by adsorption at a solid surface in contact with a liquid mobile phase defines liquid–solid chromatography (LSC), also called adsorption liquid chromatography. Solute molecules compete with eluent molecules for sites on an adsorbent such as silica or alumina; the adsorption isotherm and the eluent composition together fix the retention factor ^[1]. Mechanistic models relate retention to the mole fraction of the strong solvent in a binary eluent through displacement of adsorbed solvent from the surface ^[2]. LSC is thereby distinct from liquid–liquid (partition) chromatography, in which the stationary phase is a liquid film, and from reversed-phase bonded-phase chromatography, in which a grafted organic layer dominates sorption. Essential features are a high-surface-area solid, a mobile phase whose polarity is matched to the adsorbent so that adsorption remains reversible, and a measurable competition between analyte and eluent for surface sites. High-pressure implementations of the same adsorption mechanism were used to resolve ecdysones and other moderately polar natural products on silica ^[3]. Conceptual boundaries also exclude size-exclusion and ion-exchange modes that do not rely on surface adsorption.