The equilibrium relation between the concentration of a sample component in the stationary phase cS, and in the mobile phase cM, expressed as a function cS = f.cM. In some versions of chromatography, a distribution isotherm can be seen as a partition isotherm, an adsorption isotherm, or a combination of these, depending on the mechanism of separation.
Note:
The relation can be influenced also by concentrations of other sample components. cS and cM are usually expressed per unit volume of the phase; cS may also be expressed per mass of the dry solid phase or per unit surface area.
Source:
PAC, 1996, 68, 1591. 'Nomenclature for non-linear chromatography (IUPAC Recommendations 1996)' on page 1592 (https://doi.org/10.1351/pac199668081591)