General solution of the ideal model of chromatography for a single-compound band

A general analytical solution of the ideal model of chromatography for a single-compound band is derived. This solution is valid for any concave or convex isotherm but not for those that have an inflection point. This solution is applied to the case of a two-site adsorption model, with an equilibrium isotherm that is the sum of two Langmuir-type expressions, corresponding to each of the two sites. The isotherm for the higher energy site has a larger b coefficient (expressing the stronger solute-stationary phase affinity) and a lower saturation capacity than the isotherm for the lower energy site. As predicted by Giddings long ago, the two-site model accounts very well for the tailing that appears on the experimental band profiles, in the cases examined. Thus, tailing cannot be explained only by a slow kinetics of the retention process. The asymptotic solution of the ideal model and its properties are also discussed.