COMPARISON OF THE VARIOUS KINETIC-MODELS OF NONLINEAR CHROMATOGRAPHY

It is not possible to separate simply the influences of a slow mass transfer kinetics and of a slow kinetics of adsorption-desorption on the elution profile of a component. Four kinetic models of chromatography are studied: (i) the Thomas or reaction model, which assumes Langmuir kinetics of adsorption-desorption and no axial dispersion; (ii) the reaction-dispersive model, which uses the same Langmuir kinetics as the Thomas model but assumes a finite axial dispersion; (iii) a transport model, which uses the linear solid film driving force model to account for a slow kinetics of mass transfer and assumes no axial dispersion; and (iv) a transport-dispersive model, using the same mass transfer kinetics as the transport model and assumes finite axial dispersion. The analytical solution of the Thomas model can be fitted, with an accuracy which exceeds the precision of experimental measurements, on bands calculated using either one of the other three kinetic models of chromatography. Thus, the Thomas model, which assumes a slow adsorption-desorption kinetics and infinitely fast mass transfer kinetics, accounts very well for profiles calculated with a model making the reverse assumption. However, the values of the lumped kinetic coefficient obtained by curve fitting depend on the sample amount. Thus, the examination of an elution profile and its fitting to a model do not permit an easy solution of the inverse problem of chromatography.