Peak tailing and slow mass transfer kinetics in nonlinear chromatography

Abstract

Peak tailing has two main origins in chromatography: (i) heterogenous mass transfer kinetics, (ii) heterogenous thermodynamics with overloading of a nonlinear isotherm. The individual effects of each of these sources and the conditions under which they give rise to peak tailing are now well known. Often however, these different sources combine resulting in complex behavior. This situation is investigated in the case of an adsorbent whose surface is covered with two different types of adsorption sites. The first type of sites has a large specific surface area, a relatively weak adsorption energy and retention and fast mass transfer kinetics. The second type of sites covers a small proportion of the total surface area and has a strong retention and slow mass transfer kinetics. Tailing from different origins combine when the slow type of adsorption sites is operated under a certain degree of overloading, i.e., under nonlinear conditions. To model these phenomena, the transport-dispersive model was used with a modified solid film linear driving force model accounting for mass transfer kinetics on both types of sites. When the rate of mass transfer on the active sites is slow (k2 ≤ 10 min−1), peak tailing of thermodynamic origin is negligible. When the kinetics is fast (k2 ≥ 100 min−1, heterogenous thermodynamics is the source of peak tailing. In the intermediate region (10 < k2 < 100 min−1) peak tailing results from the combined effects of these sources, in an complicated interplay which is detailed in this study. The results of this study provide a most satisfactory explanation of the influence of the experimental conditions under which peak tailing arises in many analytical applications. This is the case, for example, of the chiral separations of small drug molecules on cellulose proteins immobilized as chiral selectors on silica.