The shock layer thickness, a new approach to the study of column performance in non-linear chromatography: I. Optimum linear velocity in frontal analysis

Abstract

In non-linear chromatography, it is common to observe very steep profiles. This happens for overloaded elution bands, for frontal analysis breakthrough curves and for the band profiles of the isotachic train in displacement chromatography. These regions where the concentration vary very rapidly are called shock layers. The relationship between the thickness of the shock layer in frontal analysis and the coefficients of the conventional terms of the plate height equation were studied experimentally. The shock layer theory of Rhee and co-workers permits the simple determination of the optimum linear velocity for minimum shock layer thickness in the case when the adsorption behavior of the feed components is described by the Langmuir model. The optimum linear velocity in frontal analysis is not only a function of the coefficients of the axial dispersion and the mass transfer resistance terms, and the retention factor ( k 0 ) ́ , as in linear chromatography, but also a function of the plateau concentrations and the second Langmuir parameter of the isotherm, b. Depending on the retention factor, the optimum velocity in frontal analysis may be larger, but is most often much smaller than in linear chromatography. Experimental results are in excellent agreement with the prediction of the theory. If they could be extended to displacement chromatography, these findings would explain some apparent contradictions found in the literature regarding the influence of the mobile phase flow velocity on the degree of separation between bands achieved in displacement chromatography, and clarify certain controversies.