Theoretical study of system peaks and elution profiles for large concentration bands in the case of a binary eluent containing a strongly sorbed additive

Abstract

The profile of system peaks when using high concentrations and a binary mobile phase was studied using the semi-ideal model of chromatography. The profile of the sample band depends on the concentration of the strong solvent and on the sample size. It is very strongly influenced by the relative adsorption strength of the strong solvent and the sample. This relative strength is measured by the ratio of (a) the origin slope of the equilibrium isotherm of the strong solvent between the stationary phase and the pure weak solvent to (b) the origin slope of the isotherm of the sample ( i.e., the ratio between their respective column capacity factors in the pure weak solvent). When this ratio is smaller than 0.2, the sample band profile depends only on the equilibrium isotherm of the sample in the binary mixture. If the ratio becomes larger, the competition between the strong solvent and the sample molecules for interaction with the stationary phase becomes more intense. For Langmuir-competitive isotherms of the strong solvent and the sample, if the concentration of the strong solvent and/or its strength are progressively increased at constant sample size, the retention time of the band decreases (as expected), whereas its typical Langmuirian asymmetry decreases and, past a narrow transition range, reverses. Bands traditionally associated with an anti-Langmuir isotherm are then obtained. Extremely unusual, broad, characteristic profiles, with a sharp front and a sharp rear, which sometimes exhibit two maxima, are obtained in the transition region. When the origin slope ratio is large, the profile of a high-concentration-sample band in the transition region has two shock layers. The occurrence of such band profiles, under experimental conditions similar to those predicted here, have been reported previously, but their origin has remained unexplained until now.