Quantitative comparison between the experimental band profiles of binary mixtures in overloaded elution chromatography and their profiles predicted by the semi-ideal model

Abstract

The experimental elution profiles of the components of a binary mixture were determined under conditions where the column is strongly overloaded and the two bands interfere markedly. These profiles were derived from the analysis of approximately 100 fractions collected during the elution of large samples of synthetic mixtures of two similar compounds having different compositions. The system investigated was the separation of 2-phenylethanol and 3-phenylpropanol on Vydac ODS silica eluted isocratically with methanol—water (50:50). The results demonstrate the displacement and the “tag-along” effects previously predicted on the basis of theoretical investigations of the elution behavior of binary mixtures in non-linear chromatography. The displacement of the first band by the second takes place essentially when the concentration of the second eluted component is the larger. The tag-along effect of the second component is observed when the major component of the mixture is the first eluted. Comparison between the experimental profiles and the profiles predicted by the semi-ideal model was performed using the competitive Langmuir isotherm model. For numerical calculations, the parameters of these isotherms were determined by measurement of the single-component isotherms of each of the two compounds by frontal analysis. The results of the comparison show that the competitive Langmuir isotherm gives only a fair approximation of the multi-component adsorption behavior of the two compounds studied. Competition between them for adsorption seems to be stronger than predicted by the competitive Langmuir model.