(341g) Local Equilibrium Theory Analysis of Chromatographic Peak Shapes in the Presence of Adsorbing Modifiers

Modifiers( or co-solvents) are often added to the mobile phase in many chromatographic separations.Very often these modifiers compete for adsorption sites along with the solute. The nature of modifier adsorption (stronger or weaker than the solute) can have a distinct effect on the shape of the solute elution profiles. Although the adsorption isotherms of both the modifier and solute can be Langmurian, the elution profiles may display either a Langmuirian (sharp front followed by a dispersed rear) or an anti-Langmuirian shape (dispersed front followed by a sharp rear). In addition, the formation of rounded peaks (that are neither strictly Langmuirian nor anti-Langmuirian) has been observed in the literature [1,2] and an explanation for this has not been thusfar available. In this study, the local-equilibrium theory of chromatography has been used to arrive at explicit conditions under which Langmuirian and anti-Langmuirian peaks are obtained and for the first time the origin of the "rounded" peaks has been fundamentally explained. The extension of this study to bi-Langmuir isotherms will also be discussed.

1. T. Fornstedt, G. Guiochon, Theoretical study of high-concentration elution profiles and large system peaks in nonlinear chromatography, Anal. Chem. 66 (1994) 2116–2128.

2. T. Fornstedt, G. Guiochon, Comparison between experimental and theoretical profiles of high concentration elution bands and large system peaks in nonlinear chromatography, Anal. Chem. 66 (1994) 2686–2693.