Local equilibrium theory analysis of chromatographic peak shapes in the presence of adsorbing modifiers

•Study of peak shapes in the presence of an adsorbing modifier.•Explanation of the origin of Langmuir and anti-Langmuir peak shapes.•Theoretical basis for the occurrence of “unded peaks”.

The nature and shapes of solute and modifier peaks for a situation when the modifier and solute adsorb competitively through the Langmuir isotherm is studied using the local-equilibrium theory. The analysis yields 10 different cases depending on the relative adsorption strengths and the concentrations of the modifier and the solute. The origin of positive and negative Langmuirian/anti-Langmuirian-type peaks, even when the adsorption isotherms are strictly Langmuirian, and explicit conditions under which each combination is observed are derived. A fundamental explanation to the origin and propagation of rounded peaks reported in liquid and supercritical fluid chromatography [1-3] is offered. A rigorous theoretical basis for the rule of thumb proposed by Fornstedt and Guiochon [1,2] about the nature of peak shapes when an adsorbing modifier is present, is established.