Ideal model of chromatography applied to charged solutes in reversed-phase liquid chromatography

The ideal model of chromatography assumes that the equilibration of a solute between the mobile and stationary phase is an infinitely rapid process. Under these assumptions, and for a given adsorption isotherm the model can be used to theoretically describe the evolution of a solute zone (peak), as it migrates along the chromatographic column. In this paper hydrophobic charged solutes are injected on a RP-18 column and the ideal model of chromatography is used to calculate the eluting peak shape. The advantage of using charged hydrophobic solutes is that the no linearity of the adsorption isotherm occurs at very low mobile phase concentrations of the solute. Another advantage is that from a theoretical point of view the adsorption isotherm is very well described by the electrostatic surface potential modified linear adsorption isotherm and the physical origin of the non-linearity is well understood. The application of this adsorption isotherm to the ideal model of chromatography is, however, mathematically more complex than for the Langmuir or Freundlich adsorption isotherms which previously have been combined with the ideal model of chromatography. It is shown that for negatively charged solutes the ideal model of chromatography combined with the electrostatically modified linear adsorption isotherm describes well the peak shape of the eluting peak. Since the non-linearity of the used adsorption isotherm is physically well defined, the experimental and theoretical study of eluting peak shapes of charged analytes may be a tool for the characterisation of the chromatographic column and also for the study of the interaction between charged solutes and the stationary phase surface.