Modeling of peak profiles. Application to the preparative liquid chromatography of steroids

The solution of the mass balance equations in liquid chromatography describes the propagation of signals of finite concentration through the column. The general numerical solution requires the prior determination of the partition isotherm. For low solute concentrations, when the isotherm equation can be replaced with a two-term expansion, an analytical solution for the peak profiles is obtained. The theory is applied to predict, as a function of solute concentrations, the elution profiles of two steroids of similar structure, cis- and trans-androsterone, with organic solvents as the mobile phase and buffered silica gel as the stationary phase. At infinite dilution both steroids are well resolved, the trans isomer being eluted before the cis isomer. At high concentrations their adsorption isotherms intercept each other and, for large amounts injected, their elution order is reversed, with marked differences in the elution peak shapes of both steroids.