Adsorption mechanisms in normal-phase chromatography. Mobile-phase modifier adsorption from dilute solutions and displacement effect.

The adsorption equilibria of methanol from diluted solutions of the alcohol in dichloromethane (the weak solvent of the binary mixture) have been investigated in normal-phase conditions on an hybrid polymeric chiral stationary phase. This was done by analyzing the shapes of nonlinear perturbations of methanol, measured with a refractive index detector from a column in equilibrium with pure dichloromethane. Experiments have been set up to demonstrate the displacement effect, predictable from the adsorption data, by a nonlinear pulse of methanol on a racemic mixture of temazepam injected at trace level. A most used method in fundamental studies of band behavior in chromatography has been used for modeling the system. An extraordinary increase of the apparent column efficiency has been observed due to the displacement phenomenon, with the peak of the second eluted enantiomer noticeably higher and narrower than that of the first eluted component.