THEORETICAL-STUDY OF HIGH-CONCENTRATION ELUTION PROFILES AND LARGE SYSTEM PEAKS IN NONLINEAR CHROMATOGRAPHY

The profiles of high concentration bands depend essentially on the equilibrium thermodynamics of the sample components in the chromatographic system. When the mobile phase contains an additive which equilibrates between the two phases, and system peaks take place, high concentration solute bands which elute closely to large primary system peaks may have very unusual band shapes. A prerequisite for the occurrence of such distortions is that the additive be more strongly retained than the solute in the pure weak solvent. Depending on the relative retention, a, of the primary system peak and the solute peak at infinitesimal sample sizes but finite additive concentrations in the mobile phase, five main types of chromatograms may arise at large sample loads. (i) If alpha is much larger than 1, the solute band has a Langmuirian profile, i.e. a steep front and a diffuse rear. (ii) For alpha slightly larger than 1, the band shape is still Langmuirian, but has a somewhat drawn-out rear, and at very large sample sizes a hump appears on the rear of the profile. (iii) For alpha equal to 1, the solute peak profiles are broad and drawn-out at both ends. (iv) For alpha slightly smaller than 1 an anti-langmuirian peak shape is observed, with a diffuse front and a steep rear; a hump appears at high sample loads, but on the peak front. (v) For alpha much smaller than 1, the peak shape is anti-langmuirian, without hump even at large sample sizes. Since ct depends on the mobile phase concentration of the additive, an adjustment of this concentration in the range where alpha is close to 1 may have a tremendous impact on the profiles of high concentration bands.