GRADIENT ELUTION CHROMATOGRAPHY AT VERY HIGH COLUMN LOADING - EFFECT OF THE DEVIATION FROM THE LANGMUIR MODEL ON THE BAND PROFILE OF A SINGLE COMPONENT

The elution profiles of very large samples of 2-phenylethanol obtained with steep concentration gradients are compared with those predicted by theory. The main difficulties encountered in this modeling stem from the needs of an accurate determination of the adsorption data at high concentrations, an accurate modeling of these data and the determination of the exact injection profile. In gradient elution the high-concentration part of the isotherm influences the band profiles more than in isocratic chromatography. The importance of an accurate equilibrium isotherm is much higher for the accurate simulation of profiles obtained in gradient elution than for those provided by isocratic elution, because in the latter case, the bands dilute steadily during their migration while in the former case they concentrate. Similarly, samples injected in gradient elution have typically a much larger volume and are more dilute. Unusual profiles are predicted and observed in agreement with these predictions, when the sample amount is of the order of the column saturation capacity and/or the sample volume is of the order of the retention volume at the mobile phase composition of the gradient start.