ROLE OF THE MODULATOR IN GRADIENT ELUTION CHROMATOGRAPHY

Mobile-phase additives are frequently used in gradient elution chromatography to modulate adsorbate retention. These additives are known to adsorb themselves onto the stationary phase, resulting in solvent demixing. In this paper, the concentration of the mobile-phase additive in the mobile phase is assumed to be high enough for it to lie in the nonlinear region of its own adsorption isotherm. Then the shape of the gradient would become deformed while passing down the column and could ultimately form a shock layer. A series of numerical simulations of a binary feed mixture is presented under conditions where a shock layer is formed, and the possible consequences are discussed. Depending on the location of the adsorbate peak with respect to the mobile-phase shock layer, leading or tralling shoulders can result, along with significant peak sharpening. The implications of these effects on separation are presented, and conditions under which the present analysis might be tested experimentally are indicated.