ISOTHERM MEASUREMENT BY FRONTAL CHROMATOGRAPHY IN THE PRESENCE OF AN ADSORBING MOBILE-PHASE MODULATOR

Frontal chromatography, in which the feed mixture is continuously fed into a chromatographic column, is commonly used td measure adsorption isotherms. Ion-exchange frontal systems with multiple feed components have been widely studied for the case where the total concentration (i n equivalents) is constant, both in the mobile phase and on the statio nary phase. In the present work, the assumption of constancy of the to tal mobile phase concentration is relaxed, which allows the frontal me asurement of single feed components in the presence of a buffer salt ( modulator) at any composition in ion exchange. Since isotherms have to be measured in the presence of the modulator, it may be better to use a range of modulator concentrations as well as a range of feed concen trations, for both ion-exchange and reversed-phase systems. It is show n that isotherms measured in the presence of modulators can provide tw o data points toward the feed-modulator multicomponent isotherm surfac e. An alternative approach gives one data point toward the feed-modula tor isotherm and one data point toward the single-component isotherm o f the modulator for reversed-phase chromatography. The selectivity of a single product and a modulator is discussed for ion-exchange chromat ography, and an explicit result is given for the curve of unit selecti vity. Simulations are presented to show that selectivity reversal, whi ch plays a crucial role in the column dynamics of the elution and disp lacement modes, is less of an issue in the frontal measurement of mult icomponent isotherms. However, due to geometric distortion, selectivit y reversal may be erroneously thought to occur in certain representati ons of adsorption data as line isotherms. Various line isotherm repres entations are discussed, and one that is free of distortion is recomme nded.