EFFECT OF MODULATOR SORPTION IN GRADIENT ELUTION CHROMATOGRAPHY - GRADIENT DEFORMATION

In gradient elution chromatography, the mobile phase composition at the column inlet is a function of time. The consequences of accounting for the adsorption of the mobile phase components themselves are investigated for the experimentally common situation of a linear inlet gradient. Surface excess adsorption data from the literature for water-acetonitrile mobile phases in reversed-phase chromatography using different octadecyl stationary phases are shown to be in good internal agreement. The resulting individual isotherms as calculated by Tani and Suzuki (1989) are fitted to Langmuir and BET forms; it is shown that the Langmuirian form fits the data well except at extremely high acetonitrile concentrations. Using these isotherm parameters, it is found that a linear inlet gradient could suffer significant distortion while moving down the column. In the extreme case, a shock front of the mobile phase modulator could result. Analytical expressions are derived describing the conditions under which such a shock could occur within the column and its subsequent path, assuming Langmuirian sorption.