Nonlinear frequency response method for estimation of single solute adsorption isotherms. Part I. Theoretical basis and simulations

A method for determination of single solute adsorption isotherms based on the analysis of the nonlinear frequency response (FR) of a chromatographic column is developed theoretically. In order to demonstrate this method, the FR of a chromatographic column is simulated for the periodical inlet concentration changes (sine waveform) around several steady-state concentrations. The frequency response functions (FRFs) up to the third order are calculated from the inlet and outlet concentration changes, which are analysed using the fast Fourier transform. Afterwards the coefficients of an adsorption isotherm are estimated from the low-frequency asymptotes of the FRFs and their derivatives. The obtained coefficients compare well with those used for simulations. Also discussed are some aspects that might affect the accuracy of the suggested method when it is applied to the analysis of experimental data, e.g. influence of velocity fluctuations, noise and size of the analysed data set.