Nonlinear frequency response method for estimation of single solute adsorption isotherms. Part II

The aim of this paper is to investigate the possibility of applying a nonlinear frequency response (FR) method described by Ilić et al. [2007b. Nonlinear frequency response method for estimation of single solute adsorption isotherms. Part I. Theoretical basis and simulations, this volume, doi:10.1016/j.ces.2007.05.004] to determine single solute adsorption isotherms. The method is based on the analysis of the nonlinear FR of a chromatographic column to a periodical change of the inlet concentration around several steady-state concentrations. The first, second and third local derivatives of single solute isotherms are estimated from the low-frequency asymptotic behaviour of the corresponding FR functions. The adsorption of 4-tert  -butylphenol and ethyl benzoate as single solutes is studied on octadecyl silica (C18)(C18) from methanol–water mixture (60:40). Experiments are performed using a standard gradient HPLC unit capable to realize inlet concentration changes in a nearly sine waveform. Small injections of compounds into the not preloaded column, as well as conventional frontal analysis experiments are performed for rough estimation of the adsorption isotherm coefficients and comparison with the results obtained using the nonlinear FR method.