Fitting competitive adsorption isotherms to the distribution data in normal phase systems with binary mobile phases

To investigate the effects of the solvents on the overloaded separations in normal-phase systems, the distribution isotherms of benzophenone and of co umarin as model compounds were determined between a silica gel column and b inary solvents containing various concentrations of 2-propanol in n-hexane and in dichloromethane and of dichloromethane in n-heptane. The distributio n data in the individual solvent systems can be formally described by the L angmuir adsorption isotherm. but the quadratic isotherm generally improves the fit to the experimental data. Various underlying adsorption models yiel ding the three-parameter quadratic isotherm an compared. The distribution d ata of mixed samples in binary solvent systems can be described by a two-co mponent quadratic isotherm with six parameters, of which four are single-co mponent isotherm coefficients. A six-parameter or a five-parameter two-comp onent quadratic isotherm should be used to describe adequately the distribu tion of a single sample compound over a wide composition range of mixed bin ary solvents, taking into account the competition of the polar solvent for the adsorption sites. The description of the distribution of a two-componen t sample mixture by a three-component quadratic isotherm requires three add itional coefficients to respect the competitive adsorption of the sample co mpounds. The validity of the equations proposed to describe the distributio n data in normal-phase systems was verified by comparison of the numericall y calculated and the experimental band profiles. (C) 1999 Elsevier Science B.V. All rights reserved.