Abstract
Methods for the calculation of a non-linear sorption isotherm from chromatographic data in the presence of significant dispersion effects were compared using chromatographic peaks, generated by a computer from a known Langmuir isotherm. The most accurate and also most easily applied method involves the use of the retention volume at the peak maximum to calculate the slope of the isotherm at the concentration which corresponds to the height of the peak. The so-called method of “elution on a characteristic point” is only accurate when the peak-shaping effects of non-linearity are overwhelming compared with those of dispersion. This condition was not generally met, not even in cases when the diffuse flanks of peaks of various sizes coincided. An equation for non-ideal, non-linear chromatographic peaks developed by other authors under the assumption that the non-linearity effects are small was fitted to the peaks studied. As expected, this is an accurate method for the determination of the isotherm at low concentrations. At higher concentrations, large deviations from the correct isotherm were observed, also when satisfactory fittings were obtained.
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