Overload behavior and apparent efficiencies in chromatography

Over the last 50 years, many analysts have attempted to analyze column overloading by measuring the drop of the column HETP with increasing sample size and to predict elution band profiles from overloaded columns by combining the influences of the thermodynamic overloading and of axial dispersion, using a simplistic perturbation model. This approach violates the principle of mass conservation. The results of the predictions do not agree with those of exact calculations made with the equilibrium-dispersive (ED) model of chromatography for constant axial dispersion. The plots of the reduced apparent column efficiency N/Nkin, versus the injected concentration, log C0, or the injected mass, log m0, may provide useful information only regarding the onset of overloading of any particular column but they are meaningless to compare the overloading behavior of columns packed with different packing materials, unless the columns used satisfy impractical requirements (same efficiency, same retention factors, and sample sizes used proportional to the volume of stationary phase in the column).

► Elution band profiles change shape progressively when the sample size is increased. ► A dispersive convolution of a thermodynamic profile may model the evolution of these profiles. ► The sum of the band variances of these two terms does not provide the variance of the elution profile. ► Because in nonlinear chromatography, these two variances are not additive. ► Only knowledge of the adsorbent saturation capacity informs on the degree of column overloading.