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When analyses of mixtures of small molecules are carried out at mobile phase velocities close to (for isocratic runs) or somewhat above (for gradient runs) the optimum velocity, the eddy diffusion term contributes to at least 75% of the band broadening. Future improvements in column performance may come only from a reduction of the eddy diffusion term. The classical models of axial dispersion of Gunn and Giddings are revisited and their predictions compared to recently reported eddy dispersion data obtained by solving numerically the Navier-Stokes equations and simulating advective-diffusive transport in the bulk region and in confined geometries of reconstructed and computer-generated random sphere packings. The Gunn model fails to describe these data. In contrast, the Giddings model succeeds, provided that his original guesses regarding the values of two parameters of his model are adjusted. Accurate measurements of real eddy dispersion data in modern high-pressure liquid chromatography (HPLC) columns were performed by applying a well established experimental protocol. Their results demonstrate that the other contribution to band broadening, sample dispersion in the homogeneous bulk region of these packed beds, accounts for less than 30% of the total eddy dispersion at velocities larger than the optimum velocity. This shows that the resolution power of modern HPLC columns is essentially controlled by wall and/or border layer trans-column eddy dispersion effects, depending on whether the column is radially equilibrated or not. Under a preasymptotic dispersion regime, the performance of short and wide HPLC columns is controlled by the border effects. As the bed aspect ratio (D/dp) increases, the column performance tends toward that of the infinite diameter column. Further improvement appears possible using radial segmentation of the outlet flow. Under an asymptotic dispersion regime, the reduced column plate height of long and thin HPLC columns is controlled by the wall effects and can be optimized only by improving the packing procedures, keeping as low as possible the bed aspect ratio and maximizing the transverse dispersion coefficient.
Gritti et al. (Fri,) studied this question.