The study of column packing homogeneity and flow profiles in packed chromatographic columns

This work continues a long history of efforts to study the flow of fluids through packed beds-- more specifically, through beds made of uniform packing particles, and generated under high pressure, -- and to probe the sources of band broadening in liquid chromatography, referred to more commonly as axial dispersion.

Any investigation of the radial distribution of local chromatographic characteristics should consider on-column, local, multichannel detection. The results presented in this work were obtained with an experimental set-up which can accommodate at least 10 optical fiber assemblies functioning as on-column, local microdetection probes when implanted into the exit frit of an HPLC column.

High performance liquid chromatographic columns are not radially homogeneous. A rather homogeneous core is surrounded by a thick layer of heterogeneous packing situated along the column wall. The thickness of this layer is variable. The flow distribution in these columns is cylindrical, nearly flat in the column center, within approximately half the column radius from the center and falls behind rather rapidly towards the wall. Because the concentration profile of an analyte along the wall passes later and is wider than in the column center, the signal of a bulk detector is bound to exhibit an increased width, and some degree of tailing. The extent to which the peak tails is proportional to the degree of deformation of the mobile phase flow profile.

The local column efficiency is at most locations higher than the overall efficiency determined with an on-line conventional detector. It varies across the column cross-section as follows: it is higher in the central core region, while in the external region it decreases with decreasing distance to the column wall. As a consequence, analytical columns are operated under experimental conditions where a significant fraction of their potential performance is wasted. A small concentration sensor placed at the column axis would see narrower, better resolved and more concentrated bands.

The consequence of this heterogeneous behavior of the column is that the conventional elution profile is a complex convolution of the convection and dispersion phenomena taking place in a chromatographic column. Obviously, two space coordinates, the column length and its radius and not only the former, are needed in a realistic model of chromatography [Guiochon, 1995].