Physical evidence of two wall effects in liquid chromatography

Using optical on-column visualization for the study of the migration of sam ple bands, the radial variations of the local migration rate were studied i n the region near the column wall. Photographs of small sample bands migrat ing along the column at various radial locations were obtained. On-column c hromatograms extracted from these photographs showed evidence of two wall e ffects. The first of these effects was present only within the immediate vi cinity of the wall. It is a direct result of the inability of the packing m aterial to form a close packed configuration against the rigid column wall surface. The second wall effect causes a systematic variation of the migrat ion rate of the sample band in the region of the wall, this rate increasing from the wall to the central region of the column. The corresponding image s portrayed the classical "wall effect" that chromatographers have long dis cussed. They also show that this effect extends further into the column tha n anticipated. As to what are the relative contributions to the results of our observations of the wall effect and of a frit effect discussed in previ ous publications, this could not be ascertained. (C) 2000 Elsevier Science B.V. All rights reserved.