termThe eddy dispersion term of the van Deemter equation has beenthe interest of many researchers and the mechanistic interpreta-tion of this parameter has improved substantially, although there isstill a degree of retrospective fitting of experimental data to allowfor optimization of the postulated theory. Specifically the use ofthe Knox equation [77] which relates an inverse cubic depend-ency of the linear velocity to the band broadening has proved verysuccessful in modeling of experimentally derived data. The disper-sion associated with this term can be rationalized as coming fromboth the size of the particles and also the quality of the packing. Ithas been shown that the packing density within a column variesradially as a result of the wall effect, which results in increasedband broadening. Thus there are two very distinct components tothe ‘A’ term, one which relates to the short range packing effectsassociated with a regular packing structure of spheres, and theother one being a longer range effect which occurs due to radialinhomogeneity across the column.In the initial marketing literature that was associated with thelaunch of a range of solid core materials, it was suggested that someof the benefits arose because of the tighter particle size distributionwhich resulted in better packing of the particles. This is an easy con-cept to visualize, however in practice the variation of the particlesize distribution that is quoted for fully porous materials and forsolid core materials is simply not large enough to cause an adverseeffect, and thus the concept is simply not applicable to the observedhigh efficiencies associated with solid core materials. A typical solidcore particle will be quoted as having a d90/10of 1.1 compared to afully porous material which has a d90/10of 1.7. It has been shownthat