IntroductionHigh-performance liquid chromatography (HPLC) has been extensively used as one of the most powerful separation techniques in the field of fine chemistry and biological science. 1 Still, however, it is now strongly required to develop a faster separation system with a high throughput. There are two major approaches for achieving the demand of the times. One is the development of an ultra-fast HPLC system consisting of a column packed with super-fine full-porous spherical particles and a special pump apparatus for delivering the mobile-phase solvent into the column under high-pressure conditions. [2][3][4] However, this approach is accompanied by new problems because the separation conditions are remarkably changed from those for conventional HPLC. Chromatography is carried out under much a faster flow velocity, hence, under extremely higher pressure conditions. The friction of the mobile phase against the packed bed of the super-fine particles leads to the generation of heat. The resulting temperature distribution taking place in both the axial and radial directions of the column provides an unfavorable influence on the peak profile, and hence on the column efficiency. [5][6][7][8] There would be a paradoxical limitation in this approach. Although the separation speed can be amplified with an increase in the mobile-phase flow velocity, the influence of the heterogeneous temperature distribution on the column efficiency must be emphasized. This is an additional problem of the ultra-fast HPLC technique, which can substantially be neglected under conventional HPLC conditions.An alternative is the development of new types of separation media, such as monoliths [9][10][11] and shell-type spherical particles. [12][13][14][15] Compared with the ultra-fast HPLC system described above, the new separation media show a different chromatographic behavior because their structural characteristics, i.e., shapes and porosities, are extremely different from those of the super-fine full-porous spherical particles. For example, in the case of shell-type particles, which consist of an inert center core and a superficial porous shell layer, the following three points can be picked up as their advantageous characteristics. At first, the whole particle size is relatively large. This is effective for preventing any unnecessary elevation of the column back pressure under high flow rate conditions. The size of interparticulate channels in a column packed with spherical particles depends on the particle diameter and their packing density. It is usually estimated to be about one third of the particle diameter, 16 and cannot independently be controlled. Second, the thickness of the superficial porous shell layer is sufficiently thin. It has been well recognized that the column efficiency under high flow-rate conditions primarily rests on the mass-transfer resistance in the stationary phase and that the reduction in the diffusion distance of sample molecules is effective for suppressing band broadening due to intra-stationary phas...