The HPLC column is usually packed with small adsorbent particles, preferably spherical in shape.

Electron microphotograph of spherical and irregular silica particles. [W.R.Melander, C.Horvath, Reversed-Phase Chromatography, in HPLC Advances and Perspectives, V2, Academic Press, 1980]

Milling of silica xerogels is the most usual way to produce irregular particles. Subsequent sieving allows to separate particles of the certain size and distribution. Chromatographic practice has shown that columns packed with irregular adsorbent particles usually have lower efficiency compare to the columns packed with spherical adsorbent particles, the last are usually more expensive though.

Several different procedures of manufacturing the spherical silica particles have been introduced. The basic principles are emulsification of a silica sol in an immiscable liquid and convertion ofthe formed droplets into gelatinous beads of silica hydrogel. The droplet size can be controlled by the viscosity of the sol.

Macroporous spherical silica particle. [K.K.Unger, Porous silica, Elsevier, 1979]