The common packing materials we use in HPLC columns are silica, hydroxyapatite media and polymeric resins, as many of types of packing used for gravity or low-pressure chromatography cannot stand the high pressures which are used in an HPLC separating system.
Due to the increased surface area, smaller-diameter beads can improve the sensitivity of separation. For an analytical HPLC column, media bead diameters are usually in the range of 1.8–5 μm. We should set the lower limit of bead size, as column pressure increases and bead diameter is reduced for a given flow rate, at the same time.
In the early time, we pack early HPLC columns with irregularly shaped silica particles to increase surface area. When scientists found that the spherical shape could provide increased efficiency and the porosity of the surface area, irregular silica was replaced by spherical porous silica.