Surface of the HPLC adsorbent is the major factor influencing the retention. Geometry of the surface define accessibility of the adsorption sites. According to the basic retention equation, adsorbent surface area is directly proportional to the retention of analyte.
Shape of silica surface is basically unknown. The only method which allows direct measurement of the surface shape is atomic force microscopy (AFM). It is based on the scanning of the surface with a special probe. AFM could be employed only for the outer surface measurement.
Shape of the silica surface
Geometry of the inner surface of the adsorbent particles is assumed to be smooth, especially after hydrothermal treatment of silica xerogel.
The most important question, is the presence or absence of the small pores, which may
produce a steric hindrance to the analyte molecules to penetrate inside. This may be
determined from the pore size distribution curve calculated from the gas adsorption