An exact knowledge of the adsorbent surface area is very important, since retention is proportional to the surface area of the adsorbent.
The most popular method of the surface area measurement is low temperature nitrogen adsorption. At the low temperature most of the adsorbent does not show any specific interactions, and adsorption of the nitrogen molecules can be approximated by the monolayer model. The level of the plato on the isotherm corresponds to the amount of the molecules in a dense monolayer. One nitrogen molecule usually occupy 16.2 Å2 on the polar surface. Thus, if we know the amount of the molecules and the surface area per one molecule, we can calculate the total surface area of the adsorbent.
These measurements give the value of the total surface area of HPLC adsorbents. The most important question is which part of the total surface actually participate in the chromatographic retention. Molecules of the HPLC analyte are usually bigger than nitrogen molecules, and they might not be able to penetrate all pores, thus an effective surface area which participated in retention is actually lower than the total surface area.
Answer on this question still remains open.
Chemical modification of the adsorbent surface should also have an influence on the
surface area and also on the pore volume of the adsorbent. This effect is very
significant, silica gel with original surface area of 350 m2/g after chemical
modification with octadecylsilane reduce its surface to 170 m2/g. This effect
strongly depends on the density of the bonded layer and length of the attached chains.