The BET technique uses adsorption of an inert gas (in our case nitrogen) to measure the outer and inner surface area of a solid sample. The sample has to be heat and vacuum pre-treated to remove the adsorbed molecules (mainly water) that might occupy the surface. After the pre-treatment procedure the sample is transferred to the measurement position. As the inert nitrogen and solids interact weakly, the solid material is cooled to the temperature of liquid nitrogen and kept under isothermal conditions during the whole measurement. The pressure of the nitrogen gas is increased gradually and a monolayer of adsorbed nitrogen molecules is built on the solid surface. The number of gas molecules in the formed monolayer is determined from the adsorbed gas volume. The “footprint” of the nitrogen molecule on the surface is known and the total surface area of sample can be calculated by using the BET equation. The mass specific surface area [m²/g] is derived from this total surface by dividing it with the mass of the sample. Multiplying this value with the skeletal density of the material [g/cm³] provides the volume specific surface area of the material.

Nitrogen adsorption measurements can provide information not only on the external surface area but also on the porosity of samples. Specific surface area affects functional properties of many different type of materials, including pharmaceuticals, catalysts, structural components.