Gas adsorption analysis based on the Brunauer–Emmett–Teller (BET) theory
Gas adsorption analysis based on the Brunauer–Emmett–Teller (BET) theory is a valuable technique for the determination of the specific surface area of lyophilized (freeze-dried) samples.
In BET, the freeze-dried sample is evacuated to remove all gases and is then cooled by using liquid nitrogen. As a next step, an inert probing gas (mostly N2) is added to the sample, which adsorbs onto the sample surface. While keeping the temperature constant, the gas pressure is increased allowing more molecules to adsorb to the sample surface (adsorption isotherm). The BET equation is now used to calculate the volume of gas needed to form a monolayer. The actual surface area can then be calculated from knowledge of the size and number of the adsorbed gas molecules.
BET analysis is employed during lyophilization process development and formulation development. It delivers valuable insights into the structure of freeze-dried samples and helps defining optimal freeze-drying conditions.