text: Using truncated BET theory (the gas adsorption theory developed by Brunauer, Emmett, and Teller), Seah and Hondros write the solid-state analogue as:
  where 
   is the solid solubility, which is known for many elements (and can be found in metallurgical handbooks). In the dilute limit, a slightly soluble substance has , so the above equation reduces to that found with the Langmuir-McLean theory. This equation is only valid for . If there is an excess of solute such that a second phase appears, the solute content is limited to  and the equation becomes
  This theory for grain boundary segregation, derived from truncated BET theory, provides excellent agreement with experimental data obtained by Auger electron spectroscopy and other techniques.