Bioequivalence and Bioavailability Forum

Is there a way/method by which we can find the extent of absorption of a particular drug from different parts of GI tract, like for Ex 20% from stomach, 60% from duodenum etc. henderson-haselbach equation may give a idea based on the ionized-unionized status. but that may not be applicable in all cases (like penicillin, even though more than 99% is unionized in somach, real absorption takes place in intestine). my question may look little theoritical, but still if some one has any idea about it? I want to know if this can be done before a generic formulation is made, so that the information may help the formulation persons also.