I am performing in vitro drug release studies using franz diffusion cells. my drug is soluble in the release medium (3mg/mL) and for the duration of my studies. I have used 1 mL of 1 mg/mL of drug in aqueous solution in donor compartment and is also stable in it for the duration of studies. I have tried different membranes, and it doesnot seem to be problem either.. but still the cumulative release at 24 hours is not 100%.. i have also filtered my release media before use.. what could be the issue? I guess, I do not need to use any solubilizers as the release media volume is 12 mL and should be enough to dissolve 100% of drug from donor compartment if released