the invention described here improves the pampa high - throughput method used in lead compound selection and optimization in pharmaceutical and biotechnological research and development , and for identifying active compounds with the right plant distribution properties in agrochemical research and development . the invention described here is a robust method and apparatus for measurement of two physical properties , permeability and membrane retention of compounds , which overcomes several of the shortcomings in the prior art . this invention includes reagents specifically designed for enhancing the sensitivity of the assay , to allow a uv detection system to be used for concentration measurements , to accurately estimate and compensate for the effects of membrane retention and unstirred water layer , and to increase the speed of the assay . in this invention , concentrated phospholipid membrane barriers are used , consisting of 10 - 74 % wt / vol commercially - available soybean lecithin extract dissolved in dodecane . sample concentrations in both the donor and the acceptor compartments of the permeation cells are quickly measured by direct uv spectrophotometry . to reduce the excessive membrane retention of compounds by the concentrated phospholipid membrane barriers , an artificial sink state is created in the acceptor compartment of the permeation cells , by using surfactants , cyclodextrins , or water - soluble lipophilic polymers , which have low absorption of uv energy . a secondary sink state is created with ionizable molecules when the permeation cell contains a ph gradient between the donor and the acceptor solutions . the resultant “ double - sink ” condition forms a basis for successful modeling of the passive - diffusion transport of molecules in the human gastrointestinal tract . also , this condition accelerates the transport of certain molecules across membranes , shortening the measurement time , and increasing the assay throughput . a new permeability equation is used , that takes into account ph gradients between the two sides of the membrane barrier and the retention of the molecules by the membrane , which the ph - gradient sink condition is unable to eliminate entirely . when combined with the prior art pampa practice of placing solubilizers , such as surfactants , bile salts , and water - soluble lipophilic polymers , into the donor compartment to overcome problems of low aqueous solubility of sample molecules , the said git model becomes modified to serve as a blood - brain barrier absorption model . this invention can also improve performance in prior art pampa membrane barriers , and in several other types of membranes , such as , nonporous synthetic membrane material , dialysis membranes with a particular molecular weight cut - off , and monolayers of cultured cells deposited onto the surface of porous microfilter supports .