a method for preparing polymeric membranes is disclosed , which has the following steps : forming a casting solution by dissolving cellulose triacetate and polyether - sulfone in a solvent mixture containing n - methyl pyrroridone and hydrogen chloride ; forming a membrane from said casting solution ; and recovering said membrane . the method of present invention provides an asymmetrical , microporous , polymeric membrane with a high recovery rate of nucleic acids and desirable bulk and surface properties . also , the membrane so - produced is useful to the application of material separations by filtration and the like , especially the condensation of nucleic acids from a crude sample by filtration .