A method for preparing polymeric membranes is disclosed, which has the following steps: (a) forming a casting solution by dissolving cellulose triacetate (CTA) and polyether-sulfone (PES) in a solvent mixture containing n-methyl pyrroridone (NMP) and hydrogen chloride (HCl); (b) forming a membrane from said casting solution; and (c) 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.