The aim of the present study was the selective precipitation of plasmid directly from alkaline lysate produced from bacterial culture with the cationic detergent, N, N', N'-polyethylene (10)-N tallow-1, 3-di amine propane. Polyethylene groups of peg-10 tallow were functionalized with tertiary amine groups and long alkyl chain. The functional groups of cationic detergent were determined by means of (1)H nuclear magnetic resonance (NMR) spectroscopy. A simple and efficient process for plasmid purification by cationic detergent was found to be dependent on acid concentration. The structure of peg-10 tallow cationic detergent after adding acid solution was characterized using Fourier transform infrared (FT-IR) spectroscopy. Inorganic acid such as HCl was introduced into the solution of peg-10 tallow. Controlling PH of precipitation system by the addition of different concentrations of HCl acid solutions on peg-10 tallow improved the partial separation of RNA from plasmid DNA (pDNA). However, different lysate masses were applied to observe the effect of 7.5% acid solution on the separation of RNA from pDNA with a peg-10 tallow cationic surfactant. The results were realized by 1% agarose gel analysis. Copyright (C) 2009 John Wiley & Sons, Ltd.