Abstract: The invention provides materials and methods for purifying nucleic acids from a biological sample held in a container (30) such as a centrifuge tube. A filtration unit (1) having a filter (7, 9) and a filtrate chamber (5) is placed within the container (30). Preferably the exterior of the filtration unit (1) engages sealingly with the interior surface of the container (30). Relative movement between the container (30) and the filtration unit (1) causes the sample to pass through the filter (7, 9) into the filtrate chamber (5), where it can be further manipulated. In preferred embodiments the filtrate is then contacted with a charge-switch nucleic acid binding agent in order to purify the nucleic acid further.