The most widely used method for concentrating DNA is precipitation with ethanol. The precipitate of DNA, which is allowed to form at low temperature (-20psy176 C or less) in the presence of moderate concentrations of monovalent cations, is recovered by centrifugation and redissolved in an appropriate buffer. The technique is rapid and is quantitative even with nanogram amounts of DNA.
0.1 volume of 3M sodium acetate (pH 5.2) are added to one volume of the aqueous
solution. This gives a final concentration of 0.3 M sodium
acetate
. 2 to 2.5 volumes of cold (-20psy176 C) absolute
ethanol are added. The tube is left at -70psy176 C for one hour or at
-20psy176 C for several hours (overnight). DNA is precipitated by
centrifugation for 20 minutes at 4psy176 C (13000 rpm), the supernatant is
discarded carefully, the remaining pellet washed with 300 
l of 80%
ethanol to remove residual salt, centrifuged again for 5 minutes at 4psy176 C,
the supernatant discarded again and the pellet dried in a vacuum evaporator
(Speed Vac, Savant). The pellet is resuspended
appropriately. It can also be
stored as a pellet stably at -20psy176 C [82, 83].