Nucleic acids absorb UV light of 250 to 270 nm wavelength, with a maximum at 260 nm. The reading at 260 nm allows calculation of the concentration of nucleic acid in the sample. An OD of 1 corresponds to approximately 50 µg/ml for double-stranded DNA and 40 µg/ml for single-stranded DNA and RNA. The ratio between the readings at 260 nm and 280 nm (A260∕280 ratio) provides an estimate for the purity of the nucleic acid. Pure preparations of DNA and RNA have ratios of 1.8 to 1.95, respectively. DNA producing a ratio below 1.6 should be re-purified3.
© 2001 Alexander Binder