What is the significance of the 260/280 and the 260/230 ratios?

260/280 Ratio

260 nm and 280 nm are the absorbance wavelengths used to assess the purity of DNA and RNA. A ratio of 1.7 – 2.0 is considered pure for DNA and a ratio of ∼2.0 is considered pure for RNA. A lower absorbance ratio may indicate the presence of protein, phenol or other contaminants that have an absorbance close to 280 nm.

Subtraction of non-nucleic acid absorbance at 320 nm, is also needed to calculate this ratio. See formula below:

DNA Purity (A₂₆₀/A₂₈₀) = (A₂₆₀ reading – A₃₂₀ reading) ÷ (A₂₈₀ reading – A₃₂₀ reading)

260/230 Ratio

The ratio of absorbance at 260 and 230 nm can be used as a secondary measure of DNA or RNA purity. In this case, a ratio between 2.0 - 2.2 is considered pure. If the ratio is lower than this expected range, it may indicate contaminants in the sample that absorb at 230nm.