How can I know if a plasmid DNA is pure?

To measure the DNA purity, the absorbance of the sample needs to be carried out from 230nm to 320nm. This could help in understanding the presence of the contaminants in the DNA sample. The most commonly used method is to find the ratio of absorbance of the sample at 260nm and 280nm, which is commonly referred to as A₂₆₀/A₂₈₀. When the A₂₆₀/A₂₈₀ is between 1.8-2.0, it is accepted as a good quality DNA sample. Lower values indicate more contaminants are present or DNA concentration is very low (<10ng/ml). High A₂₆₀/A₂₈₀ ratios are not necessarily indicative of a problem. However, a very high ratio can suggest a poor-quality blank eliminating too much signal near the 280 nm wavelength. Thus A₂₆₀/A₂₈₀ ratio would help determine the purity of the DNA sample.