What is 260 and 280 ratio

Absorbance ratios 260/280 and 260/230 for RNA

DNA and RNA absorb at 260 nm. Proteins absorb at 280 nm. The 260/280 ratio is a good estimate of how pure your sample is. For RNA, the 260/280 should be around 2. If it is lower, it could be an indication of contamination or proteins, phenol or other contaminants in your sample. The 260/230 ratio is a second measure of the purity of the sample, as the impurities absorb at 230 nm (like EDTA). The 260/230 ratio should be higher than the 260/280 ratio as it is usually between 2 and 2.2. A lower ratio can indicate contamination.

In your case I would be concerned about the purity of the sample which gave you a 260/230 ratio of 0.98. If you are using nanodrops, a look at the diagram can also be a good indication of the quality of your sample. For a pure sample, a well-defined peak (no shoulders or wobbling) is expected at 260 nm.

Please refer to the NanoDrop manual for more information.


In fact, I remember that the sample with the ratio 260/230 had two peaks. In my case, the values ​​for the 260/230 ratio are never higher than those for 260/280, which indicates that I have contamination. What can be the consequence if the correct values ​​for this ratio are not available for a PCR? I will be extracting RNA soon. How can I prevent contamination next time? What is the good practice?

Gergana Vandova

If you have two peaks this is a clear indication of contamination. Don't just look at the ratios as this gives you an idea but doesn't make a strong statement. The PCR may fail due to contamination, but you can still try. You can also try isolating your RNA again.


For the sake of completeness and for relevant details: The aromatic amino acids (Phe, Trp and Tyr) primarily contribute to the absorption capacity of proteins at 280 nm. Cysteine ​​double bonds can also absorb, but are both rare and lower in extinction coefficient than the aromatic residues.