For RNA, an A260/A280 ratio of about 2.0–2.2 is normal and generally indicates pure RNA. Many labs remember 1.8 as the “target”, but that number belongs to DNA. RNA’s different base composition gives it a higher ratio, and measuring in buffered, slightly alkaline solution pushes readings to the upper part of the range. Commonly cited acceptance ranges for RNA run from about 1.8 to 2.1, so a reading of 2.1 is not a warning sign. See the full explanation in our A260/A280 guide and the summary table in RNA quality reference values.
Why pure RNA reads higher than DNA
The ratio reflects the absorbance profile of the bases. RNA contains uracil instead of thymine and is single-stranded, so its 260/280 ratio sits near 2.0, while pure double-stranded DNA sits near 1.8. A DNA-style threshold applied to RNA therefore makes good RNA look “too high”.
When a high ratio is fine — and when to double-check
- 2.0–2.2 in Tris or TE buffer: expected for clean RNA.
- Slightly above 2.2: usually still fine; confirm the blank matched the sample buffer.
- Much higher, unstable between repeats, or with low concentration: likely a measurement artefact — a bubble, a dirty pedestal, a blank in a different buffer, or a sample too dilute for reliable ratios.
- High A260/A280 but low A260/A230: purity at 280 looks fine, but salts or guanidine may still be present; check the 230 trough on the spectrum.
What the ratio does not tell you
- Integrity. Degraded RNA can have a perfect ratio. Check 28S/18S bands on a gel (what degraded RNA looks like) or RIN.
- Genomic DNA contamination. DNA absorbs at 260 nm too, so gDNA is invisible to the ratio; use DNase treatment and a no-RT control.
- Percent purity. The ratio is not linear with contamination and cannot be converted to a percentage.
Common questions
Is an A260/A280 of 2.1 or 2.2 bad for RNA?
No. RNA measured in a buffered, slightly alkaline solution commonly reads 2.0–2.2. That is a normal result for pure RNA.
Why is the RNA ratio higher than DNA’s 1.8?
Uracil in RNA and thymine in DNA have different absorbance properties, and RNA is single-stranded, so the 260/280 ratio of pure RNA (~2.0) is higher than that of pure DNA (~1.8).
When should a very high ratio make me suspicious?
If it is far above 2.2, or comes with a very low concentration, an irregular spectrum or a poor A260/A230, check the blank, the pedestal and the sample volume before trusting it.
Can I convert A260/A280 into percent purity?
No. The ratio is not linear with contamination, so a value cannot be translated into a percentage of pure nucleic acid.
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