A260/A280 Ratio Calculator
Calculate A260/A280 purity ratio for nucleic acids and proteins. Pure dsDNA ratio: 1.8-2.0; pure RNA: 2.0-2.2. Ratios below 1.7 indicate protein or phenol contamination.
🧬 QC / Purity📐 Ratio = A260 / A280🧬 Biology
Absorbance at 260nm
Absorbance at 280nm
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| A260/A280 Ratio Calculator | — | Ratio = A260 / A280 | dimensionless |
Step-by-Step Examples
Example 1
Pure Genomic DNA
A260=1.80, A280=0.95.
- Ratio = 1.80/0.95 = 1.895
- In range 1.8-2.0: pure dsDNA
- Suitable for sequencing and restriction digestion
✓ Ratio = 1.90 — Pure dsDNA
Example 2
Protein-Contaminated DNA
A260=1.20, A280=0.85.
- Ratio = 1.20/0.85 = 1.41
- Below 1.7: significant protein contamination
- Re-purify with phenol-chloroform or column
✓ Ratio = 1.41 — Protein contamination
Example 3
RNA Sample
A260=2.10, A280=1.05.
- Ratio = 2.10/1.05 = 2.00
- For RNA, acceptable range is 2.0-2.2
- This sample is slightly low for RNA but acceptable
✓ Ratio = 2.00 — Good RNA purity
Real-World Applications
Quality Control
Every nucleic acid prep requires A260/A280 before downstream applications.
NGS Library QC
Library purity impacts sequencing quality; contaminated samples cause poor cluster generation.
PCR Reliability
Protein or phenol contamination inhibits PCR polymerase — clean template is critical.
Diagnostic Labs
Clinical DNA/RNA extractions from blood, tissue must pass purity standards before diagnostic assays.
Common Mistakes to Avoid
⚠️
Measuring at wrong dilution
A260 should be 0.1-1.0 for accurate ratio. Very dilute or concentrated samples give unreliable ratios.
⚠️
Ignoring A260/A230 ratio
A230 contamination (EDTA, carbohydrates, salts) is equally important. A260/A230 should be 2.0-2.2 for pure nucleic acid.
⚠️
Using ratio as sole purity measure
A260/A280 misses certain contaminants. Combine with gel electrophoresis and functional tests (PCR, restriction digest) for complete QC.
Frequently Asked Questions
What absorbs at 280nm? ▾
Aromatic amino acids (Trp, Tyr, Phe) absorb at 280nm. Protein contamination raises A280 relative to A260, lowering the ratio.
What causes high ratio (>2.2)? ▾
RNA contamination (RNA has higher intrinsic ratio than DNA), alkali treatment, or very pure RNA samples. Can also indicate very small oligonucleotides.
Does ratio affect concentration measurement? ▾
Indirectly. Ratio tells you about purity, not concentration. But contaminants like protein contribute to A260, falsely elevating apparent nucleic acid concentration.
What if ratio is exactly 2.0 for DNA? ▾
Could indicate RNA contamination. Run on gel or treat with RNase A and repeat quantification to confirm.
What is the A260/A230 ratio? ▾
A230 contamination (EDTA, carbohydrates, phenol, chaotropic salts) is detected by A260/A230. Should be 2.0-2.2 for clean samples. Below 1.8 indicates such contamination.