A260/A230 Ratio Calculators

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The A260/A230 ratio is a spectrophotometric purity metric for DNA and RNA that indicates contamination by organic compounds absorbing at 230 nm. The ideal A260/A230 ratio for pure nucleic acid is 2.0–2.2. Values below this threshold suggest contamination with guanidinium salts, phenol-chloroform carryover, EDTA, or other substances co-purified with the nucleic acid. Unlike the A260/A280 ratio (which detects protein contamination), the A260/A230 ratio specifically flags small organic contaminants that can inhibit downstream enzymatic reactions like PCR, reverse transcription, and restriction digestion.

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What Absorbs at 230 nm

Several common laboratory reagents and contaminants absorb strongly at 230 nm: guanidinium thiocyanate (from column lysis buffers), phenol (from phenol-chloroform extraction), carbohydrates, EDTA, and certain organic solvents. When these substances carry over into the purified nucleic acid, they elevate A230 relative to A260, depressing the A260/A230 ratio below the ideal 2.0–2.2 range.

Interpreting A260/A230 Values

  • 2.0–2.2: High purity — suitable for sensitive downstream applications
  • 1.5–2.0: Moderate purity — acceptable for many PCR applications
  • Below 1.5: Significant contamination — may inhibit enzymatic reactions
  • Above 2.3: Rare; may indicate very dilute sample or baseline drift

How to Improve A260/A230

Washing steps with wash buffers containing ethanol in spin-column protocols remove guanidinium and chaotropic salts. Increasing the number of wash steps, ensuring complete ethanol evaporation before elution, and using clean low-binding tubes improves A260/A230. For phenol-chloroform extractions, additional chloroform-isoamyl alcohol back-extraction steps remove phenol carryover.

A260/A230 vs. A260/A280

The A260/A280 ratio detects protein contamination (proteins absorb at 280 nm due to aromatic amino acids). These two ratios are complementary: a sample may have acceptable A260/A280 but poor A260/A230 if organic contaminants are present, or vice versa. Both should be checked when nucleic acid quality is critical.

Glossary

A260/A230 Ratio
A spectrophotometric purity metric for nucleic acids; ideal value 2.0–2.2; low values indicate contamination with guanidinium salts, phenol, or other organic compounds absorbing at 230 nm.
Guanidinium Thiocyanate
A chaotropic salt used in nucleic acid lysis buffers that strongly absorbs at 230 nm; incomplete removal causes low A260/A230 ratios and can inhibit downstream enzymes.
Chaotropic Agent
A chemical that disrupts non-covalent interactions in biological molecules, denaturing proteins and enabling cell lysis during nucleic acid extraction.

Frequently Asked Questions

The ideal A260/A230 ratio is 2.0–2.2 for pure DNA or RNA. Values in this range indicate minimal contamination with organic compounds that absorb at 230 nm (guanidinium salts, phenol, EDTA). A ratio below 1.8 suggests significant organic contamination that may inhibit PCR, RT-PCR, or other enzymatic reactions. Always check this ratio alongside the A260/A280 ratio when assessing nucleic acid quality.

A low A260/A230 ratio indicates contamination with substances absorbing at 230 nm, most commonly guanidinium thiocyanate or guanidinium HCl from column lysis buffers, residual phenol from phenol-chloroform extraction, EDTA, or carbohydrates. Incomplete washing of spin columns or insufficient ethanol evaporation before elution are the most common causes in column-based extractions.

For column-based extractions: add an extra wash step with wash buffer, extend centrifugation during wash steps to ensure complete buffer removal, and increase the dry-spin time before elution to evaporate residual ethanol. For phenol-chloroform extractions: add an additional chloroform-isoamyl alcohol extraction before the aqueous phase is collected. Using nuclease-free water at 65–70°C for elution can also improve yield and purity simultaneously.

The A260/A230 ratio is a fast screening tool but has limitations. It is concentration-dependent — very dilute samples (<10 ng/μL) give unreliable ratios due to low signal. It only detects contaminants absorbing at 230 nm and misses others (RNA in a DNA prep, DNA in an RNA prep). For critical applications, complement spectrophotometry with agarose gel electrophoresis (checks integrity and size) and functional tests like PCR amplification efficiency.