DNA Concentration from A260
Beer-Lambert Law: C = (A260/l) × CF × DF. dsDNA 50, ssDNA 33, RNA 40 µg/mL per A260. 260/280 ratio assesses purity.
Why This Biology Metric Matters
Why: Accurate concentration is critical for PCR, sequencing, cloning, and restriction digestion.
How: Measure A260 (and A280 for purity). Use conversion factors: dsDNA 50, ssDNA 33, RNA 40 µg/mL per A260. Oligos use C = (A260/(ε×l)) × MW × DF.
- ●Pure DNA 260/280 ≈ 1.8; pure RNA ≈ 2.0. Lower suggests protein contamination.
- ●A260 0.1–1.0 optimal; dilute if above 1.0 for accuracy.
- ●Oligonucleotides require extinction coefficient from manufacturer.
📋 Sample Scenarios
✅ Standard dsDNA Sample
High-quality double-stranded DNA with excellent purity
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🧬 RNA Extraction Quality Check
RNA sample purity assessment
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🔬 ssDNA Primer Concentration
Single-stranded DNA primer quantification
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📉 Low Concentration DNA
Dilute sample in ng/µL range
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📊 High Concentration Plasmid DNA
Concentrated plasmid DNA preparation
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🧪 Oligonucleotide Standard
Short DNA oligo with known extinction coefficient
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Enter Spectrophotometer Data
⚠️For educational use only. Always confirm dosages and care with a licensed veterinarian.
🧬 Biology Facts
Beer-Lambert: A = ε × l × c. A260 measures nucleic acid absorption.
— Spectrophotometry
dsDNA: 50, ssDNA: 33, RNA: 40 µg/mL per A260 unit (1 cm path).
— Conversion factors
260/280: DNA 1.8, RNA 2.0. <1.6 = protein; >2.2 = phenol.
— Purity
Oligos: C = (A260/(ε×l)) × MW × DF. ε from manufacturer.
— Oligonucleotides
What is DNA Concentration Measurement?
DNA concentration measurement is a fundamental technique in molecular biology that quantifies the amount of nucleic acid in a sample. The most common method uses UV spectrophotometry, which measures the absorbance of light at specific wavelengths (260 nm for nucleic acids, 280 nm for proteins).
Spectrophotometry
Uses UV light absorption at 260 nm to quantify nucleic acids based on the Beer-Lambert Law.
Purity Assessment
260/280 ratio indicates sample purity - pure DNA ~1.8, pure RNA ~2.0. Lower ratios suggest protein contamination.
Applications
Essential for PCR, sequencing, cloning, restriction digestion, and other molecular biology techniques.
How Spectrophotometry Works: Beer-Lambert Law
The Beer-Lambert Law describes the relationship between light absorption and concentration:
A = Absorbance (unitless, measured at 260 nm)
ε = Molar extinction coefficient (L/(mol·cm))
l = Pathlength (cm, typically 1 cm for standard cuvettes)
c = Concentration (mol/L)
For nucleic acids, we use conversion factors that relate A260 directly to concentration in µg/mL, simplifying the calculation:
Conversion Factors
Different nucleic acid types have different conversion factors due to their base composition and structure:
| Sample Type | Conversion Factor | Units | Notes |
|---|---|---|---|
| Double-stranded DNA (dsDNA) | 50 | µg/mL per A260 unit | Standard for genomic DNA, plasmids, PCR products |
| Single-stranded DNA (ssDNA) | 33 | µg/mL per A260 unit | Used for primers, single-stranded templates |
| RNA | 40 | µg/mL per A260 unit | Total RNA, mRNA, rRNA, tRNA |
260/280 Purity Ratio
The A260/A280 ratio is a key indicator of sample purity. Pure nucleic acids have characteristic ratios:
Pure DNA
A260/A280 ratio: 1.8
Indicates minimal protein contamination
Pure RNA
A260/A280 ratio: 2.0
Higher ratio due to RNA structure
Protein Contamination
A260/A280 ratio: < 1.6
Indicates significant protein presence
Phenol Contamination
A260/A280 ratio: > 2.2
May indicate residual extraction reagents
Oligonucleotide Concentration
For short oligonucleotides (<20 bases), the standard conversion factors are not accurate. Instead, use the extinction coefficient method:
ε260 = Molar extinction coefficient (L/(mol·cm)) - provided by manufacturer or calculated from sequence
MW = Molecular weight (g/mol) - calculated from sequence composition
l = Pathlength (cm)
DF = Dilution factor
Most oligonucleotide synthesis companies provide the extinction coefficient and molecular weight with the product.
Extinction Coefficients
Approximate extinction coefficients for common nucleotides (at 260 nm, pH 7.0):
| Nucleotide | Extinction Coefficient | Units |
|---|---|---|
| Adenine (A) | 15,400 | L/(mol·cm) |
| Guanine (G) | 11,500 | L/(mol·cm) |
| Cytosine (C) | 7,400 | L/(mol·cm) |
| Thymine (T) | 8,700 | L/(mol·cm) |
| Uracil (U) | 9,900 | L/(mol·cm) |
For an oligonucleotide, the total extinction coefficient is the sum of individual nucleotide coefficients, adjusted for nearest-neighbor effects in some cases.
Best Practices
1. Sample Preparation
Ensure samples are properly dissolved and free of particulates. Vortex gently before measurement.
2. Dilution Factor
For accurate measurements, A260 should be between 0.1 and 1.0. Dilute samples if absorbance exceeds 1.0.
3. Pathlength Correction
Standard cuvettes use 1 cm pathlength. For microvolume spectrophotometers, use the actual pathlength (often 0.2-0.5 mm).
4. Blank Correction
Always blank the spectrophotometer with the same buffer/solvent used for your sample.
Frequently Asked Questions
Why is my 260/280 ratio low?
A low ratio (<1.6) typically indicates protein contamination. Consider additional purification steps such as phenol-chloroform extraction or commercial purification kits.
What if my A260 is above 1.0?
Absorbance values above 1.0 are less accurate due to detector linearity limits. Dilute your sample and multiply the result by the dilution factor.
Can I use this for RNA?
Yes! Select "RNA" as the sample type. RNA uses a conversion factor of 40 µg/mL per A260 unit, and pure RNA has a 260/280 ratio around 2.0.
How accurate is this method?
Spectrophotometry is accurate to within 5-10% for pure samples. For more precise quantification, consider fluorometric methods (Qubit, PicoGreen) or qPCR-based quantification.
What about 260/230 ratio?
The 260/230 ratio (ideally 2.0-2.2) indicates contamination with organic compounds like phenol, guanidine, or carbohydrates. This calculator focuses on 260/280 for protein contamination assessment.