NanoDrop A260/A280: Measuring Nucleic Acid Purity
By Dr. Zubair Khalid, DVM, MS, PhD ·
Introduction to NanoDrop A260/A280
The NanoDrop spectrophotometer is a micro-volume UV-Vis instrument widely used in molecular biology laboratories to quantify nucleic acids and assess their purity. Unlike conventional cuvette-based spectrophotometers that require 50–100 µL of sample, the NanoDrop measures samples as small as 1–2 µL, making it indispensable when working with precious cDNA, plasmid preps, or RNA extractions. The instrument's name derives from its ability to measure nanoliter-scale volumes without dilution.
The A260/A280 ratio is the absorbance measured at 260 nm divided by the absorbance measured at 280 nm. This ratio serves as a rapid, first-line indicator of nucleic acid purity. A pure DNA sample typically yields a ratio of ~1.8, while pure RNA yields ~2.0. Deviations from these values signal the presence of contaminants—most commonly proteins, phenol, or chaotropic salts—that absorb light at 280 nm or elsewhere in the UV spectrum. Understanding what this ratio actually measures, how the instrument obtains it, and what can skew it is essential for generating reliable data in any molecular biology workflow.
Principle of UV Absorbance and the A260/A280 Ratio
Absorbance at 260 nm
Nucleic acids absorb ultraviolet light strongly at 260 nm due to the aromatic ring structures of their nitrogenous bases. Adenine, guanine, cytosine, thymine, and uracil all contain conjugated double-bond systems that absorb UV light in the 250–270 nm range, with a peak absorbance centered near 260 nm. The Beer-Lambert law governs this relationship:
A = ε × l × c
where A is absorbance, ε is the molar extinction coefficient (L·mol⁻¹·cm⁻¹), l is the path length (cm), and c is the concentration (mol/L). For nucleic acids, the extinction coefficient at 260 nm is approximately 50 ng/µL per absorbance unit for double-stranded DNA, 40 ng/µL per absorbance unit for single-stranded RNA, and 33 ng/µL per absorbance unit for single-stranded DNA oligonucleotides. The NanoDrop software applies these constants automatically when you select the appropriate nucleic acid type.
The absorbance at 260 nm is directly proportional to the number of bases in solution, regardless of whether they exist as free nucleotides, single-stranded nucleic acids, or double-stranded helices. However, base stacking in double-stranded DNA reduces absorbance by roughly 30–40% compared to free nucleotides—a phenomenon called the hypochromic effect. This is why the extinction coefficient for dsDNA (50) is higher than for ssDNA (33): more absorbance per unit mass means a lower coefficient is needed to convert absorbance to concentration.
Absorbance at 280 nm
Proteins absorb UV light at 280 nm primarily because of the aromatic side chains of tryptophan and tyrosine residues. Both amino acids contain indole and phenol rings, respectively, which have absorbance maxima near 280 nm. Phenylalanine also contributes, though its absorbance is weaker and peaks closer to 257 nm. A typical protein solution with a concentration of 1 mg/mL has an absorbance at 280 nm of approximately 1.0–1.5, depending on its amino acid composition.
The presence of proteins in a nucleic acid preparation therefore inflates the absorbance at 280 nm without contributing meaningfully to the absorbance at 260 nm. Since the A260/A280 ratio divides the 260 nm reading by the 280 nm reading, protein contamination decreases the ratio. A sample with a ratio below the expected value for pure nucleic acid suggests protein carryover from the extraction procedure.
Calculating the ratio
The calculation itself is trivial: the instrument measures absorbance at both wavelengths and divides the two values. For example, if a DNA sample gives A260 = 1.0 and A280 = 0.55, the A260/A280 ratio is 1.0 ÷ 0.55 = 1.82. The NanoDrop software performs this calculation automatically and displays the ratio alongside the concentration readout.
It is important to recognize that the ratio is dimensionless and independent of concentration. A 10 ng/µL DNA sample and a 500 ng/µL DNA sample of identical purity will yield the same A260/A280 ratio, provided the instrument's path length is adjusted correctly and the absorbance readings fall within the linear range of the detector.
How the NanoDrop Instrument Works
Sample loading and pedestal
The NanoDrop uses a unique sample retention system based on surface tension. The instrument has two pedestals: a lower measurement surface and an upper arm that closes over it. You pipette 1–2 µL of sample onto the lower pedestal, then lower the arm. The liquid forms a column between the two surfaces, held in place by surface tension. This eliminates the need for cuvettes or dilution, which is a major advantage when sample volume is limited.
Light from a xenon flash lamp passes through the sample column from the lower pedestal to a spectrometer in the upper arm. The spectrometer measures the transmitted light across the full UV-Vis spectrum (typically 220–750 nm), allowing the instrument to report absorbance at multiple wavelengths simultaneously. This full-spectrum capability is what enables the NanoDrop to calculate A260/A280, A260/A230, and even estimate protein concentration via the Nanodrop A280 Protein Concentration method.
Path length and dynamic range
Conventional spectrophotometers use a fixed path length of 1 cm (10 mm). The NanoDrop, however, uses a variable path length that adjusts automatically based on sample absorbance. When the arm closes, the initial path length is 1 mm. If the absorbance at 260 nm is low (below ~1.5), the instrument maintains this 1 mm path. If the absorbance is higher, the instrument reduces the path length to 0.2 mm or even 0.05 mm to keep the reading within the linear range of the detector.
This dynamic path length adjustment gives the NanoDrop an impressive dynamic range. With a 1 mm path, the instrument can measure DNA concentrations from approximately 2 to 3700 ng/µL. With the 0.05 mm path, it can measure up to 15,000 ng/µL. The software automatically applies the appropriate path length correction factor to report concentration as if measured at 1 cm. This is why the NanoDrop can measure both dilute and highly concentrated samples without dilution—a key feature for Nano-300 Nanodrop workflows and other high-throughput applications.
Interpreting A260/A280 Ratios
Expected ratios for DNA and RNA
For pure double-stranded DNA, the A260/A280 ratio is approximately 1.8. For pure RNA, the ratio is approximately 2.0. These values are empirical benchmarks established over decades of laboratory practice. The difference arises because RNA's extinction coefficient at 260 nm is lower (40 vs. 50 for dsDNA), meaning RNA absorbs less at 260 nm per unit mass, while its absorbance at 280 nm is similar to DNA. The net effect is a higher ratio for RNA.
In practice, acceptable ranges are:
| Sample Type | Acceptable A260/A280 Range | Ideal Value |
|---|---|---|
| Double-stranded DNA | 1.7–1.9 | 1.8 |
| RNA | 1.9–2.1 | 2.0 |
| Single-stranded DNA | 1.6–1.9 | ~1.7 |
Values within these ranges are generally considered pure enough for downstream applications such as PCR, restriction digestion, cloning, or reverse transcription. Values outside these ranges warrant investigation.
Effects of pH and contaminants
The A260/A280 ratio is sensitive to the pH of the solution. Nucleic acids dissolved in water typically give lower ratios than the same nucleic acids dissolved in a slightly alkaline buffer such as 10 mM Tris-Cl, pH 8.0. This pH dependence is particularly pronounced for RNA. A pure RNA sample measured in water may give a ratio of 1.8–1.9, while the same sample in Tris buffer at pH 8.0 gives 2.0–2.1. The reason is that the absorbance of nucleic acids at 280 nm decreases as pH increases, while absorbance at 260 nm remains relatively stable. This shifts the ratio upward.
For this reason, it is standard practice to elute nucleic acids in a buffered solution (typically 10 mM Tris-Cl, pH 8.0) rather than water, and to blank the NanoDrop with the same buffer used to dissolve the sample. Measuring in unbuffered water can produce misleadingly low ratios, leading you to believe your sample is contaminated when it is not.
Common Contaminants and Their Impact on A260/A280
Protein contamination
Protein is the most common contaminant in nucleic acid preparations, particularly from phenol-chloroform extractions or inadequate proteinase K digestion. Proteins absorb at 280 nm, so their presence increases the denominator of the A260/A280 ratio, driving the ratio down. A DNA sample with significant protein contamination might give a ratio of 1.5–1.6 instead of 1.8.
The extent of the effect depends on the protein's amino acid composition. Proteins rich in tryptophan and tyrosine absorb strongly at 280 nm and will depress the ratio more than proteins composed primarily of aliphatic amino acids. Bovine serum albumin (BSA), a common additive in restriction enzyme buffers, is particularly problematic because it contains multiple tryptophan residues.
If protein contamination is suspected, you can re-purify the sample using phenol-chloroform extraction followed by ethanol precipitation, or use a commercial spin column cleanup kit. Alternatively, proteinase K digestion (0.1–0.5 mg/mL at 50–55°C for 30–60 minutes) followed by re-extraction can remove residual protein.
Phenol and chaotropic salts
Phenol absorbs strongly at 270 nm, which is close enough to 260 nm to inflate the A260 reading. Guanidine hydrochloride and guanidinium thiocyanate, commonly used in RNA extraction buffers, absorb in the 230 nm region and can also contribute to absorbance at 260 nm. These contaminants typically cause the A260/A280 ratio to rise above the expected value—sometimes to 2.2 or higher—because they add to the numerator more than the denominator.
Phenol contamination is best detected by examining the full absorbance spectrum. Phenol produces a characteristic shoulder or peak around 270 nm. If you see this, the sample should be re-extracted with chloroform to remove residual phenol, then ethanol-precipitated to remove chaotropic salts.
Carbohydrates and other organics
Carbohydrates, including glycogen and polysaccharides, absorb weakly in the UV range but can contribute to background absorbance across the spectrum. They are more problematic for the A260/A230 ratio than for A260/A280, but heavy contamination can depress both ratios. Carbohydrates are common contaminants in plant DNA preparations and in RNA extracted from tissues with high glycogen content (e.g., liver).
Other organic compounds, including EDTA, can also interfere. EDTA absorbs at 230 nm and, at high concentrations, can contribute to absorbance at 260 nm. This is one reason why nucleic acids should be eluted in low-EDTA buffers (e.g., 10 mM Tris-Cl, 0.1 mM EDTA) rather than standard TE buffer (10 mM Tris, 1 mM EDTA).
Best Practices for Accurate NanoDrop Measurements
Sample preparation and mixing
Before measuring, ensure your sample is thoroughly mixed. Nucleic acids can form concentration gradients in solution, especially after freezing and thawing or after ethanol precipitation. Vortex the sample briefly (3–5 seconds) and centrifuge it for 2–3 seconds to collect the liquid at the bottom of the tube. For viscous samples or those containing high concentrations of nucleic acids, pipette the sample up and down 5–10 times before taking an aliquot.
The volume you pipette onto the pedestal should be 1–2 µL. Volumes smaller than 1 µL may not form a complete column between the pedestals, leading to inaccurate readings. Volumes larger than 2 µL do not improve accuracy and can cause overflow, which risks cross-contamination.
Blanking and baseline correction
Blanking is the most critical step in obtaining accurate NanoDrop measurements. The blank solution should be the exact buffer in which your nucleic acid is dissolved—not water, not a different buffer. If your DNA is in 10 mM Tris-Cl, pH 8.0, blank with that buffer. If your RNA is in nuclease-free water, blank with nuclease-free water.
To blank, pipette 1–2 µL of the blank solution onto the lower pedestal, lower the arm, and select "Blank" on the instrument. The instrument measures the absorbance spectrum of the blank and subtracts it from all subsequent sample measurements. After blanking, wipe the pedestal clean with a lint-free lab wipe before measuring your sample.
The NanoDrop also performs a baseline correction by subtracting the absorbance at 340 nm from all readings. This corrects for any light scattering caused by particulates in the sample or slight misalignment of the pedestal. Nucleic acids and proteins do not absorb at 340 nm, so this correction does not affect the A260/A280 ratio.
Replicates and consistency
Always measure each sample in at least duplicate, preferably triplicate. Pipette a fresh aliquot for each measurement rather than reusing the same drop. This controls for pipetting errors, sample inhomogeneity, and evaporation. The NanoDrop software can calculate the average and standard deviation of replicate measurements, which helps you assess the precision of your readings.
Consistency in technique matters. Always use the same pipette, the same pipette tips, and the same volume for all measurements. Wipe the pedestal between every measurement, including between replicates. A contaminated pedestal is one of the most common sources of erroneous readings, as described in the Nanodrop Eight User Manual and the Nanodrop Ultra User Manual.
Limitations and Alternative Purity Assessments
A260/A230 ratio
The A260/A230 ratio is a secondary purity indicator that measures absorbance at 260 nm relative to absorbance at 230 nm. For pure nucleic acids, this ratio should be between 2.0 and 2.2. Lower values indicate contamination with substances that absorb at 230 nm, including chaotropic salts (guanidine), EDTA, carbohydrates, and phenol. The A260/A230 ratio is often more sensitive than A260/A280 for detecting certain contaminants, particularly those introduced by silica column-based purification kits. See the Nanodrop A260/a230 article for a detailed treatment of this ratio.
A sample can have an acceptable A260/A280 ratio but a poor A260/A230 ratio, indicating contamination that the 280 nm reading does not detect. Conversely, a sample with a good A260/A230 but poor A260/A280 likely contains protein. For this reason, experienced researchers always report both ratios.
Fluorometric quantification
The NanoDrop measures total absorbance at 260 nm, which includes contributions from free nucleotides, degraded nucleic acid fragments, and contaminants. It cannot distinguish between intact nucleic acids and degraded fragments. Fluorometric quantification using dyes such as PicoGreen (for dsDNA), RiboGreen (for RNA), or Qubit assays provides a more accurate measurement of intact nucleic acid concentration because these dyes bind specifically to double-stranded DNA or RNA and do not fluoresce when bound to free nucleotides or single-stranded fragments.
Fluorometric methods are particularly recommended when the nucleic acid will be used for applications sensitive to degradation, such as next-generation sequencing library preparation or quantitative PCR. However, fluorometric assays require a standard curve and additional reagents, making them more expensive and time-consuming than spectrophotometric measurement.
Gel electrophoresis
Agarose gel electrophoresis is the gold standard for assessing nucleic acid integrity. Running 100–200 ng of your sample on a 1% agarose gel stained with ethidium bromide or a safer DNA stain like SYBR Safe allows you to visualize the nucleic acid. Genomic DNA should appear as a single high-molecular-weight band near the well. RNA should show distinct ribosomal bands (28S and 18S for eukaryotic RNA, 23S and 16S for prokaryotic RNA) with minimal smearing. Degraded samples show smearing and loss of high-molecular-weight bands.
Gel electrophoresis also reveals contaminating RNA in DNA preparations (visible as a low-molecular-weight smear) and contaminating DNA in RNA preparations (visible as a high-molecular-weight band above the ribosomal RNA). The NanoDrop cannot provide this information.
Common Pitfalls and Troubleshooting
Contaminated pedestal
The most frequent cause of unexpected NanoDrop readings is a dirty pedestal. Residual sample from a previous measurement can remain on the pedestal surface, especially if the sample was viscous or contained high concentrations of nucleic acids. This residue absorbs UV light and contaminates subsequent measurements.
Solution: Wipe both the upper and lower pedestals with a lint-free lab wipe between every measurement. If the pedestal appears dirty, clean it with a small volume of distilled water or 70% ethanol, then dry it completely before use. Some protocols recommend cleaning with 10% bleach followed by water to remove protein deposits, but check your instrument manual first—bleach can damage some optical surfaces.
Improper blanking
Blanking with the wrong solution is a common error. If you blank with water but your sample is in Tris buffer, the buffer's absorbance will be added to your sample reading. Tris-Cl at pH 8.0 has minimal absorbance at 260 nm but does absorb slightly at 230 nm, which can affect the A260/A230 ratio. More importantly, if your blank contains any nucleic acid—even trace amounts from a contaminated buffer—your sample readings will be artificially low.
Solution: Always use the exact buffer from your sample preparation for blanking. Prepare a fresh aliquot of buffer for blanking rather than using buffer that has been open for extended periods. If you suspect your buffer is contaminated, measure its absorbance spectrum without blanking—a clean buffer should give near-zero absorbance across the UV range.
Sample carryover
Pipetting errors can introduce air bubbles into the sample drop, which scatter light and produce erratic readings. Air bubbles are visible as bright spots in the sample column. Similarly, pipetting too vigorously can create foam, which also scatters light.
Solution: Pipette gently and slowly. After dispensing the sample onto the pedestal, inspect it visually for bubbles before lowering the arm. If you see bubbles, wipe the pedestal and re-pipette a fresh aliquot. Also ensure that the pipette tip is clean and does not contain residual liquid from a previous step.
Low A260/A280 ratio despite clean sample
If your sample gives a low A260/A280 ratio but you have ruled out protein contamination, check the pH of your solution. As discussed earlier, nucleic acids in unbuffered water give lower ratios than in buffered solutions. If your sample is in water, the low ratio may be an artifact of pH rather than true contamination.
Solution: Measure the pH of your sample solution. If it is below 7.0, consider adding a small volume of 1 M Tris-Cl, pH 8.0, to bring the pH to 8.0, then re-measure. Alternatively, elute future samples in 10 mM Tris-Cl, pH 8.0, instead of water.
Frequently Asked Questions
What is a good A260/A280 ratio for DNA?
A good A260/A280 ratio for double-stranded DNA is approximately 1.8. Values between 1.7 and 1.9 are generally considered acceptable for most downstream applications. A ratio significantly below 1.7 suggests protein contamination or the presence of other contaminants that absorb at 280 nm. A ratio above 1.9 may indicate RNA contamination (if you are measuring DNA) or the presence of phenol or other organic compounds that absorb near 260 nm.
What does A260/A280 ratio indicate?
The A260/A280 ratio indicates the purity of a nucleic acid sample with respect to protein contamination. Nucleic acids absorb maximally at 260 nm, while proteins absorb maximally at 280 nm. A high ratio (around 1.8 for DNA, 2.0 for RNA) indicates that the sample is relatively free of protein. A lower ratio suggests protein contamination. The ratio does not provide information about nucleic acid integrity, concentration accuracy, or contamination with substances that absorb at other wavelengths.
Why is my A260/A280 ratio low?
A low A260/A280 ratio is most commonly caused by protein contamination in the sample. Other causes include: measuring in unbuffered water (which lowers the ratio due to pH effects), the presence of phenol or other organic solvents that absorb at 280 nm, or the presence of carbohydrates that contribute to background absorbance. If your ratio is low, first check the pH of your solution, then consider re-purifying the sample to remove protein.
Can I use A260/A280 to quantify RNA?
Yes, the NanoDrop can quantify RNA using the absorbance at 260 nm with an extinction coefficient of 40 ng/µL per absorbance unit. The A260/A280 ratio for pure RNA should be approximately 2.0. However, the NanoDrop cannot distinguish between intact RNA and degraded RNA fragments, nor can it distinguish RNA from contaminating DNA. For applications requiring precise RNA quantification, such as quantitative PCR or RNA sequencing, fluorometric quantification with RiboGreen or a similar dye is recommended.
How does pH affect A260/A280 readings?
The pH of the solution affects the absorbance of nucleic acids at 280 nm. As pH increases, absorbance at 280 nm decreases, which increases the A260/A280 ratio. Nucleic acids dissolved in unbuffered water (which is often slightly acidic due to dissolved CO₂) give lower ratios than the same nucleic acids dissolved in a slightly alkaline buffer such as 10 mM Tris-Cl, pH 8.0. This effect is more pronounced for RNA than for DNA. To obtain consistent, comparable readings, always measure samples in the same buffer and blank with that buffer.
What is the difference between A260/A280 and A260/A230?
A260/A280 measures absorbance at 260 nm relative to 280 nm and primarily indicates protein contamination. A260/A230 measures absorbance at 260 nm relative to 230 nm and indicates contamination with substances that absorb at 230 nm, including chaotropic salts (guanidine hydrochloride, guanidinium thiocyanate), EDTA, carbohydrates, and phenol. For pure nucleic acids, A260/A230 should be between 2.0 and 2.2. A low A260/A230 with an acceptable A260/A280 suggests contamination from column purification reagents or residual organic solvents. Both ratios should be evaluated together when assessing nucleic acid purity.
Key Takeaways
- The A260/A280 ratio is a rapid, first-line purity indicator: ~1.8 for pure DNA and ~2.0 for pure RNA, measured in a buffered solution at pH 8.0.
- The NanoDrop measures 1–2 µL samples using surface tension to hold the liquid between two pedestals, with automatic path length adjustment (1 mm to 0.05 mm) to accommodate a wide concentration range.
- The ratio is calculated by dividing absorbance at 260 nm (nucleic acids) by absorbance at 280 nm (proteins); protein contamination lowers the ratio, while phenol and chaotropic salts can raise it.
- Always blank with the exact buffer used to dissolve your sample, and measure in 10 mM Tris-Cl, pH 8.0, rather than water, to avoid pH-dependent ratio artifacts.
- The A260/A230 ratio complements A260/A280 by detecting guanidine salts, EDTA, and carbohydrates; always report both ratios for a complete purity assessment.
- The NanoDrop measures total absorbance and cannot distinguish intact nucleic acids from degraded fragments; use fluorometric quantification or gel electrophoresis when integrity matters.
- Wipe the pedestal between every measurement, use fresh aliquots for replicates, and inspect the sample drop for air bubbles to ensure accurate, reproducible readings.