Nanodrop A280 Protein Concentration: A Practical Guide

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to Nanodrop A280 Protein Concentration

Protein quantification is a fundamental step in nearly every molecular biology workflow, from downstream enzymatic assays to structural studies and biophysical characterization. Among the various methods available—Bradford assay, BCA assay, Lowry assay, and UV spectrophotometry—the measurement of absorbance at 280 nm (A280) offers a rapid, label-free, and non-destructive approach that requires no chemical reagents or standard curves. This method exploits the intrinsic UV absorbance of proteins, making it particularly attractive when sample volume is limited or when the sample must be recovered for subsequent use.

The NanoDrop spectrophotometer has become a standard instrument in teaching and research laboratories because it measures A280 in microvolume samples (1–2 µL) without the need for cuvettes or dilution. Understanding the principles, calculations, and limitations of A280 measurement is essential for generating reliable protein concentration data. This guide covers the molecular basis of A280 absorbance, the operational details of the NanoDrop, the calculations involved, and the practical considerations that determine measurement accuracy.

What is A280?

A280 refers to the absorbance of light at a wavelength of 280 nanometers. When a beam of monochromatic UV light passes through a protein solution, a fraction of that light is absorbed by specific chemical groups within the protein. The amount of absorbed light is directly proportional to the number of absorbing groups in the light path. For proteins, the primary chromophores at 280 nm are the aromatic side chains of the amino acids tryptophan, tyrosine, and, to a lesser extent, phenylalanine. Additionally, disulfide bonds between cysteine residues contribute weakly to absorbance in this region.

The absorbance value itself is a dimensionless number, typically reported in absorbance units (AU). A pure protein solution with an A280 of 1.0 in a 1 cm path length cuvette corresponds to a specific protein concentration that depends on the protein's amino acid composition. For example, a 1 mg/mL solution of bovine serum albumin (BSA) has an A280 of approximately 0.66, while a 1 mg/mL solution of immunoglobulin G (IgG) has an A280 of approximately 1.35. This variability underscores why the extinction coefficient—a protein-specific constant—is essential for accurate quantification.

Why Use a Nanodrop?

The NanoDrop spectrophotometer was designed to address a common problem in biological research: precious samples that exist in minute volumes. Traditional cuvette-based spectrophotometers require 50–100 µL of sample in a standard 1 cm path length cuvette, which is often more than the total volume available after purification steps such as affinity chromatography or size-exclusion chromatography. The NanoDrop solves this by using a patented pedestal technology that creates a very short optical path length (typically 0.5–1.0 mm), allowing measurements of 1–2 µL samples without dilution.

Beyond the microvolume capability, the NanoDrop offers several practical advantages. It does not require cuvettes, eliminating the need for cleaning and the risk of cross-contamination. The instrument automatically calculates the path length based on the sample volume and can measure across a wide concentration range—from approximately 0.1 mg/mL to 100 mg/mL for BSA—without the need for serial dilutions. The measurement takes only a few seconds, making it ideal for high-throughput workflows. For a deeper comparison of UV-based methods, see Quantification of Protein Concentration.

Principle of A280 Absorbance

The absorbance of light by a protein at 280 nm is a direct consequence of the electronic structure of certain amino acid side chains. When UV photons strike these chromophores, electrons in the aromatic rings are promoted from a ground state to a higher-energy excited state. The energy difference between these states corresponds to the wavelength of absorbed light. For aromatic amino acids, this transition occurs in the UV range, with absorption maxima near 280 nm.

Aromatic Amino Acids

Three amino acids contribute to A280 absorbance, each with a different molar extinction coefficient (ε), which is a measure of how strongly a molecule absorbs light at a given wavelength:

• Tryptophan (Trp, W): The strongest absorber at 280 nm, with a molar extinction coefficient of approximately 5,500–5,690 M⁻¹ cm⁻¹. Tryptophan contains an indole ring, a bicyclic structure with a high degree of π-electron conjugation, which makes it an efficient chromophore. Proteins with a high tryptophan content will have a correspondingly high A280 for a given mass concentration.

• Tyrosine (Tyr, Y): The second strongest absorber, with a molar extinction coefficient of approximately 1,280–1,490 M⁻¹ cm⁻¹. Tyrosine contains a phenol ring. Its absorbance is pH-dependent; at high pH (above 10), the phenolic hydroxyl group ionizes, shifting the absorption maximum to longer wavelengths and increasing absorbance at 280 nm.

• Phenylalanine (Phe, F): The weakest absorber at 280 nm, with a molar extinction coefficient of approximately 100–200 M⁻¹ cm⁻¹. Phenylalanine contains a benzene ring. Its absorption maximum is actually at 257 nm, but its tail extends into the 280 nm region, contributing minimally to A280.

The relative contribution of these amino acids to the total A280 of a protein depends on their abundance. For example, the enzyme lysozyme (from chicken egg white) contains 6 tryptophan, 3 tyrosine, and 3 phenylalanine residues per molecule, giving it a molar extinction coefficient at 280 nm of approximately 37,650 M⁻¹ cm⁻¹. In contrast, a protein with few aromatic residues, such as collagen (which is rich in glycine and proline), has a much lower A280 per unit mass.

In addition to the aromatic amino acids, disulfide bonds (S–S) formed between cysteine residues contribute weakly to absorbance at 280 nm. The absorbance arises from the n→σ* transition of the sulfur atoms, with a molar extinction coefficient of approximately 100–150 M⁻¹ cm⁻¹ per disulfide bond. While this contribution is small compared to tryptophan and tyrosine, it becomes significant for proteins with many disulfide bonds, such as antibodies.

Beer-Lambert Law

The relationship between absorbance and concentration is described by the Beer-Lambert Law:

A = ε × c × l

Where:

• A is the absorbance (dimensionless)
• ε is the molar extinction coefficient (M⁻¹ cm⁻¹)
• c is the molar concentration (M)
• l is the path length (cm)

For protein quantification, the Beer-Lambert Law is often rearranged to solve for concentration:

c = A / (ε × l)

When working with mass concentration (mg/mL) rather than molar concentration, the extinction coefficient is expressed as a mass extinction coefficient (ε₁%, also written as E¹% or A¹%₁cm), which is the absorbance of a 1% (10 mg/mL) solution in a 1 cm path length. The concentration is then calculated as:

c (mg/mL) = A × dilution factor / (ε₁% × l)

The Beer-Lambert Law assumes that the absorbing species is the only component in solution that absorbs at the measurement wavelength, that the solution is homogeneous, and that there are no intermolecular interactions that alter absorbance. In practice, these assumptions are often violated, which is why careful blanking and sample preparation are critical.

Nanodrop Instrumentation and Measurement

The NanoDrop spectrophotometer is a specialized instrument designed for microvolume UV-Vis absorbance measurements. Its optical design and operational workflow differ substantially from traditional cuvette-based spectrophotometers, and understanding these differences is essential for obtaining accurate results.

Pedestal Technology

The defining feature of the NanoDrop is its pedestal measurement system. The instrument consists of a lower pedestal (the measurement surface) and an upper arm that contains a fiber optic cable. To measure a sample, the user pipettes 1–2 µL of solution directly onto the lower pedestal. The upper arm is then lowered, bringing the upper fiber optic into contact with the sample. Surface tension holds the liquid column between the two optical surfaces, creating a defined path length.

The path length in a NanoDrop is not fixed at 1 cm as in a cuvette. Instead, the instrument automatically adjusts the path length based on the absorbance of the sample. At the start of each measurement, the NanoDrop measures the absorbance at a short path length (approximately 0.2 mm) and then extends the path length until the signal is within the linear range of the detector. For most measurements, the path length is between 0.5 mm and 1.0 mm, but it can be as short as 0.05 mm for highly concentrated samples. The instrument reports the absorbance normalized to a 1 cm path length, which is the standard convention in spectrophotometry.

The short path length serves two purposes. First, it allows measurement of highly concentrated samples without dilution, because the absorbance is proportional to the product of concentration and path length. A sample with an A280 of 50 in a 1 cm cuvette would have an A280 of only 0.5 in a 0.1 mm path length. Second, it minimizes the sample volume required, as the optical path is defined by the gap between the two pedestal surfaces.

Sample Loading and Measurement

The measurement process follows a standardized workflow:

1. Blank measurement: Before measuring samples, the instrument must be blanked. The blank solution should be the exact buffer in which the protein is dissolved (e.g., 50 mM Tris-HCl, pH 8.0, 150 mM NaCl). A 1–2 µL aliquot of blank buffer is pipetted onto the lower pedestal, the arm is lowered, and the instrument measures the absorbance spectrum. This spectrum is stored as the baseline and is automatically subtracted from all subsequent sample measurements.

1. Sample measurement: After the blank measurement, the pedestal is wiped clean with a lint-free lab wipe. A 1–2 µL aliquot of the protein sample is pipetted onto the pedestal, the arm is lowered, and the measurement is initiated. The instrument records the absorbance spectrum from approximately 220 nm to 350 nm and reports the absorbance at 280 nm, normalized to a 1 cm path length.

1. Post-measurement cleanup: After each measurement, both the upper and lower pedestals must be wiped clean to prevent sample carryover between measurements.

For detailed operational instructions specific to your instrument model, consult the Nanodrop Eight User Manual or the Nanodrop Ultra User Manual. The Nanodrop 8 User Manual also provides useful guidance on software settings and data export.

Calculating Protein Concentration from A280

Converting an A280 reading to a protein concentration requires knowledge of the protein's extinction coefficient and the path length used for the measurement. The NanoDrop software performs this calculation automatically when the user specifies the appropriate parameters, but understanding the underlying math is essential for troubleshooting and for manual calculations.

Extinction Coefficient

The extinction coefficient is a measure of how strongly a protein absorbs light at 280 nm. It is typically expressed in one of two ways:

• Molar extinction coefficient (ε): Units of M⁻¹ cm⁻¹. This is the absorbance of a 1 M solution in a 1 cm path length. For a protein with a known amino acid sequence, ε can be calculated using the Edelhoch method, which sums the contributions of tryptophan, tyrosine, and cystine (disulfide bonds):

ε₂₈₀ = (n_Trp × 5,500) + (n_Tyr × 1,490) + (n_Cys-S-S × 125) M⁻¹ cm⁻¹

where n_Trp, n_Tyr, and n_Cys-S-S are the numbers of tryptophan, tyrosine, and disulfide bonds in the protein, respectively.

• Mass extinction coefficient (ε₁%): Units of (mg/mL)⁻¹ cm⁻¹. This is the absorbance of a 1% (10 mg/mL) solution in a 1 cm path length. It is related to the molar extinction coefficient by:

ε₁% = ε / (10 × MW)

where MW is the molecular weight in daltons.

For proteins with unknown sequences, a generic extinction coefficient is often used. Common values include:

The choice of extinction coefficient has a direct impact on the calculated concentration. For example, using the BSA coefficient (ε₁% = 6.6) for a protein that actually has ε₁% = 10.0 would underestimate the concentration by approximately 34%. Therefore, it is critical to use the correct coefficient for the protein being measured.

Path Length Correction

As noted earlier, the NanoDrop measures absorbance at a short path length and then normalizes the result to a 1 cm path length. The normalization is based on the Beer-Lambert Law:

A₁cm = A_measured × (1 cm / l_actual)

Where l_actual is the actual path length used for the measurement. The NanoDrop determines l_actual by measuring the absorbance of the sample at a wavelength where the buffer has known absorbance characteristics, or by using the position of the upper arm relative to the lower pedestal.

The path length correction is performed automatically by the instrument software. However, it is important to be aware that the correction assumes the sample is homogeneous and that the absorbance is linearly related to concentration across the measured range. At very high concentrations (above approximately 100 mg/mL for most proteins), the linear relationship can break down due to intermolecular interactions, leading to inaccurate results.

The concentration is then calculated using the corrected absorbance:

c (mg/mL) = A₂₈₀ (1 cm) / ε₁%

For example, if a protein solution has an A₂₈₀ of 0.5 (normalized to 1 cm) and the protein has ε₁% = 10.0, the concentration is 0.5 / 10.0 = 0.05 mg/mL.

Factors Affecting A280 Accuracy

The A280 method is straightforward in principle, but several factors can compromise accuracy. Understanding these interferences is essential for interpreting results and for deciding when an alternative method, such as the Bradford or BCA assay, might be more appropriate.

Nucleic Acid Contamination

Nucleic acids (DNA and RNA) absorb strongly at 260 nm, but their absorbance tails into the 280 nm region. A solution containing both protein and nucleic acids will therefore have an inflated A280 reading, leading to an overestimation of protein concentration.

The extent of interference depends on the nucleic acid concentration and the protein-to-nucleic acid ratio. As a rough guide, a nucleic acid solution with an A₂₆₀ of 1.0 has an A₂₈₀ of approximately 0.5–0.6. This means that even modest nucleic acid contamination can significantly skew protein concentration measurements.

The ratio of absorbance at 260 nm to 280 nm (A₂₆₀/A₂₈₀) is commonly used to assess purity. For pure protein, this ratio is typically 0.5–0.6. For pure nucleic acid, it is approximately 2.0. A ratio above 0.6 suggests nucleic acid contamination, and the protein concentration should be interpreted with caution. For a detailed discussion of this ratio, see Nanodrop A260/a280 and Nanodrop 260/280.

If nucleic acid contamination is suspected, options include:

• Treating the sample with a nuclease (e.g., DNase I at 1 U/µL for 15 minutes at 37°C) followed by re-purification.
• Using a correction factor: protein concentration (mg/mL) = 1.55 × A₂₈₀ − 0.76 × A₂₆₀ (the Warburg-Christian formula). This correction is approximate and should be used with caution.
• Switching to a colorimetric assay such as Bradford or BCA, which are less sensitive to nucleic acid interference.

Buffer Components

Many common buffer components absorb UV light and can interfere with A280 measurements. The most problematic are:

• Dithiothreitol (DTT): A reducing agent commonly used at 1–10 mM in protein buffers. DTT absorbs strongly below 280 nm, with significant absorbance at 280 nm at concentrations above 10 mM. At 1 mM, DTT contributes approximately 0.01 AU at 280 nm, which is negligible for most purposes. However, at 100 mM (used in some refolding buffers), the contribution can be 1.0 AU or more.

• β-Mercaptoethanol (BME): Another reducing agent, typically used at 1–5 mM. BME absorbs at 280 nm, with a contribution of approximately 0.005 AU per mM.

• Imidazole: Used for elution of His-tagged proteins from nickel affinity columns, typically at 250–500 mM. Imidazole has significant absorbance at 280 nm, with a contribution of approximately 0.05 AU per 100 mM. This means that a protein eluted with 500 mM imidazole will have an artificially high A280 unless the blank contains the same concentration of imidazole.

• EDTA: A chelating agent used at 0.5–5 mM in many buffers. EDTA has minimal absorbance at 280 nm at these concentrations, but it can contribute at higher concentrations.

• Triton X-100 and other detergents: Non-ionic detergents such as Triton X-100 and NP-40 absorb strongly in the UV range, including at 280 nm. A 0.1% solution of Triton X-100 has an A₂₈₀ of approximately 0.5 AU. This makes A280 measurement essentially impossible in the presence of these detergents unless the detergent is removed or the blank is matched exactly.

The key to mitigating buffer interference is to use the exact buffer as the blank. If the protein is in 50 mM Tris-HCl, pH 8.0, 150 mM NaCl, 1 mM DTT, then the blank should be that same buffer. This subtracts the buffer's absorbance from the sample measurement. However, this approach fails if the buffer composition changes during the experiment (e.g., if the protein is eluted with a gradient of imidazole, each fraction will have a different imidazole concentration, and a single blank will not be accurate for all fractions).

Other Contaminants

• Phenol red: A pH indicator sometimes present in cell culture media. It absorbs at 280 nm and can interfere if not removed during protein purification.

• Lipids and lipoproteins: These can scatter light, causing an apparent increase in absorbance across the UV spectrum. Light scattering is wavelength-dependent (proportional to λ⁻⁴), so it is most pronounced at shorter wavelengths but can still affect A280.

• Particulate matter: Precipitated protein or other insoluble material can scatter light, inflating absorbance readings. Centrifugation or filtration (0.22 µm) before measurement can help.

Best Practices for Accurate A280 Measurements

Obtaining reliable A280 measurements requires attention to sample preparation, instrument operation, and data interpretation. The following practices will help ensure accurate results.

Sample Preparation

1. Ensure the protein is fully dissolved: Aggregates or precipitates will scatter light and inflate absorbance. If the sample appears cloudy, centrifuge at 15,000 × g for 10 minutes at 4°C and use the supernatant.

1. Match the blank to the sample buffer: The blank must be the exact buffer in which the protein is dissolved, including all additives (reducing agents, imidazole, glycerol, etc.). If the buffer contains a component that absorbs at 280 nm (e.g., imidazole), the blank must contain the same concentration of that component.

1. Remove nucleic acids if present: If the A₂₆₀/A₂₈₀ ratio is above 0.6, consider whether nucleic acid contamination is likely (e.g., from cell lysates). If so, treat with a nuclease or use an alternative quantification method.

1. Dilute if necessary: The NanoDrop can measure concentrations up to approximately 100 mg/mL (for BSA), but the linear range depends on the protein. If the A280 reading is above 2.0 (after path length correction), dilute the sample with the same buffer and re-measure. Dilution reduces the risk of non-linear absorbance due to intermolecular interactions.

Blanking and Replicates

1. Blank before each set of measurements: The blank should be measured at the start of each measurement session, and ideally re-measured if the buffer composition changes. Some protocols recommend blanking before each sample, but this is not necessary if the buffer is consistent.

1. Measure at least three replicates: Pipetting errors, air bubbles, and sample evaporation can cause variability between measurements. Measuring 2–3 µL aliquots in triplicate and averaging the results provides a more reliable estimate. The coefficient of variation (CV) between replicates should be less than 5% for a well-prepared sample.

1. Check the spectrum: The NanoDrop software displays the full absorbance spectrum. A smooth spectrum with a peak near 280 nm and a minimum near 250 nm is typical for a pure protein. An irregular spectrum or a rising baseline toward shorter wavelengths may indicate contamination or light scattering.

1. Record the path length: The NanoDrop reports the path length used for each measurement. If the path length is unusually short (below 0.3 mm) or unusually long (above 1.0 mm), this may indicate a problem with the sample (e.g., air bubbles) or the instrument.

Common Pitfalls and Troubleshooting

Even experienced researchers encounter problems with A280 measurements. The following are common failure modes and their solutions.

Air Bubbles

Air bubbles in the sample are a frequent source of erroneous readings. A bubble in the light path will scatter light and cause an artificially high absorbance. Bubbles can also prevent the sample from forming a proper liquid column between the pedestals, leading to an incorrect path length.

Symptoms: Absorbance readings that are highly variable between replicates, or a spectrum with an irregular, spiky appearance.

Solutions:

• Pipette the sample gently to avoid introducing bubbles.
• After pipetting onto the pedestal, inspect the sample visually for bubbles before lowering the arm.
• If a bubble is present, wipe the pedestal and re-pipette a fresh aliquot.
• Centrifuge the sample briefly (5,000 × g for 1 minute) to remove dissolved air before pipetting.

Concentration Limits

The NanoDrop has both lower and upper limits of detection. Below approximately 0.1 mg/mL (for BSA), the A280 signal is close to the noise level of the detector, and measurements become unreliable. Above approximately 100 mg/mL, the absorbance may exceed the linear range of the Beer-Lambert Law, and the path length correction may become inaccurate.

Symptoms: At low concentrations, readings may be negative or highly variable. At high concentrations, readings may plateau or show a non-linear relationship with dilution.

Solutions:

• For dilute samples (below 0.1 mg/mL), concentrate the sample (e.g., using a centrifugal concentrator with a 10 kDa molecular weight cutoff) or use a more sensitive method such as a fluorescence-based assay.
• For concentrated samples (above 50 mg/mL), dilute the sample with the same buffer and re-measure. Multiply the result by the dilution factor.

Incorrect Blanking

Using the wrong blank is one of the most common errors. If the blank does not match the sample buffer, the baseline will be incorrect, and all sample readings will be offset.

Symptoms: All sample readings are consistently too high or too low, or the A280 values are negative.

Solutions:

• Always use the exact buffer as the blank. If the buffer contains imidazole, DTT, or other UV-absorbing components, the blank must contain the same concentrations.
• If the sample was dialyzed or buffer-exchanged, use the final dialysis buffer as the blank.
• If the buffer composition is unknown, measure the buffer alone (without protein) to check its absorbance at 280 nm. If it is above 0.1 AU, the buffer itself will interfere with the measurement.

Sample Carryover

Residual sample on the pedestal from a previous measurement can contaminate the next measurement.

Symptoms: The first replicate of a new sample gives a reading that is inconsistent with subsequent replicates, or readings drift over time.

Solutions:

• Wipe both the upper and lower pedestals with a lint-free wipe between every measurement.
• After measuring a highly concentrated sample, consider wiping the pedestal twice.
• Run a blank measurement after a highly concentrated sample to verify that the pedestal is clean.

Negative Readings

A negative A280 reading can occur for several reasons:

1. The blank has a higher absorbance than the sample. This can happen if the blank buffer contains a component that absorbs at 280 nm (e.g., imidazole) and the sample has been diluted or buffer-exchanged into a different buffer.

1. The sample is too dilute. If the protein concentration is below the detection limit, the absorbance may be close to zero, and small variations can produce negative values.

1. The blank was measured incorrectly. If the blank contained an air bubble or was otherwise compromised, the baseline will be incorrect.

Solutions:

• Verify that the blank matches the sample buffer.
• Concentrate the sample if it is too dilute.
• Re-blank with a fresh aliquot of buffer.

Summary and Key Takeaways

The A280 method for protein quantification is a rapid, non-destructive technique that exploits the intrinsic UV absorbance of aromatic amino acids. The NanoDrop spectrophotometer extends this method to microvolume samples, making it ideal for precious or limited-volume samples. However, the accuracy of A280 measurements depends on a thorough understanding of the underlying principles and careful attention to experimental details.

The Beer-Lambert Law (A = ε × c × l) forms the basis of the calculation, and the extinction coefficient is the critical protein-specific parameter. Nucleic acids, buffer components, detergents, and particulate matter can all interfere with A280 measurements, and the use of an appropriate blank is essential for correcting buffer contributions. The NanoDrop's short path length and automatic path length correction enable measurements across a wide concentration range, but the linear range has limits, and samples outside this range require dilution or concentration.

By following best practices—matching the blank to the sample buffer, removing contaminants, measuring replicates, and checking the absorbance spectrum—you can obtain reliable protein concentration data from A280 measurements. When in doubt, compare A280 results with an independent method such as the Bradford or BCA assay.

Frequently Asked Questions

What does A280 measure in a protein sample?

A280 measures the absorbance of UV light at 280 nm, which is primarily due to the aromatic amino acids tryptophan and tyrosine, with minor contributions from phenylalanine and disulfide bonds. The absorbance is proportional to the number of these chromophores in the light path, and thus to the protein concentration.

How do I calculate protein concentration from A280?

Protein concentration is calculated using the Beer-Lambert Law: c = A₂₈₀ / (ε₁% × l), where A₂₈₀ is the absorbance at 280 nm (normalized to 1 cm path length), ε₁% is the mass extinction coefficient, and l is the path length (1 cm after normalization). For example, if A₂₈₀ = 0.5 and ε₁% = 10.0, then c = 0.5 / 10.0 = 0.05 mg/mL.

Why is my A280 reading negative?

A negative A280 reading typically indicates that the blank has a higher absorbance than the sample. This can occur if the blank buffer contains a UV-absorbing component (e.g., imidazole) that is not present in the sample, or if the sample is too dilute to produce a measurable signal. Check that the blank matches the sample buffer and that the sample concentration is within the detection range.

Can I use A280 for any protein?

A280 can be used for any protein that contains at least one tryptophan or tyrosine residue, which includes virtually all naturally occurring proteins. However, the accuracy depends on knowing the extinction coefficient. For proteins with few aromatic residues, the A280 signal will be weak, and the measurement may be unreliable at low concentrations. For proteins with unknown sequences, a generic coefficient (e.g., BSA or IgG) can be used, but the result will be approximate.

What is the extinction coefficient for a protein?

The extinction coefficient is a measure of how strongly a protein absorbs light at 280 nm. It is expressed as either a molar extinction coefficient (ε, in M⁻¹ cm⁻¹) or a mass extinction coefficient (ε₁%, in (mg/mL)⁻¹ cm⁻¹). For a protein with a known sequence, ε can be calculated from the number of tryptophan, tyrosine, and disulfide bonds. For BSA, ε₁% = 6.6; for IgG, ε₁% = 14.0.

How does nucleic acid contamination affect A280?

Nucleic acids absorb at 260 nm, but their absorbance tails into the 280 nm region. Contamination with DNA or RNA will inflate the A280 reading, leading to an overestimation of protein concentration. The A₂₆₀/A₂₈₀ ratio can help detect contamination: pure protein has a ratio of 0.5–0.6, while pure nucleic acid has a ratio of approximately 2.0.

What is the typical path length in a Nanodrop?

The NanoDrop uses a variable path length that is automatically adjusted based on the sample's absorbance. The typical path length is between 0.5 mm and 1.0 mm, but it can range from 0.05 mm to 1.0 mm. The instrument normalizes all readings to a 1 cm path length for reporting.

Why do I need to blank the Nanodrop?

Blanking establishes the baseline absorbance of the buffer and the instrument. The blank measurement is subtracted from all subsequent sample measurements, correcting for the absorbance of buffer components, the optical system, and any contaminants on the pedestal. Without proper blanking, all sample readings will be offset by the buffer's absorbance.

Key Takeaways

• A280 measures protein concentration by exploiting the UV absorbance of tryptophan, tyrosine, and phenylalanine at 280 nm.
• The Beer-Lambert Law (A = ε × c × l) governs the relationship between absorbance and concentration, with the extinction coefficient being protein-specific.
• The NanoDrop uses pedestal technology to measure 1–2 µL samples with a variable, automatically corrected path length.
• Nucleic acids, buffer components (DTT, imidazole, detergents), and particulate matter can interfere with A280 measurements.
• Always blank with the exact sample buffer and measure at least three replicates to ensure reliability.
• The A₂₆₀/A₂₈₀ ratio is a useful indicator of nucleic acid contamination, with values above 0.6 warranting caution.
• A280 is non-destructive and requires no reagents, but it is only as accurate as the extinction coefficient used and the quality of the sample preparation.

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