Question 1

What is the Beer-Lambert law?

Accepted Answer

The Beer-Lambert law (also called Beer’s law) describes the linear relationship between absorbance and concentration of an absorbing species in solution: $A = \varepsilon l c$, where A is absorbance (unitless), $\varepsilon$ is the molar extinction coefficient ($	ext{L} \cdot 	ext{mol}^{-1} \cdot 	ext{cm}^{-1}$), l is the optical path length (cm), and c is concentration (mol/L). It is the foundation of quantitative UV-Vis spectrophotometry.

Question 2

When does the Beer-Lambert law fail?

Accepted Answer

The law assumes dilute solutions with non-interacting chromophores. It breaks down at high concentrations (typically A > 2) due to molecular interactions, aggregation, and changes in refractive index. Scattering samples, fluorescent compounds, and chemical equilibria that shift with concentration also cause deviations. Always verify linearity with a standard curve.

Question 3

What is molar absorptivity (extinction coefficient)?

Accepted Answer

Molar absorptivity ($\varepsilon$), also called the molar extinction coefficient, is a measure of how strongly a chemical species absorbs light at a given wavelength. It is an intrinsic property of the molecule in a specific solvent, expressed in $	ext{L} \cdot 	ext{mol}^{-1} \cdot 	ext{cm}^{-1}$. Higher $\varepsilon$ values mean stronger absorption and thus greater analytical sensitivity.

Question 4

Why is absorbance above 2 unreliable?

Accepted Answer

An absorbance of 2 means only 1% of light reaches the detector (10^-2 = 0.01). At A > 2, the detected signal is so small that electronic noise dominates, and stray light in the instrument causes systematic underestimation of the true absorbance. Most spectrophotometers are accurate only up to A $\approx$ 1.5–2.0. Dilute the sample to bring readings into the linear range (A = 0.1–1.0).

Question 5

How does the A260 method work for DNA/RNA quantification?

Accepted Answer

Nucleic acids absorb UV light strongly at 260 nm due to their purine and pyrimidine bases. By convention, an A260 of 1.0 in a 1 cm cuvette corresponds to approximately 50 µg/mL of dsDNA, 33 µg/mL of ssDNA, or 40 µg/mL of RNA. The A260/A280 ratio assesses purity: ~1.8 for pure DNA, ~2.0 for pure RNA. Ratios below 1.7 suggest protein or phenol contamination.

Question 6

How do I correct for path length differences?

Accepted Answer

Standard cuvettes have a 1 cm path length, but microvolume instruments (e.g., NanoDrop) use much shorter paths (0.05–1 mm). The Beer-Lambert law accounts for this: simply enter the correct path length value. For microplate readers, path length varies with well volume and must be corrected — many instruments offer an automatic path length correction feature using the A977/A900 water absorbance ratio.

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