Beer-Lambert Law
Definition
A = εbc. Absorbance equals molar absorptivity times path length times concentration. The linear relation between absorbance and concentration that makes UV-Vis quantitation possible across a calibrated range, typically two to three orders of magnitude before non-linearity sets in.
- Equation
- A = εcl (absorbance equals molar absorptivity times concentration times path length)
- Core application
- Quantitative determination of sample concentration using light absorption in UV-Visible, IR, and atomic absorption spectroscopy
- Linear range
- Valid across approximately 2-3 orders of magnitude before deviations occur
Common questions
What does the Beer-Lambert law equation actually measure?+
The equation A = εcl links absorbance (how much light a sample stops) to three things: the sample's concentration, the path length (how far light travels through it), and the material's molar absorptivity (how eagerly that material absorbs light at a given wavelength). In the lab, this is how you turn a light measurement into a chemical quantity.
Why is Beer-Lambert law so important in forensic spectrophotometry?+
It is the foundation of quantitative analysis. Every UV-Visible, infrared, and atomic absorption calibration in a forensic lab relies on this linear relationship between absorbance and concentration. It lets you build a calibration curve and then determine unknown concentrations from their absorbance readings.
Does the law work for all concentration levels?+
It holds true across a useful range, typically two to three orders of magnitude, before non-linearity sets in at very high concentrations. Staying within the linear range is critical for reliable quantitation in forensic analysis.
Related terms
- Chromophore
- The molecular subunit responsible for a dye's colour, typically a conjugated double-bond system; chemical erasure destroys the chromophore while leaving the decolourised...
- Absorption
- The purification technique in which a crude antiserum is mixed with antigen preparations from cross-reacting species. Cross-reactive antibodies bind those antigens and...
- Atomisation
- Conversion of a sample into free gaseous atoms in the ground state (AAS) or excited state (AES). The mandatory first step of...
- ATR (Attenuated Total Reflectance)
- An FTIR sampling mode in which the sample is pressed against a diamond or ZnSe crystal and the evanescent wave probes the...
- Band Spectrum
- A continuous-looking envelope of closely spaced lines produced by molecules. Electronic, vibrational and rotational transitions couple together so a single electronic excitation...
- Electron Impact (EI) Ionisation
- A hard ionisation mode at 70 eV used in GC-MS that produces extensive, reproducible fragmentation. The fragmentation pattern is matched against libraries...
- Fingerprint Region
- The 1500 to 400 cm⁻¹ stretch of an infrared spectrum where bending and combination bands produce a pattern unique to a given...
- Fingerprint Region (IR)
- 1500 to 400 cm⁻¹. The complex lower-wavenumber zone where the whole-molecule pattern is unique enough to identify a substance by overlay.
- Fluorescence Lifetime
- The average time a fluorophore spends in the S1 excited state before emitting a fluorescence photon; typically 1-10 ns for organic fluorophores,...
- Fluorescence Quantum Yield (Phi)
- The fraction of absorbed photons that result in fluorescence emission (0 to 1); determines the brightness of a fluorescent signal and is...
- Hollow Cathode Lamp (HCL)
- A sealed lamp whose cathode is made of the element being analysed, filled with argon or neon at low pressure. Strikes a...
- ICP Source
- Inductively coupled argon plasma at 6000 to 10000 K. The hottest practical excitation source for AES, giving simultaneous multi-element analysis (ICP-OES, ICP-MS).
Explained in these topics
- Atomic and Molecular Spectra FundamentalsA = εbc. Absorbance equals molar absorptivity times path length times concentration. The universal absorption-quantitation equation that runs every UV-Vis, IR...
- Light-Matter Interaction: Absorption, Reflection, FluorescenceA = εcl; relates absorbance to the molar extinction coefficient (ε), concentration (c), and path length (l) of an absorbing solution; the foundation of quantit...
- Spectrometric Detection: UV-Vis, FTIR and Mass SpectrometryA = εbc. Absorbance equals molar absorptivity times path length times concentration. The linear relation between absorbance and concentration that makes UV-Vis...
- Spectrophotometry for Forensic Science: UV, Visible, IR, Raman, AAS and AESA = εcl. Absorbance equals molar absorptivity times concentration times path length. The single key equation in UV-Visible spectrophotometry.