Molar Extinction Coefficient (ε)
Definition
The intrinsic absorptivity of a molecule at a given wavelength, in L/(mol·cm); oxyhaemoglobin at 415 nm has ε ≈ 125,000, making it one of the most strongly absorbing molecules used in forensic work.
- Symbol
- ε (epsilon)
- Units
- L/(mol·cm)
- Example value
- Oxyhaemoglobin at 415 nm, ε ≈ 125,000
- Governing law
- Beer-Lambert law
Common questions
Why does a high molar extinction coefficient matter for detecting bloodstains?+
A high epsilon means even trace concentrations of haemoglobin produce a strong, measurable absorption signal, which is why haemoglobin-based methods such as the Soret band absorption are sensitive enough to detect very small or diluted bloodstains.
How is the molar extinction coefficient used quantitatively in casework?+
Combined with measured absorbance and the Beer-Lambert relationship, it allows an analyst to estimate the concentration of a target chromophore such as haemoglobin in a sample rather than only confirming its presence.
Related terms
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- Beer-Lambert Law
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- Chromophore
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- Fluorescence Lifetime
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- Fluorescence Quantum Yield (Phi)
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- Jablonski Diagram
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- Kubelka-Munk Model
- A diffuse-reflectance theory relating the measured reflectance of a scattering-plus-absorbing layer to its absorption and scattering coefficients; used in forensic paint and...
- Phosphorescence
- Light emission from the triplet excited state (T1) via a spin-forbidden radiative transition; characterised by emission persisting after the excitation source is...
- Raman Scattering
- Inelastic scattering of a photon from a molecular vibration; the scattered photon carries a frequency shift equal to the vibrational frequency, producing...
- Specular Reflection
- Mirror-like reflection from a smooth surface where angle of incidence equals angle of reflection; minimised in forensic photography with crossed polarisers to...
- Stokes Shift
- The wavelength difference between the excitation peak and the emission peak of a fluorophore. A positive Stokes shift (emission at longer wavelength...