Molar Absorptivity (ε)
Definition
The proportionality constant in the Beer-Lambert law, in L mol⁻¹ cm⁻¹. A property of the analyte at a given wavelength, solvent and pH. Tabulated in pharmacopoeias and primary literature; an analyst rarely measures it from scratch.
- Units
- L mol⁻¹ cm⁻¹
- Wavelength dependent
- Different for each wavelength used in spectrophotometry
- Key role in detection
- High ε indicates low detection limits; enables identification at smaller concentrations
Common questions
What does molar absorptivity tell you about a compound?+
Molar absorptivity (ε) tells you how strongly a chemical absorbs light at a specific wavelength. A high value means the compound is a strong chromophore and will be easier to detect even at low concentrations. Low molar absorptivity means you need more of the compound present to see a measurable signal.
How do you find molar absorptivity values for analysis?+
You don't usually measure it from scratch. Instead, you look it up in pharmacopoeias and primary literature, where ε values are already tabulated for common compounds. Since it's a property of the specific analyte at a given wavelength, solvent, and pH, you need to match your exact experimental conditions to the reference value.
Why does molar absorptivity have those specific units?+
The units L mol⁻¹ cm⁻¹ come from the Beer-Lambert law, which relates absorbance to concentration and path length. Molar absorptivity is the proportionality constant in that equation. The units ensure that when you multiply ε by concentration (mol/L) and path length (cm), you get absorbance, which is unitless.
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...
- Absorbance (A)
- The base-10 logarithm of the ratio of incident to transmitted light intensity, A = log10(I0/I). Dimensionless. Linear with concentration in the working...
- Atomisation
- Conversion of a sample into free gaseous atoms in the ground state (AAS) or excited state (AES). The mandatory first step of...
- Bandpass and Slit Width
- The wavelength interval the monochromator delivers to the sample. Routine forensic UV-Vis runs at 1 nm or 2 nm bandpass; pharmacopoeial methods...
- Beer-Lambert Law
- A = εbc. Absorbance equals molar absorptivity times path length times concentration. The linear relation between absorbance and concentration that makes UV-Vis...
- Diode Array Detector (DAD)
- A UV-Vis detector that records the full 190 to 800 nm absorbance spectrum of every peak as it elutes, instead of a...
- 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.
- 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).
- Photomultiplier Tube (PMT)
- Cascaded electron-multiplication detector behind the monochromator slit on a dispersive scanning instrument. Reads one wavelength at a time at very high sensitivity.
- Stokes Shift (Raman)
- The Raman-scattered photon at lower energy than the incident photon. The Stokes line is the one routinely measured because it is stronger...
Explained in these topics
- Spectrophotometry for Forensic Science: UV, Visible, IR, Raman, AAS and AESWavelength-specific constant for a given absorbing species, units L mol⁻¹ cm⁻¹. High ε means a strong chromophore and a low detection limit.
- UV-Vis Spectrophotometry and the Beer-Lambert LawThe proportionality constant in the Beer-Lambert law, in L mol⁻¹ cm⁻¹. A property of the analyte at a given wavelength, solvent and pH. Tabulated in pharmacopo...