Molar Absorption Coefficients - Spectra And Spectral Data

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Molar Absorption Coefficients

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Part of the problem when looking for molar absorption coefficients is the confusion around correct terminology. Many students and researchers still use obsolete terms like "extinction coefficient." Here are some definitions for clarity.

Molar absorption coefficient (ε) Synonyms: Molar extinction coefficient, Molar absorptivity "The recommended term for the absorbance for a molar concentration of a substance with a path length of l cm determined at a specific wavelength. Its value is obtained from the equation ε = A / cl Strictly speaking, in compliance with SI units the path length should be specified in meters but it is current general practice for centimeters to be used for this purpose. Under defined conditions of solvent, pH and temperature the molar absorption coefficient for a particular compound is a constant at the specified wavelength." -- Denney, R.C. Dictionary of Spectroscopy, 2nd ed.; Wiley: New York, 1982; pp 119-20.

Molar absorptivity "Synonym: Molar (decadic) absorption coefficient. Decadic absorbance divided by the path-length l and mole concentration c, of the absorbing material.

ε = A10 / cl.

The molar absorptivity is a Beer-Lambert absorption coefficient. SI unit: m2 mol-1." -- Handbook of Vibrational Spectroscopy; Chalmers, J.M., Griffiths, P.R. Eds.; Wiley: New York, 2002; Vol.5, p 3772.

"The term molar absorptivity for molar absorption coefficient should be avoided." -- IUPAC Gold Book

Extinction coefficient "A term that has been widely used for the molar absorptivity, unfortunately often with values given in ill-defined units. Use of this term has been discouraged since the 1960s, when international agreement with non-chemical societies reserved the word "extinction" for diffusion of radiation, i.e. the sum of the effects of absorption, scattering, and luminescence." -- Handbook of Vibrational Spectroscopy; Vol.5, p 3760.

"Seldom, if ever, is it safe to assume adherence to Beer's law and use only a single standard to determine the molar absorptivity. It is never a good idea to base the results of an analysis on a literature value for the molar absorptivity." --Skoog, D.A., Holler, F.J., Crouch, S.R. Principles of Instrumental Analysis, 6th ed.; Brooks/Cole, 2007; p 375.

• Reaxys This link opens in a new window The UV.EAC field contains numerically searchable extinction/absorption coefficient data for organic compounds at specified wavelengths and solvents, with source literature references. This table is found under the Spectra-UV/VIS property heading in the substance record display.
• Properties of Organic Compounds This link opens in a new window Entries often contain ε in the UV data field following the peak wavelength. Covers about 29,000 important organics.
• Combined Chemical Dictionary This link opens in a new window Database containing information on over 600,000 substances. The UV data fields for some organic compounds display ε in context with λmax, but ε is not numerically searchable.

• NIST Chemistry WebBook Interactive UV-VIS spectra included in this database graph log ε on the Y axis. Place the mouse pointer over the desired wavelength to see log ε. The WebBook contains an unknown number of UV-VIS spectra compiled from older sources.

• SciFinder This link opens in a new window Searching the wider literature for ε values is imprecise at best. The easiest approach is to search for references using the query phrase "molar absorptivity of [CAS Registry Number]" and choose hits where these terms are closely associated with each other. (SciFinder will automatically truncate "absorpt*.) Another method is to first search for the compound by Registry Number, name, or structure; get All References, and then refine the results by topic, first using the phrase "molar absorption coefficient" and then, for older papers, "extinction coefficient." Note: These textual search methods depend on the absorption coefficient being mentioned in the abstract, title, or indexing terms. If ε is not a key point of the paper, it usually is not mentioned. A fallback would be to search for papers containing ultraviolet data on the compound. The original document usually has to be consulted, and much of the relevant literature is not recent. See the Literature Searching page for more details. more... less...

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Literature values of ε and log ε can be found scattered in various handbooks, although they are not always clearly labeled. They generally appear in context with absorption peaks or as the Y axis in spectral plots. A bibliography of legacy print sources is provided here as a PDF.

• Molar Absorption Coefficients - Print Sources

• Last Updated: Aug 25, 2025 1:31 PM
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