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Spectra and Spectral Data

Molar Absorption Coefficients

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.

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. 

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