Molar Absorption Coefficients
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.
"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
"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.
Where to Look for Them - Online
- Combined Chemical Dictionary
- 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.
- Properties of Organic Compounds
- Entries often contain ε in the UV data field. Covers about 29,000 important organics.
- 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 results display.
- SciFinder (Chemical Abstracts)
- Searching the wider literature for ε values is imprecise at best. The easiest approach is to search for references using the 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, using the phrases "molar absorption coefficient" and then, for older papers, "extinction coefficient." Note: These 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.
Where to Look for Them - Print Sources
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.
Users outside UT-Austin can click the button to find other owning libraries.
- Atlas of spectral data and physical constants for organic compounds.
- QD 272 S6 G73 1975 Spectra (6 vols., 2nd ed., CRC, 1975)
- Molar absorption coefficients appear in parentheses next to the wavelengths in the Ultraviolet column.
- Correlation tables for the structural determination of organic compounds by ultraviolet light absorptiometry.
- QD 272 S6 P447 1974 Spectra (Verlag Chemie, 1974)
- log ε is given in parentheses after the wave number. Compounds are ordered by R (ring or unsaturation) number, then by number and kind of heteroatoms in alphabetical order. No indexes.
- Data for biochemical research.
- QP 520 D37 1986 Reference (3rd ed., Oxford, 1986)
- ε or log ε is occasionally given in parentheses next to the λmax in the General Remarks column in the chapters on amino acids, amines, amides, peptides; carboxylic acids, alcohols, aldehydes, ketones; phosphate esters; vitamins and coenzymes, etc.
- Gmelin handbook of inorganic and organometallic chemistry.
- Reference Stacks (8th ed., Springer, 1924-98)
- ε may be found in the Optical Absorption (Lichtabsorption) section of a compound's entry. See the guide to using Gmelin.
- Handbook of biochemistry and molecular biology.
- QP 514.2 H34 1975 Reference (3rd ed., CRC, 1975-77)
- Vol. A2, pp.383-545, table titled "Molar absorptivity and A(1% 1cm) values for proteins at selected wavelengths of the ultraviolet and visible region." This table is reprinted in the Practical handbook of biochemistry and molecular biology (CRC, 1989), pp.196-358.
- Handbook of biochemistry and molecular biology.
- QP 514.2 H34 2010 Reference (4th ed., CRC, 2010)
- Table on pp.269-358 contains acidic, neutral and alkaline UV and pK data and references for 246 purines, pyrimidines, nucleosides and nucleotides.
- Handbook of data on organic compounds.
- QD 257.7 H36 1994 Reference (3rd. ed., CRC, 1994)
- Molar absorption coefficients when available are given in parentheses next to major bands in the UV section of the entry. Also available online as "Properties of Organic Compounds" (see above).
- Handbook of natural products data.
- QD 415.7 R34 1990 Spectra (Elsevier, 1990-94)
- Includes UV (λmax and log ε) data for plant-derived natural products. v.1 - Diterpenoid and steroidal alkaloids; v.2 - Pentacyclic triterpenoids; v.3- Isoquinoline alkaloids.
- Handbook of ultraviolet and visible absorption spectra of organic compounds.
- QC 462.85 H5 1967 Spectra (Plenum, 1967)
- Part 1 is a table of absorption maxima arranged by chemical structure of the absorbing chromophore. Part 2 is the same list grouped by absorption maximum. log ε appears in the column next to the λmax. No indexes.
- Merck index.
- Handbook Table
- ε is given in parentheses next to the UV max in the properties paragraph of some entries.
- Organic electronic spectral data.
- QC 459 O64 Spectra (Wiley, 1946-89)
- log ε is given in parentheses next to the λmax. There is no cumulative index to this annual series, so you have to search year by year, making it a last resort.
- Practical handbook of spectroscopy.
- QD 95 P73 1991 Spectra (CRC, 1991)
- The UV table (pp.565-646) data are sorted into groups based on the intensity of the strongest band, then in decreasing order by wavelength of the strongest band. If you already have ε you can identify an unknown this way.
- Sadtler ultraviolet spectra.
- Sadtler Alcove
- Molar absorptivities are usually provided in the spectrum's data table, denoted as am. Use the main index set to locate the compound, as described on the Sadtler page. A one-volume abridgement with 1600 important compounds, The Sadtler handbook of ultraviolet spectra, was published in 1979.
- Specifications and criteria for biochemical compounds.
- QP 520 N38 1972 Storage (3rd ed., NAS, 1972)
- ε is given at selected wavelengths for a number of carotenoids, coenzymes, nucleotides, and porphyrins. A 1977 supplement gives ε for 30 biogenic amines.
- Tables of spectrophotometric absorption data of compounds used for the colorimetric determination of elements.
- QD 113 I55 Spectra (Butterworths, 1963)
- Entries for anions and cations include extinction coefficient values "with respect to one gram-atom of the element to be determined per liter of solution, not to one mole of the colored complex."
- UV-VIS atlas of organic compounds.
- QC 462.85 P47 1992 Spectra (2nd ed., VCH, 1992)
- Exact values of εmax are indicated on the spectral traces.