The molar attenuation coefficient is a measurement of how strongly a chemical species attenuates light at a given wavelength. It is an intrinsicproperty of the species. The SI unit of molar attenuation coefficient is the square metrepermole, but in practice, it is usually taken as the M−1⋅cm−1 or the L⋅mol−1⋅cm−1. In older literature, the cm2/mol is sometimes used with corresponding values 1,000 times larger. In practice these units are the same, with the difference being expression of volume in either cm3 or in L. The molar attenuation coefficient is also known as the molar extinction coefficient and molar absorptivity, but the use of these alternative terms has been discouraged by the IUPAC.
The absorbance of a material that has only one attenuating species also depends on the pathlength and the concentration of the species, according to the Beer–Lambert law where
ε is the molar attenuation coefficient of that material;
Different disciplines have different conventions as to whether absorbance is decadic or Napierian, i.e., defined with respect to the transmission via common logarithm or a natural logarithm. The molar attenuation coefficient is usually decadic. When ambiguity exists, it is best to indicate which one applies. When there are N attenuation species in a solution, the overall absorbance is the sum of the absorbances for each individual species i: The composition of a mixture of N attenuating species can be found by measuring the absorbance at N wavelengths. The wavelengths chosen are usually the wavelengths of maximum absorption for the individual species. None of the wavelengths must be an isosbestic point for a pair of species. The set of the following simultaneous equations can be solved to find the concentrations of each attenuating species: The molar attenuation coefficient is directly related to the attenuation cross section via the Avogadro constantNA:
In biochemistry, the molar attenuation coefficient of a protein at depends almost exclusively on the number of aromatic residues, particularly tryptophan, and can be predicted from the sequence of amino acids. Similarly, the extinction coefficient of nucleic acids at 260 nm can be predicted given the nucleotide sequence. If the molar attenuation coefficient is known, it can be used to determine the concentration of a protein in solution.
