The molar extinction coefficient and mass extinction coefficient (defined below) should not be confused with the different definition of "extinction coefficient" used more commonly in physics, namely the imaginary part of the complex index of refraction (which is unitless). In fact, they have a straightforward but nontrivial relationship; see Mathematical descriptions of opacity.
In biochemistry, the extinction coefficient of a protein at 280 nm depends almost exclusively on the number of aromatic residues, particularly tryptophan, and can be predicted from the sequence of amino acids. If the extinction coefficient is known, it can be used to determine the concentration of a protein in solution.
Another measure of the extinction coefficient is E 1% which gives the mass extinction coefficient. E1% is the absorbance of a 1% solution by mass and has the units g-1L cm-1.
When there is more than one absorbing species in a solution, the overall absorbance is the sum of the absorbances for each individual species (X, Y etc.):
The composition of a mixture of N components can be found by measuring the absorbance at N wavelengths (the values of ε for each compound at these wavelengths must also be known). The wavelengths chosen are usually the wavelengths of maximum absorption (absorbance maxima) for the individual components. None of the wavelengths must be an isosbestic point for a pair of species. For N components with concentrations and wavelengths , absorbances are obtained:
This set of simultaneous equations can be solved to find concentrations of each absorbing species.
The molar extinction coefficient is directly related to the Absorption cross section via: