Molar absorptivity
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In analytical chemistry, the molar absorptivity or molar extinction coefficient ε of a chemical species at a given wavelength is a measure of how strongly the species absorbs light at that wavelength. It is an intrinsic property of the species; the actual absorbance of a sample is dependent on its thickness L and the concentration c of the species.
When there is more than one absorbing species in a solution, the absorbance is the sum of the absorbances for each individual species. The absorbance at a given wavelength of a mixture of species X, Y, ... is given by
,
where A is the absorbance of the mixture. The composition of a mixture of two or more compounds can be found by using two or more wavelengths and by knowing ε at each wavelength for each compound. These wavelengths are usually chosen as the wavelengths of maximum absorption (absorbance maxima) for the individual components. For N components with concentrations ci (i=1,...,N) and N wavelengths λi, absorbances Ai are obtained:
.
This set of equations can be solved for the concentrations ci, provided that none of the wavelengths is an isosbestic point for any pair of species. An isosbestic point of two species is a wavelength where the two species have equal extinction coefficients.
