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

$A\; =\; L\backslash cdot\; (\backslash epsilon\_\{\backslash mathrm\; X\}\; c\_\{\backslash mathrm\; X\}\; +\; \backslash epsilon\_\{\backslash mathrm\; Y\}\; c\_\{\backslash mathrm\{Y\}\}\; +\; \backslash cdots)$,

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 $c\_i$ (i=1,...,N) and N wavelengths $\backslash lambda\_i$, absorbances $A\_i$ are obtained:

$A(\backslash lambda\_i)\; =\; L\backslash sum\_\{j=1\}^N\; \backslash epsilon\_j(\backslash lambda\_i)\; c\_j$.

This set of equations can be solved for the concentrations $c\_i$, 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.

Analytical chemistry