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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\cdot (\epsilon_{\mathrm X} c_{\mathrm X} + \epsilon_{\mathrm Y} c_{\mathrm{Y}} + \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 Wavelength s 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 \lambda_i, absorbances A_i are obtained:

:A(\lambda_i) = L\sum_{j=1}^N \epsilon_j(\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.