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. Analytical chemistry is the analysis of material samples to gain an understanding of their chemical composition and structure. ... Chemical species is a common, general name for atoms, molecules, molecular fragments and ions as entities being subjected to a chemical process or to a measurement. ... The wavelength is the distance between repeating units of a wave pattern. ... In spectroscopy, the absorbance A is defined as , where I is the intensity of light at a specified wavelength λ that has passed through a sample (transmitted light intensity) and is the intensity of the light before it enters the sample (or incident light intensity). ... In spectroscopy, the absorbance A is defined as , where I is the intensity of light at a specified wavelength λ that has passed through a sample (transmitted light intensity) and is the intensity of the light before it enters the sample (or incident light intensity). ...

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 c_i (i=1,...,N) and N wavelengths λ_i, absorbances A_i are obtained: The wavelength is the distance between repeating units of a wave pattern. ...

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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. It has been suggested that isobestic point be merged into this article or section. ...