Molar absorptivity
|
|
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
- <math>A = L\cdot (\epsilon_{\mathrm X} c_{\mathrm X} + \epsilon_{\mathrm Y} c_{\mathrm{Y}} + \cdots)<math>,
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 <math>c_i<math> (i=1,...,N) and N wavelengths <math>\lambda_i<math>, absorbances <math>A_i<math> are obtained:
- <math>A(\lambda_i) = L\sum_{j=1}^N \epsilon_j(\lambda_i) c_j<math>.
This set of equations can be solved for the concentrations <math>c_i<math>, 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.