Quantitative Chemometrics Using Refractive Index Spectra

Classical quantitative chemometrics based on absorbance spectra has been routinely performed for approximately 40 years. Since absorbance is a function of the absorption index, it is natural to extend chemometric methods to the refractive index function. This function, related to the absorption index via the Kramers--Kronig relations, is derived from corrections applied to absorbance spectra to ensure compliance with wave optics principles. In this note, we demonstrate that, at least in the quasi-thermodynamically ideal binary system of benzene and toluene, classical quantitative chemometrics performs better when based on refractive index spectra than when based on absorption index spectra. The primary reason for this difference is that the refractive index at a given wavenumber integrates all changes resulting from absorptions at higher wavenumbers. This property is particularly advantageous in non-absorbing regions, where absorption index spectra provide no information about the system's composition.