Influence of anomalous dispersion on the ATR spectra of aqueous solutions

Transmission spectra of aqueous solutions are difficult to obtain because in the 3 mu m infrared region, the high absorptivity of water requires the use of thin films. In contrast, attenuated total reflectance (ATR) spectra of aqueous solutions, not requiring thin films, are easily obtained. Unfortunately, compared to measurements made by transmission, ATR measurements cause some variation in the band shape due to the anomalous dispersion (AD) effect. To evaluate these variations, we studied pure water and KCI aqueous solutions. We then compared the ATR spectra of both substances with the real and imaginary refractive index spectra calculated by using the Kramers-Kronig relation. As long as the proper material is used in the ATR cell and the concentration of water is not significantly decreased, then the IR-ATR measurements directly reflect the chemistry of the sample.