Influence of anomalous dispersion on the ATR spectra of aqueous solutions

Transmission spectra of aqueous solutions are difficult to obtain because i n the 3 mu m infrared region, the high absorptivity of water requires the u se of thin films. In contrast, attenuated total reflectance (ATR) spectra o f aqueous solutions, not requiring thin films, are easily obtained. Unfortu nately, compared to measurements made by transmission, ATR measurements cau se some variation in the band shape due to the anomalous dispersion (AD) ef fect. To evaluate these variations, we studied pure water and KCI aqueous s olutions. We then compared the ATR spectra of both substances with the real and imaginary refractive index spectra calculated by using the Kramers-Kro nig relation. As long as the proper material is used in the ATR cell and th e concentration of water is not significantly decreased, then the IR-ATR me asurements directly reflect the chemistry of the sample.