Effective path length in attenuated total reflection spectroscopy

Attenuated total reflection (ATR) spectroscopy is now the most popular sampling technique for the measurement of infrared spectra of condensed phase samples. Most practitioners of ATR spectroscopy use the equation for depth of penetration, d(p), to estimate the path length of the evanescent wave through the sample. However, the effective path length, de, of the evanescent wave in an ATR measurement, i.e., the equivalent path length in a transmission measurement that would lead to an absorption band of the same intensity, is a more accurate metric than dp. In measurements designed to obtain the absorptivity of bands in the spectrum of a strongly absorbing viscous liquid, we have shown that the refractive index used in the expressions for d(e), must be modified to take into account the effect of anomalous dispersion before accurate effective path lengths and band absorptivities can be measured.