13.17 : Attenuated Total Reflectance (ATR) Infrared Spectroscopy: Overview

Attenuated total reflectance (ATR) infrared spectroscopy is a powerful analytical technique used to study the composition of materials. It is widely employed in chemistry, materials science, forensic science, and other fields where sample characterization is required. ATR has several advantages over traditional transmission IR spectroscopy, including the requirement of little to no sample preparation and the ability to analyze a wide range of samples.

The ATR process begins by directing a beam of IR radiation onto a diamond or germanium crystal. The high refractive index leads to the total internal reflection of the beam within the ATR crystal. When the beam hits the interface between the high refractive index crystal and the lower refractive index sample, an evanescent wave is generated that extends beyond the surface of the crystal into the sample. As the evanescent wave interacts with the sample, some of the energy from the wave is absorbed. The depth to which this wave penetrates the sample depends on the incident light's wavelength, the incidence angle, and the refractive indices of the crystal and the sample. The absorbed energy corresponds to the vibrational frequencies of the molecules in the sample, creating a unique absorption spectrum. Each molecule has a unique combination of bond vibrations and absorbs IR radiation at unique wavelengths. Comparing the absorption spectrum of the sample to known spectra helps identify the molecular composition of the sample.