Attenuated Total Reflectance (ATR) Infrared Spectroscopy: Overview

ATR infrared spectroscopy is an analytical technique for obtaining infrared spectra of challenging samples such as solids with limited solubility, films, threads, pastes, and adhesives. It requires little to no sample preparation and can analyze a wide range of materials, making it valuable in chemistry, materials science, and forensic science for sample characterization and identifying molecular composition.

When an infrared beam passes from a high refractive index crystal, such as diamond or germanium, to a lower refractive index sample at an angle greater than the critical angle, total internal reflection occurs. During this process, the beam penetrates a short distance beyond the crystal into the sample, generating an evanescent wave that interacts with the sample molecules.

The penetration depth of the evanescent wave depends on the beam wavelength, the refractive indices of both the crystal and sample, and the angle of the beam relative to the interface. These variables determine how far the evanescent wave extends into the sample and how effectively it interacts with the molecular vibrations.

The evanescent wave is absorbed by the sample at specific wavelengths corresponding to the vibrational frequencies of molecules. Each molecule has a unique combination of bond vibrations and absorbs infrared radiation at unique wavelengths. Comparing the absorption spectrum to known spectra helps identify the molecular composition of the sample.

ATR spectra contain the same absorption bands as conventional transmission infrared spectra but differ in relative intensities. This difference arises because the evanescent wave interacts with the sample surface differently than a transmitted beam, affecting how strongly different molecular vibrations are represented in the final spectrum.

ATR infrared spectroscopy requires minimal sample preparation compared to transmission methods, allowing direct analysis of solids, films, and pastes without dissolving or grinding samples. It can analyze a wide range of sample types and provides rapid characterization, making it more practical for routine analysis in research and industrial applications.

The evanescent wave generated at the crystal-sample interface penetrates into the sample and interacts with its molecules. The sample absorbs this short-lived radiation at specific wavelengths corresponding to molecular vibrational frequencies, causing the beam intensity to decrease. This reduction in beam intensity is called attenuated total reflectance.