Fourier Transform Infrared Spectroscopy (FT-IR) Diamond Attenuated Total Reflection (ATR) Measurements: The Good, the Bad, and the (Really) Ugly

Fourier transform infrared spectroscopy (FT-IR) measurements using a diamond attenuated total reflection (ATR) accessory have become the primary sampling method for routine analyses. The accessibility and simplicity of the ATR sampling technique have made it available to a broad cross-section of casual users that are not aware of the spectral distortions that typically occur, particularly for strong absorption bands when compared to transmission experiments. This study discusses the basic principles of ATR measurements and presents simple criteria to measure good quality ATR FT-IR spectrum. The qualitative changes typical of good- and poor-quality ATR FT-IR spectra are presented so the analyst can easily identify photometrically accurate spectral data. These results highlight the significant spectral distortions that can occur for many materials when the analyst limits the sampling options to only a modern diamond ATR accessory. Because the difference in the refractive indices between the sample and ATR internal reflection element must be optimized, the user of a fixed incidence ATR accessory (typically 45°) should include both a diamond (and/or zinc selenide) as well as a germanium internal reflection element to ensure that high-quality ATR FT-IR spectra of a wide variety of samples can be measured.