Key Parameters for High-Quality ATR-FT-IR Spectroscopy of Liquids and Solids

Selecting an optimal infrared sampling approach is critical to unlock the problem-solving capabilities of Fourier transform infrared (FT-IR) spectroscopy. This work evaluates how the physical state of the sample, the internal reflection element (IRE)/sample interface as well as differences between the sample and IRE refractive indices impact the attenuated total reflection (ATR) infrared spectral quality. Anomalous dispersion within infrared absorption bands has a critical effect on the ATR infrared spectral quality relative to classical transmission preparations. Using a single-bounce ATR accessory at 45° incidence, infrared spectra were acquired with diamond and Ge IREs and compared against transmission-based measurements (capillary films for liquids; Nujol mulls and KBr discs for solids). Liquid immersion oil standards spanning refractive index values of η = 1.45–1.70 were used to systematically examine refractive index effects in the ATR infrared spectra of the samples. Water was also examined as a liquid with lower refractive index (η = 1.33) but with very large absorbance values. Commonly encountered, industrially relevant inorganics species spanning refractive indices of η ≈ 1.45–2.5 were measured to assess the additional challenges of ATR infrared measurements of solid-state materials. Characteristic spectral distortions of ATR infrared spectra include baseline offsets and slope, anomalous-dispersion-based infrared band asymmetry and broadening and in extreme cases complete loss of diagnostic infrared bands. These effects were far less pronounced in liquids than in solid materials. The advantages of using higher refractive index IREs such as Ge are illustrated with application examples including carbon black-filled rubber crumb, inorganic-containing polyolefin roofing tile, and lyophilized mining pregnant leach solution. In these cases, Ge-ATR infrared spectral measurements consistently reduce anomalous dispersion-based spectral artifacts and improve spectral information content, resulting in significantly improved interpretability and digital library spectral matching compared with diamond-ATR FT-IR measurements.