Mid‐Infrared Spectroscopy: Anomalies, Artifacts and Common Errors

Abstract Understanding, appreciating and recognizing the influences that anomalies, artifacts and common errors have on the appearance and spectral contrast of a mid‐infrared spectrum is vital if data are to be interpreted and quantified correctly. This article presents examples of some of the common imperfections that may be observed in mid‐infrared spectra, and discusses concisely their origin. It covers the effects of atmospheric intrusion (water vapour and carbon dioxide), and the influences of stray light on the spectra of both liquid and solid samples. It includes a discussion of the superimposition on a spectrum of a sinusoidal waveform, such as ‘channel spectra’ sometimes seen in high‐resolution gas phase spectra or ‘interference fringes’ observed in polymer film transmission spectra. Simple examples of spectra of common contaminants are poly(dimethyl siloxane) – silicone grease/oil‐, phthalate plasticizers from poly(vinyl chloride) (PVC) sources – gloves, pipes ‐, and nitrate from non‐spectroscopic grade KBr. The effects of anomalous dispersion (Christiansen effect) and scatter are illustrated for both a KBr disk and a Nujol ® mull spectrum; the influence of anomalous dispersion and angle of incidence on attenuated total reflection (ATR) spectra is also discussed, as is the overlay of a specular reflection spectrum on diffuse reflection and transflection spectra. The influence of particle size on spectral contrast for a range of sample presentation methods is illustrated. The origins of saturation effects in photoacoustic FT‐IR spectra, inverted bands in emission spectra, and diffraction effects in FT‐IR microscopy measurements are outlined. Other considerations include choice of single‐beam background, detector non‐linearity, data manipulation, plot software characteristics, instrument linearity, and external reflection mixed‐mode spectra.