Infrared absorption spectrometry

By exploiting the interactions between matter and light, optical spectroscopies can be used to obtain the chemical composition and characterize the physical and chemical properties of samples. Depending on the spectral range used (or the energy of the photons used), this interaction probes the various types of energy levels of matter. In the X-ray or hard ultraviolet range, the electron levels of core electrons, electrons close to nuclei, are excited, while in the 200-400 nm range of standard ultraviolet, electrons involved in chemical bonds are probed. In the infrared, vibrational levels are probed to characterize both the chemical groups making up the sample and its molecular structure, as well as the physical properties of order, disorder, mechanical stress, optics and so on. However, the nature of the information obtained depends on the method used to record the spectrum and the physical state of the sample (gas, liquid, solid, etc.).

In practice, today's Fourier transform spectrometers (FTIR) have qualities that enable a wide range of analysis possibilities. Analysis can be qualitative, to identify a compound on the basis of its spectral signature (complete spectral fingerprint) or, using changes in a restricted zone of this fingerprint, to establish fine changes in structure or molecular interactions depending on the energy levels "probed" by the radiation. In addition to identification, the analysis will be quantitative, enabling a substance to be dosed, thanks to the evolution of its spectral signature as a function of the quantity of material measured.

This article is specifically dedicated to infrared absorption spectroscopy or vibrational absorption spectroscopy. After a brief review of general notions on matter-radiation interaction, a second section presents the main methods of recording an infrared spectrum, comparing their advantages and disadvantages. In the third part, the methods currently readily available for recording a spectrum, depending on whether the sample studied is solid (solid, film or powder), liquid or gaseous, will be detailed for their current applications. The fourth part concludes with a few methods for qualitative analysis of infrared spectral data, or for quantitative prediction of a variable, such as concentration, but also physico-chemical quantities such as acidity, hygrometry, the degree of organization of an assembly, the intrinsic pressure undergone by a sample, and so on. This type of computer processing is integrated into a large number of acquisition software packages, and is becoming increasingly widespread in industry for users with no particular pre-training.