# Infrared Spectroscopy-Quo Vadis?
## 2 Infrared Spectroscopy: The Past  
Given the close resemblance to spectroscopy in the visible spectral region, NIR spectroscopy has been developed in a straightforward fashion. Fused silica is highly transparent in the NIR and thus is an excellent choice for NIR optics. Bright light sources as well as silicon based (e.g., line array) detectors for NIR spectroscopy are readily available at a fair price. A conceptual drawing of the basic elements of a spectroscopic setup is given in Figure 3. NIR spectroscopy provides excellent information, e.g., on the water content in analyte matrices [3]. Furthermore, carbohydrates, proteins, fats as well as plastics can be identified in the NIR [4]. Hence, industrial sorting machines are a main application of NIR spectroscopy as long as the broad and overlapping features in the NIR spectra are sufficient for identification, classification and quantification [5]. Hence, NIR has its capitalizing on highly sensitive and cheap photodetector technologies, which are not useable in MIR. Worthy of note is that the NIR spectral features are the overtones of the fundamental MIR vibrations, and thus, broader--and consequently more overlapping/less discriminatory--and significantly weaker in signal strength vs. the fundamental vibrational signatures prevalent in the MIR [6]. Figure 3: Conceptual illustration of an instrument for infrared spectroscopy. Commonly, such a setup comprises a light source, a wavelength selective element, a signal transducer, which defines the interaction path length, and a radiation detection device. This core setup is typically accompanied by sampling accessories such as sample feed conduits, flow cells, etc. as well as a robust housing and system control, signal acquisition and data evaluation electronics and appropriate–in current times multivariate–data evaluation, mining, and classification software packages. Figure 2: Growing number of scientific publications concerned with infrared spectroscopy. Results were retrieved from a SciFinder search querying for the term “infrared”, limiting the results to scientific journal publications. With a total number of over 1.2 M publications, the growth of infrared-related topics from the year 1924 to 2022 (2023 preliminary) is clearly evident. CAplus. CAS. [[https://scifinder.cas.org/](https://scifinder.cas.org/)]([https://scifinder.cas.org/](https://scifinder.cas.org/)) (accessed on 6 May 2022). MIR spectroscopy has clearly found widespread use in academic research facilities such as universities as well as industrial laboratories. The basic concept of accessing fundamental rotational, vibrational and roto-vibrational molecular transitions has long matured into convenient-to-operate devices, albeit requiring more specialized optics, materials, light sources and detectors than in the NIR. With IR spectroscopy gaseous, liquid and solid phase (i.e., condensed phase) samples can be accessed. Here, MIR spectroscopy enables the non-destructive probing of analytes in various matrices via their well-pronounced MIR signatures providing fundamental transitions of much higher signal strength vs. the NIR. In the early days of MIR spectroscopy, bulky devices requiring sophisticated and delicate optics, including refractive and diffractive optical components, limited the applicability or the wide acceptance of MIR spectroscopic devices as on field tools, and complementary techniques-as for example NIR spectroscopy benefitting from the optical telecommunications revolution-had progressed faster towards lower cost and portable devices. Since then, slow scanning speeds, limited energy throughput and spectral resolution since have been overcome by the development of Fourier-transform infrared (FTIR) spectroscopy. The Fellgett (i.e., high signal-to-noise ratio), Jacquinot (i.e., high throughput) and Connes (i.e., wavelength accuracy) advantages are well-known hallmarks of FTIR technologies and are classical textbook knowledge.