Introduction

Infrared spectroscopy is a technique that distinguishes between chemical compositions based on their vibrational frequencies. It is a very used method due to its simplicity. Infrared spectroscopy is a very rapid method for accurate quantitative results. However, some structures require also the use of the Raman spectroscopy to be analyzed accurately.  Especially molecules, that are non-polar and very symmetrical, require the use of Raman spectroscopy. Therefore, infrared spectroscopy is commonly used in conjunction with Raman spectroscopy to gain more complete knowledge of the vibrational modes of the molecule.^[1]

IR spectroscopy can be used for samples in the gaseous, liquid and solid states. Different solid structures, such as powders, films or fibers, can be measured, thus making infrared spectroscopy a very versatile method. The results, however, are strongly dependent from the environment of the molecule.^[1] Usually liquid and solid samples are used.^[2]

Fourier Transform Infrared spectrometers (FT-IR) are very common in chemistry laboratories. Modern FT-IR spectrometers can be very compact in size.^[3]
 A tabletop FTIR-spectrometer is presented in the Figure 1.

Figure 1. Fourier Transform-Infrared spectrometer. Figure from Wikipedia (License: CC BY-SA 4.0).

General working principle

The basis for infrared spectroscopy lies in the fact that almost all materials can absorb light in the infrared region.^[2] The absorbed light in a given wavelength in the infrared region may cause a vibrational excitation in the sample. These excitations are possible, when the energy of the absorbed infrared light corresponds to the difference in energy between states within the vibrational modes. The wavelengths of the absorbed light are characteristic for a given molecule, and therefore composition of the sample can be determined. The vibrational modes have a theoretical basis in the group theory and quantum mechanics. However, theoretical approach is not usually required. This is because characteristic frequencies of the functional groups of the molecules can be obtained simply by using tables found in literature.^[1]

Figure 2. Example table of characteristic vibrational frequencies of functional groups. Figure from Wikipedia (License: CC BY-SA 4.0).

Instrumentation

Most IR machines today are Fourier transform Infrared spectrometers (FTIR). Their function is described in detail below.

The single frequency source of the IR-light is made of a glower or a heated element. This light is then directed to the beam splitter. This beam splitter is usually made of a thin film of germanium on a potassium bromide substrate. The splitter will split incident light so that half of the light will be transmitted through while other half will be reflected, thus creating two paths for the light. One of these beams is directed to a fixed mirror and then returned to the splitter. Other will be directed to a movable mirror and then send to the splitter. After both beams reach the splitter, half of this light is transmitted back to the source and half gets directed to the sample. Behind the sample is the detector, that is usually pyroelectric and is made of triglycine sulfate (DTGS). It is used to measure the intensity of the light. The FTIR machine processes the signal by combining the signal intensities of both the sample and the separately measured background. Thus created interferogram is transformed to an observable IR spectra (both adsorption and transmission spectra are available) using Fourier transformation.^[4]

Figure 3. A schematic picture of the working principle of the FTIR. (picture: Mario Mäkinen)

Example case

Mishra et al.^[5]used X-Ray diffraction and Fourier transformation infrared spectroscopy to analyze copper (II) thiourea complexes. Figure 4 shows the results of FTIR.

Figure 4. FTIR results for CuCl2 (II) Thiourea and CuSO4 (II) Thiourea samples^[5]. (License: CC BY 3.0)

References