• Engineering

  Infrared spectroscopy

  Posted by Encyclios on May 16, 2023 at 1:51 PM

  Infrared spectroscopy is the study of the interaction of infrared light with matter; it is the analysis of infrared light interacting with a molecule. This can be analyzed in three ways by measuring absorption, emission, and reflection. The fundamental measurement obtained in infrared spectroscopy is an infrared spectrum, which is a plot of measured infrared intensity versus wavelength (or frequency) of light. The main use of this technique is in organic and inorganic chemistry. It is used by chemists to determine functional groups in molecules. Infrared spectroscopy measures the vibrations of atoms, and based on this it is possible to determine the functional groups. Generally, stronger bonds and light atoms will vibrate at a high stretching frequency (wavenumber). Infrared spectroscopy is an analytical technique that takes advantage of the vibrational transitions of a molecule, has been of great significance to scientific researchers in many fields such as protein characterization, nanoscale semiconductor analysis, and space exploration.

  Fourier transform infrared (FTIR) spectroscopy is a measurement technique that allows one to record infrared spectra. FTIR spectrometers (Fourier Transform Infrared Spectrometer) are widely used in organic synthesis, polymer science, petrochemical engineering, pharmaceutical industry, and food analysis. In addition, since FTIR spectrometers can be hyphenated to chromatography, the mechanism of chemical reactions and the detection of unstable substances can be investigated with such instruments. Up till FTIR spectrometers, there have been three generations of infrared radiation spectrometers:

  • the first generation infrared radiation spectrometer was invented in the late 1950s. It utilizes a prism optical splitting system. The prisms are made of NaCl. The requirement of the sample’s water content and particle size is extremely strict. Furthermore, the scan range is narrow. Additionally, the repeatability is fairly poor. As a result, the first generation infrared radiation spectrometer is no longer in use;
  • the second generation IR spectrometer was introduced to the world in the 1960s. It utilizes gratings as the monochrometer. The performance of the second generation infrared radiation spectrometer is much better compared with infrared radiation spectrometers with prism monochrometer, But there are still several prominent weaknesses such as low sensitivity, low scan speed and poor wavelength accuracy which rendered it out of date after the invention of the third generation infrared radiation spectrometer;
  • The invention of the third generation infrared radiation spectrometer, Fourier transform infrared spectrometer, marked the abdication of monochrometer and the prosperity of interferometer. With this replacement, infrared radiation spectrometers became exceptionally powerful. Consequently, various applications of infrared radiation spectrometers have been realized.

  The molecules of chemical compounds selectively absorb infrared radiation of certain wavelengths. Therefore, by crossing a sample of the compound under examination successively by infrared radiation of different wavelengths and recording on a graph in which measure each of them is absorbed, it is obtained a trace that takes the name of absorption spectrum.

  Infrared absorption spectra are called vibration and rotation spectra, because infrared radiations convey quantities of energy able to cause in molecules vibrations of atoms or rotational motions of an atom or a group of atoms around the axis that binds it to another atom. Vibrations can occur in the sense of temporarily varying both the distances between two atoms linked to each other, and the angle that two atoms form with a third atom to which they are both linked.

  Spectra in ultraviolet or visible are generally very simple because they are due to vibrations corresponding to electrons of double and triple bonds. Infrared spectra are also originated by simple bonds; consequently any organic molecule presents an infrared spectrum much more complex with a large number of absorption bands. For these characteristics the infrared absorption spectrum can quickly provide a series of information on the structural elements present in a molecule of unknown structure, also because of its complication, is an absolutely typical property of the molecule of a compound: while two molecules also very different can present various physical properties and even a spectrum of ultraviolet absorption in practice identical, there are no two compounds that present equal absorption spectrum in the infrared. This can therefore be useful to ascertain the identity of a compound of which is available a quantity even in the order of a few milligrams by comparing the spectrum with those already known.

  Infrared absorption spectra can be measured on substances in solid, liquid or solution state, or even on gaseous substances.

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