Spectroscopy for Quality Control: Fourier & Raman Techniques

This thesis analyses Fourier Transform Spectroscopy and Raman Spectroscopy techniques, which are used in determining the unique properties of unidentified substances. Spectrometry is often used in physics and analytical chemistry for the identification of elements by the spectrum emitted or absorbed by them. The analytical techniques are particularly essential in the quality control in the manufacture of drugs, as they help obtain information on both the physical and chemical characteristics thereof through all the stages of their production.

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Fourier Transformed Infrared Spectrometry (FTIR), a very versatile tool used to detect common contaminants, byproducts of oil degradation, and additives. FTIR is one of the most used tools in drug analysis laboratories. Its principal value lies mainly in the fact that it is a test where the instrument is the basis of the entire test (a sample preparation or wet chemical is not required), runs quickly and is capable of simultaneously detecting several parameters, including water, fuel, glycol, oxidation, soot and some additives. In fact, from its value, it seems the ideal solution to provide fast and economic analyzes.

FTIR is based on the principles of molecular spectroscopy. This wide-ranging area in physics and chemistry covers a large number of experimental techniques, some of which are found in other lubricant analysis tests and others that are so sophisticated that they are only relevant in research laboratories. The basic principle behind molecular spectrometry is that molecules absorb energy from light at specific wavelengths, known as their resonance (vibration) frequencies. For example, water molecules resonate (vibrate) around the 3450 wavenumbers (indicated as cm-1) in the infrared region of the electromagnetic spectrum.

The Raman spectroscopy is a spectroscopic technique used in condensed matter physics and chemistry to the study of vibrational, rotational, and other low-frequency modes in a system. It is based on the inelastic dispersion, or Raman dispersion, of monochromatic light, which usually comes from a laser in the visible range, near-infrared, or near-ultraviolet. The laser light interacts with phonons or other excitations in the system so that the energy of the laser photons moves up or down. Usually, the sample is illuminated with a laser beam. The illuminated spotlight is collected with a lens and sent through a monochromator.

Although FTIR spectroscopy and Raman spectroscopy are often interchangeable and provide complementary information, there are practical differences that influence which of them will be optimal. Most of the molecular symmetry will allow both FTIR and Raman activity. In a molecule that contains an inversion center, the IR bands and Raman bands are mutually exclusive (i.e., the link will be active Raman or active IR but not both). A general rule is that functional groups that have substantial changes in the dipoles are strong in IR, while those that have weak changes in the dipoles or a high degree of symmetry and no net change in the dipoles will look better in the Raman spectra. Fourier transform spectroscopy and Raman spectroscopy techniques are capable of being applied in real-time so that they can be beneficial for the pharmaceutical industry.