Infrared Spectroscopy

• Infrared Spectroscopy is an analytical method used for identifying and studying chemicals. It involves the interaction of infrared radiation with matter, absorbing radiation and exciting molecules into a higher vibrational state.
• It can be categorised into Near Infrared (NIR), Mid Infrared (MIR), and Far Infrared (FIR). This categorization is based on the absorption of different wavelengths of infrared light.
• The technique employs the principle that each type of bond within a molecule vibrates at a specific frequency. This frequency changes with the type of atom and the nature of the bonding.
• An infrared spectrometer contains a source of infrared radiation, a sample holder, a diffraction grating or prism monochromator, and a detector.
• The absorption spectrum produced provides a ‘fingerprint’ of the molecule, which can be compared with known spectra to identify or confirm the identity of the molecule in the sample. Peaks in the infrared spectrum corresponds to the frequencies of vibrations between the bonds of the atoms making up the material.
• Infrared Spectra are read by interpreting peaks. The number of absorption bands, their intensities and their shapes can provide valuable information about the sample. The peaks in the left side of the spectrum (>1500 cm^-1) corresponds to functional group vibrations, while the right side corresponds to the fingerprint region.
• Different types of vibrations include stretching (change in bond length) and bending (change in bond angle). These are further divided into symmetric and asymmetric types.
• Infrared Spectroscopy can be used in both qualitative and quantitative analysis. Qualitative analysis involves identifying the functional groups present in the sample. Quantitative analysis uses the Beer-Lambert Law to determine the concentration of a particular molecule.
• Limitations of IR Spectroscopy include that it cannot be used for samples that do not absorb IR radiation, such as homonuclear diatomic molecules. It also generally requires a relatively large amount of sample for analysis.
• Infrared Spectroscopy has wide range of applications in fields such as pharmaceuticals, environmental chemistry, forensic analysis, and food quality testing.
• It is crucial to understand how to interpret the Infrared Spectra; being able to identify the different types of bonds from the peaks in the spectra is key to successful identification of an unknown molecule.
• Practise interpreting spectra and comparing to known spectra will lead to increased familiarity and competence with this technique.
• Infrared Spectroscopy is a non-destructive technique which makes it useful for precious samples.
• It’s important to remember the units in which wavelength is measured in IR spectrometry. The units are wavenumbers (cm^-1) and not wavelength (nm). The wavenumber is the reciprocal of the wavelength.