Infrared spectroscopy or IR spectroscopy is essentially a way to see what is apparently invisible, which in this case refers to atoms and molecules. It is the study of matter in which how infrared light interacts with a molecule is observed. Infrared light refers to the type of light which has a frequency lower than visible light but has a longer wavelength. Absorption, reflection and emission are three ways in which the analysis is conducted.
In other words, this kind of spectroscopy focuses on the electromagnetic spectrum’s infrared region. Widely used in the fields of organic and inorganic chemistry, infrared spectroscopy determines the functional groups of molecules. This way, different chemical compounds can be identified and their structures determined.
IR spectroscopy has varied applications including measuring carbon dioxide concentrations in greenhouses, analyzing forensic material, and detecting alcohol content in blood in case of drunk driving. With this method, different paint pigments in an artwork can also be identified and studied and the degree of polymerization can be measured. In the food industry, IR spectroscopy can be used to measure the concentration of different compounds in a particular food item. Even devices that detect gas leaks make use of IR spectroscopy.
A detailed understanding of IR spectroscopy
With the IR spectroscopy method, the infrared light frequencies absorbed by a molecule can be accurately detected. Since these light frequencies match the vibration of bonds inside the molecule, the latter absorbs the frequencies. The energy needed for exciting this molecular bond and making them vibrate with greater amplitude is only found in the infrared region. However, it must be noted that only a polar molecular bond will interact with electromagnetic infrared radiation.
Now, a molecule has separate zones of partial negative and positive charge, and this helps the electric field of the electromagnetic wave to cause the vibration of the molecular bond. A change occurs in the molecule’s dipole moment due to the change in the vibration energy. And the polarity of the molecule’s bond determines the intensity with which infrared light is absorbed. Do note that O=O or N≡N bond cannot absorb radiation as they are nonpolar and symmetrical in nature and cannot interact with electric fields.
The infrared spectrum can be essentially divided into near, mid and far regions, based on how they are related to the visible spectrum. The high energy and near IR region have 0.8-2.5 μm wavelengths or 14000-4000 cm-1 and can lead to harmonic or overtone vibration. The mid IR region on the other hand has 2.5-25 μm wavelength or 4000-400 cm-1 and can help in studying fundamental vibrations as well as associated rotational vibrational structure. The far IR region is right next to the microwave region and has 25-1000 μm wavelengths or 400-10 cm-1. Being low energy, this is useful for rotational spectroscopy.
The regions of the infrared spectrum can also be classified in a different manner than what is mentioned above. Meaning, the region from 4000 cm-1 to 1300 cm-1 comprises bands that help identify an unknown compound’s functional group. And the region from 1300 cm-1 to 400 cm-1(also known as fingerprint region) features bands that are unique to every molecule, just like a fingerprint. These bands are useful for comparing a particular compound’s spectra with another.
IR spectroscopy can make use of samples in different physical states, such as solid, liquid and gas. The methods of sample preparation are different for each and are elaborated below:
Solid film technique–Appropriate for amorphous solids, this technique involves melting of the sample and then cooling and depositing it like a thin film on the KBr cell.
Solid run in solution technique –The sample must be dissolved in solvents like carbon tetrachloride, carbon disulfide, or dichloromethane to prepare a concentrated solution. This solution is then spread on KBr plates as a thin film for scanning. Do note that no infrared radiation should be absorbed by the solvent.
Pressed pellets technique –Pellets of the sample and KBr (dry KBr powder must be crushed for this) need to be prepared in this technique. The sample concentration in KBr can range from 0.2 to 1%. The mixture of sample and KBr then must be transferred to a die set, where a hydraulic press will put pressure and create compressed pellets.
Mull technique –The powdered sample is combined with nujol agent or mineral oil in this method and mulled to create a paste. This is then applied on the NaCl, AgCl or KBr plates.
There is no need to mix liquid samples with any solvent, since solvents have an absorption spectrum of their own and can interfere with correct results. Highly polished salt plates of KBr, NaCl or AgCl are used to scan the sample, which is placed in the form of a drop and spread like a thin film.
For a gaseous sample, a specially designed cell made of KBr and NaCl is used. This has a 5 to 10 cm path length. The gaseous vapors are placed in the cell and the cell is then positioned in the path of infrared radiation.
The IR spectroscopy principle is based on some fundamental concepts. It is common knowledge that atoms connected by chemical bonds make up a molecule. The motion of atoms and bonds can be compared to springs and balls, which involve vibration and this vibration is known as the natural frequency of vibration.
Now, application of infrared radiation leads to the vibration between atoms of a molecule. When the infrared frequency is equal to the natural frequency of vibration, the infrared radiation gets absorbed. As a result, a peak is noticed, the value of which depends on the functional group involved. This is because various functional groups can absorb different frequencies of infrared radiation. Due to this uniqueness, the IR spectrum of any chemical compound is like a molecule’s fingerprint.
Apart from understanding the IR principle, understanding the IR spectroscopy instrumentation is also very helpful for researchers. This involves splitting an IR light beam into two and passing them through the sample and reference. Both these beams are then reflected so that they pass through a splitter and detector in sequence. Once the processor interprets the data that passes through the detector, the final reading is printed.