Fourier Transform Infrared Spectroscopy
Fourier Transform Infrared Spectroscopy
FTIR is known to every chemist for the simple reason, it resolves and identifies various functional groups present in a chemical compound. FTIR can be taken for solid samples and liquid samples. A KBr pellet is made for solid samples while a quartz liquid cell is used for liquid samples.
Types of samples for FTIR analysis
Two types of sample can be studied for FTIR.
1) The first one is a solid sample. The sample is mixed with KBr and a pelletizer is used to make a sample.
2) In the case of a liquid sample, a liquid cell made of quartz material is used. Quartz material will allow IR is passed hence it is selected.
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FTIR: Solid Sample
The sample under investigation is taken and powdered to a tiny size. This reduces the IR beam to get scattered, further, the baseline remains almost parallel to the x-axis. A sample amount of KBr of spectroscopic grade is added to the sample under investigation. KBr acts as a binder and it is a window to frequency from 400 cm-1 to 4000 cm-1. The mixed sample with KBr is pelleted using a pelletizer. The procedure is as follows: The mixed sample with KBr is filled into a die and the die is placed in a pelletizer as shown in figure 1. To the die, the pressure is applied by the machine shown in figure 1. Once the pressure is removed, carefully the die is demounted and the pellet formed is removed. This pellet needs to be preserved in a desiccator if FTIR spectra are not recorded immediately. Keeping the pelt in desiccator prevents it from accumulating water vapour present in the atmosphere.
Precaution to be followed
Various precautions are needed to be taken during pellet preparation.
1. The die should be clean and dry.
2. The sample in the die should be properly filled, the walls of the die should always be free from a sample.
3. Exact pressure required by the sample to form a pellet should be given.
4. Pellet should not be either too thin or too thick, the appropriate amount of sample should be filled in the die.
5. The piston of the should always be immersed in oil to prevent from corroding.
Reference spectra
Always a reference spectra of pure KBr is required before recording the FTIR of the sample. A pellet made of pure KBr (spectroscopic grade) is made into a pellet as described above and the spectra of this are used as a reference.
Whenever FTIR spectra of a sample is recorded, the instrument uses the FTIR of KBr and negates the peaks of KBr with respect to the sample.
FTIR: Liquid Sample
Recording a spectrum of the liquid sample is quite easy compared to solid material. The cuvette, the rectangular-shaped transparent hollow cube made of quartz is used. Quartz is transparent to IR radiation and hence quartz is used. The sample is filled in the cuvette and placed in the path of IR radiation and the spectra are recorded.
Interpretation of FTIR spectra
When an FTIR spectrum of a sample is recorded it appears as shown in figure 2. The spectra can be recorded either in transmittance or absorption modes. If the pellet is optically opaque, the instrument chooses absorption mode and if the pellet is transparent to IR radiation, then the transmittance mode has opted.
Each peak corresponds to a functional group. Peaks are classified as mild, intense, strong. Every textbook of FTIR will give a table of functional groups and corresponding peaks they give rise to. Hence when analyzing the FTIR spectra the peaks and corresponding wavenumbers are taken and mapped with the available/ reported data. Each peak thus can be identified and attributed to the chemical/functional group.
Conclusion
FTIR spectrum analysis is discussed. Technique of recording FTIR for solid and liquid samples are also deliberated. Interpretation of the spectra is discussed.
image source
- FTIR: https://www.specac.com/en/products/ftir-acc/kits/sample
- 9IBA038282_400x400: https://new.abb.com/products/measurement-products/analytical/ft-ir-and-ft-nir-analyzers/laboratory-spectrometers/mb3000