KBr is used as a material for pellet formation in IR-spectroscopy for several reasons.
Firstly, KBr is optically transparent in the fingerprint region of IR spectroscopy. This means that it allows IR radiation to pass through it without absorbing or interfering with the signal. This transparency is essential for obtaining accurate IR spectra with sharp peaks, good intensity, and high resolution.
Secondly, KBr is a commonly used alkali halide that becomes plastic when subjected to pressure. This property allows it to form a sheet that is transparent in the infrared region when pressed into a pellet. Other alkali halides, such as cesium iodide (CsI), can also be used for pellet formation, especially for measuring the infrared spectrum in the low-wavenumber region.
The process of KBr pellet formation involves mixing a small percentage (approximately 0.1 to 1.0%) of the sample into fine KBr powder. This mixture is then finely pulverized and put into a pellet-forming die. A force of approximately 8 tons is applied under a vacuum for several minutes to form transparent pellets. Degassing is performed to eliminate air and moisture from the KBr powder, as inadequate vacuum can result in easily broken pellets that scatter light.
Before forming the KBr powder into pellets, it is important to pulverize it to a specific mesh size (200 mesh max.) and then dry it at approximately 110 °C for two to three hours. Rapid heating should be avoided, as it can oxidize some of the KBr powder to KBrO3, causing a brown discoloration. After drying, the powder should be stored in a desiccator to prevent moisture absorption.
KBr pellets are used in IR-spectroscopy because they allow for variable pathlengths of the compound of interest. This means that the thickness of the pellet can be adjusted to control the amount of sample that the IR radiation passes through. This flexibility in pathlength is advantageous for obtaining accurate and reliable results.
In addition, when performing measurements, background measurements can be taken using an empty pellet holder or a pellet of KBr only. These measurements help correct for infrared light scattering losses in the pellet and for moisture adsorbed on the KBr.
Overall, KBr is used as a material for pellet formation in IR-spectroscopy because of its optical transparency, plasticity under pressure, and ability to form transparent pellets. It allows for accurate and reliable analysis of solid samples in the fingerprint region of IR spectroscopy.
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