The range of KBr in IR is from wave number 4000-400 cm-1. In this range, KBr has a transmittance of 100% and does not exhibit absorption. KBr is optically transparent for light in the range of IR measurement, which means that it does not interfere with the absorbance of the sample.
KBr is commonly used as a carrier for the sample in IR spectroscopy because it allows the IR radiation to pass through it without interference. It is often mixed with the sample in a ratio of 0.2 to 1 percent. The concentration of the sample in KBr should be lower than in liquid films because the pellet is thicker and a higher concentration can cause difficulties in obtaining clear spectra.
The preparation of KBr pellets involves mixing the KBr powder with the sample, grinding them together to form a fine powder, and then pressing them into a disc using a pellet die. The typical conditions for KBr pellet preparation are a 100:1 ratio of KBr to sample by weight, a 13 mm pellet die, and a pressing load of 10 tons. For FTIR applications, a 7 mm pellet with a pressing load as little as 2 tons can be used.
The KBr pellet method takes advantage of the property of alkali halides, such as KBr, becoming plastic under pressure and forming a transparent sheet in the infrared region. The sample is mixed with fine KBr powder, pulverized, and then put into a pellet-forming die. A force of approximately 8 tons is applied under vacuum to form transparent pellets. Degassing is performed to remove air and moisture from the KBr powder.
Inadequate vacuum during pellet formation can result in easily broken pellets that scatter light. Before forming the KBr powder into pellets, it is important to pulverize it to a maximum mesh size of 200 and dry it at approximately 110 °C for two to three hours. Rapid heating can cause some of the KBr powder to oxidize, leading to brown discoloration.
During measurements, the background can be measured with an empty pellet holder or a pellet holder with a KBr pellet that contains no sample. This can correct for infrared light scattering losses in the pellet and for moisture adsorbed on the KBr.
Overall, KBr is used in IR spectroscopy as a carrier for the sample because of its optical transparency and lack of absorption in the IR range. It is prepared as pellets by mixing it with the sample, grinding them together, and pressing them under vacuum. The resulting pellets are mostly transparent to infrared light but contain a dilute quantity of the sample for analysis.
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