KBr is used for FTIR primarily because it is transparent to infrared light, allowing for accurate measurements without blocking the light path. This property makes KBr an ideal material for creating pellets that can contain a sample without significantly affecting the transmission of infrared radiation.
Transparency to Infrared Light: KBr, like other alkali halides, is transparent in the infrared region. This transparency is crucial for FTIR because the technique relies on the interaction of infrared light with the sample. If the material used to hold the sample is opaque to infrared light, it would block the necessary radiation, making the analysis impossible.
KBr Pellet Method: The KBr pellet method involves mixing a small amount of the sample with KBr (typically about 1% by weight) and then compressing this mixture under high pressure to form a transparent pellet. This method is effective because the alkali halides, including KBr, become plastic under pressure and form a transparent sheet that does not interfere with the infrared light. This allows the light to pass through the sample, interacting with its chemical bonds and producing a spectrum that can be analyzed.
Sample Preparation: Proper sample preparation is critical in FTIR spectroscopy. The sample must be prepared in such a way that it is transparent to IR radiation. Using salts like KBr, NaCl, or AgCl ensures that the sample will not absorb the necessary wavelengths of infrared light, allowing for clear and accurate spectra. The use of KBr pellets is particularly common in FTIR because of its effectiveness and ease of use.
Hydroscopic Nature of KBr: It is important to note that KBr is hydroscopic, meaning it absorbs water from the air. This can affect the FTIR measurements if the KBr absorbs too much moisture. Therefore, it is recommended to prepare KBr pellets in a controlled environment, such as a glovebox, to minimize exposure to moisture. This precaution ensures that the FTIR measurements are not influenced by water absorption, maintaining the accuracy of the analysis.
In summary, KBr is used in FTIR because of its transparency to infrared light, its ability to form transparent pellets under pressure, and its effectiveness in sample preparation. These properties make KBr an essential component in the FTIR analysis process, ensuring accurate and reliable results.
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