KBr does absorb IR radiation, but it is transparent to a significant range of infrared light, making it suitable for use in FTIR spectroscopy.
Explanation:
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Transparency of KBr to Infrared Light: KBr is commonly used in FTIR spectroscopy because it is transparent to a significant range of infrared radiation. This property allows it to be used as a medium to prepare samples for analysis without significantly interfering with the infrared light needed for the spectroscopic measurements. The text mentions that KBr pellets, typically containing just 1% sample by weight, are used to ensure the right amount of sample is introduced into the system without blocking the path of the infrared light.
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Preparation and Handling of KBr: KBr is hygroscopic, meaning it absorbs water from the air. This property can affect the quality of FTIR measurements if not properly managed. The text suggests that grinding and pressing KBr should ideally be done in a controlled environment like a glovebox to minimize moisture absorption. The use of a vacuum die is also mentioned as a method to reduce the impact of moisture on the KBr pellets. Proper preparation techniques are crucial to prevent issues like cloudy disks, which can be caused by factors such as insufficient grinding of the KBr mixture, moisture in the sample, or incorrect sample-to-KBr ratio.
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Application in FTIR Spectroscopy: In FTIR spectroscopy, KBr is used not only as a medium to prepare samples but also in diffuse reflectance measurements. The sample is mixed with KBr powder and packed into a sample plate for infrared spectrum measurement. The diffuse reflectance method involves repeated transmission of light through the sample, emphasizing low absorption bands. A Kubelka-Munk transformation is then applied to the diffuse reflectance spectrum to allow comparison with transmission spectra and for quantitative analysis.
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Limitations and Precautions: Despite its usefulness, the use of KBr in FTIR requires careful handling to avoid issues related to its hygroscopic nature and to ensure the quality of the spectroscopic data. Overuse of the sample or poor preparation techniques can lead to unreliable results, emphasizing the need for precise and controlled sample preparation methods.
In summary, while KBr does absorb some IR radiation, its transparency to a significant range of infrared wavelengths makes it invaluable in FTIR spectroscopy for sample preparation and analysis. Proper handling and preparation techniques are essential to mitigate its hygroscopic properties and ensure accurate spectroscopic data.
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