KBr is used as a reference standard in FTIR primarily because it is transparent to infrared light and forms stable, transparent pellets when mixed with samples and subjected to pressure. This transparency allows for accurate transmission of infrared radiation through the sample, facilitating precise measurement of the sample's spectral characteristics.
Transparency to Infrared Light: KBr, like other alkali halides, is transparent in the infrared region of the electromagnetic spectrum. This property is crucial for FTIR analysis because the technique relies on the transmission of infrared light through the sample to measure its absorption characteristics. When KBr is used to prepare a sample pellet, it ensures that the sample does not block or significantly attenuate the infrared light, which would otherwise lead to inaccurate or incomplete spectral data.
Formation of Transparent Pellets: The KBr pellet method involves mixing a small amount of the sample with KBr (typically about 1% by weight) and then pressing this mixture under high pressure to form a transparent pellet. This method is effective because alkali halides, including KBr, become plastic under pressure and can form a coherent, transparent sheet. The transparency of the pellet is essential for allowing the infrared light to pass through the sample without significant scattering or absorption, which could distort the spectral data.
Sample Preparation for Accurate Spectra: Proper sample preparation is critical in FTIR spectroscopy. The use of KBr in sample preparation ensures that the sample is in a form that is conducive to obtaining clear, sharp, and high-resolution spectra. The KBr pellet method is particularly favored because it allows for the inclusion of just the right amount of sample, preventing overloading of the sample which could lead to spectral artifacts.
Hydroscopic Nature and Handling Considerations: KBr is hydroscopic, meaning it can absorb moisture from the air. This property can affect the quality of the FTIR measurements if not properly managed. To mitigate this, sample preparation is often conducted in controlled environments such as gloveboxes or with the use of vacuum dies to prevent moisture absorption. This careful handling ensures that the KBr remains dry and does not introduce water-related spectral features that could interfere with the analysis of the sample.
In summary, KBr is used as a reference standard in FTIR due to its transparency to infrared light, its ability to form transparent pellets with samples, and its role in facilitating accurate and high-quality spectral measurements. Proper handling and preparation techniques are essential to maintain the integrity of the KBr and ensure reliable FTIR results.
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