KBr is used for IR (infrared) spectroscopy primarily because it is transparent to infrared light and forms a stable, transparent pellet when mixed with a sample and subjected to pressure. This allows for the effective analysis of solid samples in the infrared region.
Transparency to Infrared Light: Potassium bromide (KBr) is an alkali halide that exhibits transparency in the infrared region of the electromagnetic spectrum. This property is crucial for its use in IR spectroscopy, as it allows infrared light to pass through the sample without significant absorption. This transparency ensures that the sample's interaction with the infrared light can be accurately measured, providing clear and interpretable spectra.
Formation of Stable Pellets: KBr becomes plastic under pressure, allowing it to be easily formed into a pellet with the sample. This pelletization process is essential for handling solid samples, which might not be amenable to analysis in other forms. The pellet method involves grinding the sample with KBr and then pressing this mixture at high pressure to form a disk. This disk is then placed in the spectrometer for analysis. The uniformity and stability of the KBr pellet ensure consistent and reproducible results.
Sample Dilution and Measurement: The KBr pellet method also allows for the dilution of the sample within the pellet, typically at concentrations ranging from 0.1% to 10% by weight. This dilution is crucial as it prevents overloading the detector and ensures that the sample's spectral features are not obscured by excessive absorption. The method enables the analysis of very small sample volumes, as little as 50 to 100 ng, which is particularly useful for rare or precious samples.
Advantages Over Other Techniques: Compared to newer techniques like attenuated total reflectance (ATR), the KBr pellet method offers the advantage of variable pathlength, which can be adjusted by changing the thickness of the pellet. This adjustment capability is significant for optimizing the detection of various types of samples, especially those with weak or strong absorptions.
Practical Considerations: The use of KBr pellets in FTIR (Fourier Transform Infrared) spectroscopy ensures that the sample does not block the path of the infrared light, which would otherwise lead to unreliable results. The transparency of KBr and the controlled dilution of the sample within the pellet ensure that the sample's spectral features are accurately represented without overwhelming the detector.
In summary, KBr is used in IR spectroscopy due to its transparency in the infrared region, its ability to form stable and reproducible pellets with samples, and its practical advantages in handling and analyzing solid samples. These features make the KBr pellet method a fundamental technique in the field of infrared spectroscopy.
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