Potassium bromide (KBr) is commonly used in Fourier Transform Infrared (FTIR) spectroscopy because it is transparent to infrared light, allowing for accurate and reliable measurements of sample spectra. The use of KBr pellets enables the proper dilution of samples, ensuring that the sample does not block the infrared light path and that the measurement is not affected by the sample's opacity or excessive thickness.
Transparency to Infrared Light: KBr is transparent to infrared light, which is essential for FTIR spectroscopy. This transparency allows the infrared radiation to pass through the sample and reach the detector without significant absorption or scattering. This property ensures that the measured spectrum is primarily due to the sample's interaction with the infrared light, rather than being influenced by the matrix or substrate material.
Sample Preparation and Dilution: In FTIR spectroscopy, it is crucial to have the right amount of sample in the system to obtain reliable results. Using too large a sample can block the path of the infrared light, leading to unreliable comparisons. KBr pellets are typically prepared by mixing the sample with KBr in a 1:100 ratio, which dilutes the sample to an appropriate concentration. The mixture is then compressed into a pellet using a hydraulic press. This process creates a solid pellet that is primarily transparent to infrared light, containing a dilute sample within the detection range of the FTIR instrument.
Preparation Considerations: KBr is hygroscopic, meaning it absorbs moisture from the air. This can affect the FTIR measurement if the KBr absorbs a significant amount of water, especially in humid environments. To mitigate this, sample preparation, including grinding and pressing, is preferably done in a controlled environment such as a glovebox. Alternatively, using a vacuum die during pressing can help reduce the impact of moisture.
Alternative Materials: While KBr is the most common alkali halide used in FTIR spectroscopy, other materials like cesium iodide (CsI) can also be used, particularly for measuring the infrared spectrum in the low-wavenumber region (400 to 250 cm-1). The choice of material depends on the specific requirements of the analysis, including the spectral range of interest and the properties of the sample.
In summary, KBr is used in FTIR spectroscopy due to its transparency to infrared light, its ability to dilute samples effectively, and its compatibility with standard sample preparation methods. These properties ensure that FTIR measurements are accurate, with sharp peaks and high resolution, making KBr an indispensable tool in infrared spectroscopy.
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