Potassium bromide (KBr) is widely used in Fourier Transform Infrared (FTIR) spectroscopy. This is because it has several key properties that make it ideal for this type of analysis.
1. Transparency to Infrared Light
KBr is transparent to infrared light. This is crucial for FTIR spectroscopy. It allows infrared radiation to pass through the sample and reach the detector without significant absorption or scattering. This 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.
2. Sample Preparation and Dilution
In FTIR spectroscopy, it is essential 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. This 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.
3. 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.
4. 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.
5. Ensuring Accurate Measurements
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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