To prepare a KBr pellet for IR spectroscopy, follow these steps:
1. Pulverize the KBr powder into a 200-mesh size. This can be done by grinding it in a mortar and pestle or using a grinding mill.
2. Dry the KBr powder at 110°Celsius to remove any bound water molecules. This step is important to ensure that the pellets are transparent to IR radiation.
3. Determine the sample/KBr ratio. The concentration of the sample in KBr should be in the range of 0.1 to 1.0 percent. This lower concentration is required because the pellet is thicker than a liquid film, and a higher concentration can result in the complete absorption or scattering of the IR beam, leading to noisy spectra.
4. Mix the sample with the powdered KBr. For a 13 mm diameter pellet, mix 0.1 to 1% of the sample with 200 to 250 mg of powdered KBr. It is recommended to work quickly to minimize the absorption of humidity by the KBr powder.
5. Grind the mixture to a fine powder. If the sample is very hard, add the sample first, grind it, then add the KBr and grind again.
6. Place the powdered mixture into a pellet-forming die. The die should have a diameter of 13 mm.
7. Apply a force of approximately 8 tons to the die under a vacuum of several mm Hg. This pressure and vacuum help the alkali halides, such as KBr, become plastic and form a transparent sheet.
8. Maintain the pressure and vacuum for several minutes to form transparent pellets. Inadequate vacuum can result in easily broken pellets that scatter light.
9. Perform degassing to eliminate air and moisture from the KBr powder. This can be done by applying heat or using a vacuum chamber.
10. Before using the pellets, store them in a desiccator to prevent moisture absorption.
It is important to note that the quality of the KBr or halide salt powder used will greatly affect the overall quality of the pellet. Therefore, always use a spectroscopic grade of purity for accurate IR spectra.
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