Preparing a KBr pellet for IR spectroscopy involves several meticulous steps to ensure the pellet is transparent and free from impurities. This process is crucial for obtaining accurate and reliable IR spectra.
How do you prepare a KBr pellet for IR spectroscopy? (10 Steps to Perfect Pellets)
1. Pulverize the KBr Powder
Start by pulverizing the KBr powder into a 200-mesh size. This can be achieved by grinding it in a mortar and pestle or using a grinding mill.
2. Dry the KBr Powder
Dry the KBr powder at 110°Celsius to remove any bound water molecules. This step is essential to ensure that the pellets are transparent to IR radiation.
3. Determine the Sample/KBr Ratio
Determine the sample/KBr ratio. The concentration of the sample in KBr should be between 0.1 to 1.0 percent. This lower concentration is necessary because the pellet is thicker than a liquid film, and a higher concentration can lead to the complete absorption or scattering of the IR beam, resulting in noisy spectra.
4. Mix the Sample with KBr Powder
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
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 Mixture in a Die
Place the powdered mixture into a pellet-forming die. The die should have a diameter of 13 mm.
7. Apply Pressure and Vacuum
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 Pressure and Vacuum
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
Perform degassing to eliminate air and moisture from the KBr powder. This can be done by applying heat or using a vacuum chamber.
10. Store the Pellets
Before using the pellets, store them in a desiccator to prevent moisture absorption.
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