Preparing KBr pellets for infrared spectroscopy involves several meticulous steps to ensure the pellets are of high quality and suitable for accurate spectral analysis.
6 Steps to Perfect KBr Pellets
1. Preparation of KBr Powder
- Start by obtaining fine KBr powder. Ensure it is pulverized to a maximum of 200 mesh to ensure a homogenous mixture when combined with the sample.
- Dry the KBr powder at approximately 110°C for two to three hours. This step is crucial to remove any moisture and to prevent the oxidation of KBr to KBrO3, which can cause discoloration.
- After drying, store the KBr powder in a desiccator to maintain its dryness.
2. Mixing the Sample with KBr Powder
- Mix the sample with the KBr powder. The sample should constitute approximately 0.1 to 1.0% of the total mixture by weight. This ensures that the sample's spectral features are not overwhelmed by the KBr matrix.
- Thoroughly mix the sample and KBr powder to achieve a uniform distribution.
3. Pellet Formation
- Place the mixed powder into a pellet-forming die. Ensure the die is clean and dry to prevent contamination or poor pellet formation.
- Apply a force of approximately 8 tons under a vacuum of several mm Hg for several minutes. This high pressure and vacuum help in forming transparent and sturdy pellets by eliminating air and moisture.
- Ensure adequate vacuum is maintained during the process to prevent the formation of brittle pellets that scatter light, which can affect the quality of the infrared spectra.
4. KBr Pellet Technique
- Pour the KBr powder/sample matrix into the hole in the stainless steel (SS) collar, with one anvil already inserted.
- Insert the second anvil and place the assembly in the press handle.
- Apply pressure by squeezing the handle until the pellet is formed.
- Remove the anvils and extract the formed KBr pellet from the collar.
- Optionally, place the collar in a V-shaped sample holder that fits standard slide mounts for spectrometers.
5. Post-Formation Handling
- After analysis, the pellet can be rinsed out of the collar with water or ejected and saved for further use or analysis.
6. Equipment and Precautions
- Use a pellet press die set, pestle and mortar (preferably made of agate for minimal sample loss and contamination).
- Heat the anvils and the body of the die set before pellet making to ensure everything is as dry as possible.
- Ensure all components (anvils, die set, and powder) are at the same temperature to avoid cloudy or wet pellets.
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