To prepare KBr pellets for IR spectroscopy, follow these steps:
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Sample/KBr Ratio: The sample should be mixed with KBr at a concentration of 0.2 to 1 percent. This low concentration is necessary because the pellet is thicker than a liquid film, and a higher concentration can lead to noisy spectra due to complete absorption or scattering of the IR beam.
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KBr Pellet Preparation: The sample and KBr must be transparent to IR radiation to allow accurate detection of the IR spectrum. Commonly, salts like KBr, NaCl, or AgCl are used for this purpose. For a 13 mm-diameter pellet, mix approximately 0.1 to 1.0% of the sample with 200 to 250 mg of fine KBr powder. Pulverize the mixture finely and place it into a pellet-forming die. Apply a force of about 8 tons under a vacuum of several mm Hg for several minutes to form transparent pellets.
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Degassing and Drying: Before pellet formation, ensure the KBr powder is degassed to remove air and moisture, which can cause fragile pellets that scatter light. Pulverize the KBr to a maximum of 200 mesh and dry it at approximately 110 °C for two to three hours. Rapid heating can oxidize some KBr to KBrO3, causing discoloration. After drying, store the powder in a desiccator.
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Compression: Use a hydraulic press to compress the KBr and sample mixture in a pellet die chamber. The typical conditions for KBr sample preparation involve a 100:1 KBr to sample ratio by weight, a 13 mm Pellet Die, and a pressing load of 10 tons. For FTIR applications, a 7 mm pellet might require as little as 2 tons of pressing load.
These steps ensure the preparation of KBr pellets that are suitable for obtaining clear, high-resolution IR spectra, allowing for accurate analysis of the sample's molecular structure.
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