To prepare samples for IR spectroscopy as KBr disks, the following steps are crucial:
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Sample/KBr Ratio: The sample should be mixed with KBr at a concentration ranging from 0.2 to 1 percent. This low concentration is necessary because the pellet is thicker than a liquid film, and thus requires less sample to avoid issues like complete absorption or scattering of the IR beam, which can lead to noisy spectra.
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Preparation of the Pellet: The mixture of sample and KBr is then pressed into a pellet. This process involves grinding the sample and KBr together to achieve a fine powder, which is then placed in a die and compressed under high pressure to form a clear, transparent disk. The transparency is essential for allowing the IR radiation to pass through the sample effectively.
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Spectrum Acquisition: After preparing the KBr disk, it is placed in the spectrometer to obtain the IR spectrum. The ideal spectrum should show the strongest band with a transmission of 0 to 10 percent and should not be totally absorbing for more than 20 cm−1. If the bands appear distorted, indicating that the particle size is too large, the sample should be further ground to reduce the particle size and improve the spectrum quality.
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Cleaning of KBr Plates: After the procedure, the KBr plates must be thoroughly cleaned to prevent contamination of future samples. This involves wiping the windows with a tissue, washing several times with methylene chloride followed by ethanol, and polishing the surface with a polishing kit. Gloves should be worn to prevent fogging, and the cleaned surface should be clear and free from scratches.
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Alternative Methods: An alternative method for obtaining IR spectra of solids is using Nujol (mineral oil) mulls between KBr plates. This method involves a background measurement on KBr or other diluent powder, followed by mixing the sample powder at a concentration of 0.1% to 10% in the KBr powder and packing it into the sample plate for infrared spectrum measurement.
These steps ensure that the sample is prepared correctly for IR spectroscopy, allowing for accurate and high-quality spectral data.
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