Preparing samples for IR spectroscopy using KBr disks is a meticulous process that requires careful attention to detail.
5 Essential Steps to Prepare Samples for IR Spectroscopy as KBr Disks
1. 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.
Using less sample helps avoid issues like complete absorption or scattering of the IR beam.
These issues can lead to noisy spectra.
2. 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.
The fine powder 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.
3. 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.
It 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.
4. 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.
Polishing the surface with a polishing kit.
Gloves should be worn to prevent fogging.
The cleaned surface should be clear and free from scratches.
5. 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.
Packing it into the sample plate for infrared spectrum measurement.
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