The KBr pellet method is a technique used to prepare solid samples for infrared (IR) spectroscopy.
This method involves mixing the sample with potassium bromide (KBr) powder, pressing the mixture into a pellet, and analyzing the pellet in an IR spectrometer.
The key steps in this process include preparing the KBr powder, mixing the sample with KBr, pressing the mixture into a pellet, and analyzing the pellet.
How do you do the KBr pellet method? 5 Key Steps Explained
1. Preparing the KBr Powder
Before forming the KBr powder into pellets, it must be pulverized to a fine consistency, typically to a mesh size of 200 or less.
This pulverization ensures that the KBr particles are small enough to form a transparent pellet when pressed.
The powder is then dried at approximately 110 °C for two to three hours to remove any moisture.
Rapid heating should be avoided as it can oxidize some of the KBr powder to KBrO3, causing a brown discoloration.
After drying, the powder is stored in a desiccator to maintain its dry state.
2. Mixing the Sample with KBr
The sample, typically in a concentration of 0.2 to 1 percent, is mixed with the KBr powder.
This mixture is prepared by weighing the required amounts of sample and KBr, ensuring the ratio is correct for the desired pellet thickness and clarity.
The mixture is then finely pulverized to ensure an even distribution of the sample within the KBr matrix.
3. Pressing the Mixture into a Pellet
The pulverized mixture is placed into a pellet-forming die.
A significant force, usually around 8 tons, is applied under a vacuum of several mm Hg for several minutes to form transparent pellets.
The vacuum is crucial as it helps eliminate air and moisture, which can cause the pellets to be brittle and scatter light.
The pressure and vacuum conditions are adjusted based on the size of the pellet and the specific requirements of the analysis.
4. Analyzing the Pellet
Once the pellet is formed, it is placed in a sample holder compatible with the IR spectrometer.
The pellet is then analyzed to obtain the IR spectrum of the sample.
After analysis, the pellet can be rinsed out of the collar with water or ejected and saved for further analysis if needed.
5. The Versatility of the KBr Pellet Method
This method is particularly useful for solid samples that are not volatile or do not dissolve in common solvents, making it a versatile technique for obtaining IR spectra of a wide range of materials.
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