Preparing KBr pellets for observation is a crucial process in spectroscopic analysis.
This detailed guide will walk you through the essential steps to ensure clear and effective KBr pellets.
4 Key Steps to Clear Spectroscopy
1. Sample/KBr Ratio and Concentration
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.
Higher concentrations can lead to noisy spectra due to complete absorption or scattering of the IR beam.
2. Preparation of Pellets
Mixing the Sample with KBr
Use a ratio of 100:1 KBr to sample by weight.
For a 13mm diameter die, use 200 - 300 mg of KBr and 1 mg of the sample.
Ensure the sample is thoroughly blended with KBr powder using a mortar and pestle or a grinding mill.
Pressing the Pellet
Place the mixture into the hole in the stainless steel (SS) collar.
Insert the shorter anvil first, followed by the longer anvil.
Assemble the collar in the press handle and apply pressure, typically around 8 tons for 13mm pellets or 2 tons for 7mm pellets used in FTIR applications.
Remove the anvils, and the KBr pellet will be retained inside the collar.
Finalizing and Storing the Pellet
Place the collar with the pellet in a V-shaped sample holder that fits standard 2 × 3in. slide mounts of spectrometers.
After analysis, the pellet can be rinsed out of the collar with water or ejected and saved.
3. Additional Considerations
Pulverization and Drying
Pulverize the KBr powder to a maximum of 200 mesh.
Dry it at approximately 110 °C for two to three hours.
Rapid heating can oxidize some KBr to KBrO3, causing discoloration.
Vacuum and Degassing
Apply a vacuum of several mm Hg during the pressing process to eliminate air and moisture.
This helps in forming transparent and durable pellets.
Inadequate vacuum can result in fragile pellets that scatter light.
Storage
After drying, store the KBr powder in a desiccator to maintain its quality and prevent moisture absorption.
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