The KBr pellet technique is a method used in infrared spectroscopy for the analysis of solid samples. This technique involves the preparation of a transparent pellet by mixing a small amount of the sample with potassium bromide (KBr) powder, applying high pressure to form a pellet, and then analyzing the pellet using infrared spectroscopy.
Summary of the KBr Pellet Technique:
- Preparation of the Sample Mixture: A small percentage (0.1 to 1.0%) of the solid sample is mixed with finely ground KBr powder. This mixture is then further pulverized to ensure uniformity and compatibility.
- Formation of the Pellet: The mixture is placed in a pellet-forming die and subjected to high pressure (approximately 8 tons) under vacuum conditions for several minutes. This process forms a transparent pellet that is suitable for infrared analysis.
- Analysis and Post-Analysis Handling: The pellet is placed in a sample holder compatible with standard spectrometers. After analysis, the pellet can be washed out or saved for further use.
Detailed Explanation:
- Sample Preparation: The sample is mixed with KBr powder at a controlled ratio, typically 0.2 to 1 percent. This low concentration is necessary because the pellet's thickness can lead to high absorption or scattering of the IR beam if the sample concentration is too high. The mixture is pulverized to ensure a homogeneous distribution of the sample within the KBr matrix.
- Pellet Formation: The mixture is placed in a die and subjected to high pressure under vacuum. The vacuum is crucial to remove air and moisture, which can cause the pellet to be opaque or easily breakable. The pressure applied is significant, around 8 tons, which is necessary to achieve the plasticity in the KBr that allows it to form a transparent sheet.
- Post-Formation Handling: After the pellet is formed, it is placed in a V-shaped sample holder that fits into standard spectrometer mounts. The analysis of the pellet provides the infrared spectrum of the sample. Post-analysis, the pellet can be easily removed from the holder and either discarded or saved for further analysis.
Significance and Advantages: The KBr pellet technique is particularly useful for analyzing solid samples that are not soluble in common solvents or are too small in quantity for other techniques. The ability to adjust the pathlength of the sample in the pellet offers flexibility in the analysis, making it a valuable tool in infrared spectroscopy. Despite newer techniques like ATR (Attenuated Total Reflectance), the KBr pellet method remains popular due to its simplicity, effectiveness, and the ability to handle a wide range of sample types.
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