Preparing a KBr sample for infrared spectroscopy involves several critical steps to ensure accurate and reliable results. Here’s a detailed guide to help you through the process.
How do you prepare a KBr sample for infrared spectroscopy? 4 Essential Steps
1. Preparation of KBr and Sample Mixture
Start by taking finely powdered KBr from an oven set at 100 °C. Be cautious to avoid burns.
Transfer the KBr into a mortar.
Add between 1 to 2 percent of your sample to the KBr.
Mix and grind the mixture to a fine powder.
For hard samples, grind the sample first, then add KBr and grind again. This ensures a homogeneous mixture that minimizes scattering losses and absorption band distortions.
2. Formation of Pellets
The finely ground mixture is then used to form pellets.
Pour the KBr-sample matrix into a pellet-forming die.
Subject the die to a pressing load, typically around 8 tons under a vacuum of several mm Hg for a few minutes.
This process forms transparent pellets.
It's important to note that the KBr powder should be pulverized to a maximum of 200 mesh and dried at approximately 110 °C for two to three hours before pellet formation. Rapid heating can oxidize KBr to KBrO3, causing discoloration.
3. Pellet Handling and Analysis
After the pellets are formed, they are retained inside a collar.
This collar can be placed in a V-shaped sample holder compatible with standard spectrometer slide mounts.
After analysis, the pellet can be rinsed out with water or ejected from the collar for storage or further analysis.
4. Special Considerations for Liquids and Solids in Solution
For liquid samples, a small drop is placed on a KBr plate.
Another plate is placed on top, rotated to create an even film.
These plates are then inserted into a sample holder for spectroscopy.
For solids in solution, a concentrated solution of the compound is prepared in a suitable solvent like CH2Cl2.
Throughout these processes, it's essential to work quickly to minimize the hygroscopic KBr's exposure to humidity, which can increase background noise in certain spectral ranges.
Additionally, proper cleaning of KBr plates and tools is necessary to prevent contamination of future samples.
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