When it comes to IR spectroscopy, the amount of sample you need can vary based on the type of sample and the concentration you're aiming for.
How much sample is needed for IR? (4 Key Methods Explained)
1. Mull Technique for Solid Samples
For solid samples, one common method is the mull technique.
In this method, the sample is finely crushed and mixed with a mulling agent like Nujol.
A thin film of this mixture is then applied onto salt plates and mounted in the path of the IR beam for analysis.
This technique is often used for Fourier Transform Infrared Spectroscopy (FTIR) and usually requires a few milligrams of sample.
2. KBr Pellet Preparation for Solid Samples
Another popular method for solid samples is preparing KBr pellets.
In this method, a small amount of sample powder (typically 1-2 mg) is mixed with KBr powder.
The mixture is then ground together to create a homogeneous powder.
Around 200-250 mg of this mixture is enough to produce a pellet approximately 2 mm thick with a diameter of ½ inch (12.7 mm).
The exact amount of powder needed can vary depending on the desired pellet size.
3. Ratio of KBr to Sample
The ratio of KBr to sample in the pellet should be around 100:1.
This ensures that the majority of the pellet is made up of KBr, which is an IR-transparent material.
A lower concentration of the sample is required to prevent difficulties in obtaining clear pellets and to avoid complete absorption or scattering of the IR beam.
4. Grinding and Pressing Techniques
It's important to properly grind the KBr mixture to ensure a clear and transparent pellet.
Over grinding can increase surface area and lead to increased moisture absorption, resulting in a higher background in certain ranges.
The pellet should be pressed with a force per unit area of around 8,000 to 10,000 psi (55-69 MPa) to achieve the desired thickness and clarity.
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