For IR spectroscopy, the amount of sample needed can vary depending on the type of sample and the desired concentration.
For solid samples, one method is to use a mull technique where 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 commonly used for Fourier Transform Infrared Spectroscopy (FTIR) and typically requires a few milligrams of sample.
Another common method for solid samples is to prepare 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 sufficient to produce a pellet approximately 2 mm thick with a diameter of ½ inch (12.7 mm). The exact amount of powder needed may vary depending on the desired pellet size.
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.
It is 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.
In summary, for IR spectroscopy, the amount of sample needed can vary depending on the preparation method and the desired concentration. Generally, a few milligrams of sample are sufficient for solid samples, either using the mull technique or preparing KBr pellets with a ratio of around 100:1. Proper grinding and pressing techniques are important to obtain clear and transparent pellets for accurate spectral analysis.
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