Preparing samples for infrared spectroscopy requires different methods depending on the state of the material—whether it's solid, liquid, or gas.
The primary requirement is that the material must be transparent to infrared radiation.
This is why salts like NaCl and KBr are commonly used.
1. Solid Samples
1.1 Mull Technique
This method involves crushing the sample finely and mixing it with Nujol, a mulling agent, to form a thick paste.
The paste is then spread onto salt plates, typically made of KBr or NaCl, and placed in the path of the IR beam for spectral analysis.
The sample should be ground to an average particle size of about 1 to 2 microns to ensure good transmission of IR light.
1.2 Pellet Preparation
Another common method for solid samples is the preparation of KBr pellets.
Approximately 10 mg of the sample is mixed with KBr in a 1:100 ratio and then compressed using a hydraulic press.
The resulting pellet is transparent to infrared light and is suitable for FTIR analysis.
This method is particularly useful for powder samples and allows for the detection of dilute sample quantities.
2. Liquid and Gas Samples
For liquid and gas samples, the preparation typically involves the use of cells with specific path lengths to accommodate the sample and ensure transparency to IR radiation.
The cells are usually made of materials like KBr or NaCl, which are transparent in the IR region.
3. General Considerations
3.1 Transparency
The sample must be transparent to IR radiation.
This is why salts like KBr and NaCl, which are transparent in the IR region, are used in sample preparation.
3.2 Particle Size
For solid samples, the particle size is crucial.
It should be small enough (typically 1 to 2 microns) to allow the IR light to pass through without significant scattering.
3.3 Absorption by Nujol
When using Nujol as a mulling agent, it's important to note that Nujol itself has a characteristic spectrum.
This can interfere with the analysis of the sample, so it's a consideration in the interpretation of the spectra.
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