IR spectroscopy is a technique used to characterize solid, liquid, or gas samples by analyzing their interaction with infrared radiation. The key to successful IR spectroscopy is proper sample preparation, ensuring that the sample is transparent to the IR radiation. Common materials used for this purpose are salts like NaCl and KBr.
Solid Samples:
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Mull Technique: This method involves crushing the sample finely and mixing it with Nujol, a mulling agent, in a mortar using a pestle to form a thick paste. A thin film of this paste is applied onto salt plates, typically made of NaCl or KBr, and then placed in the path of the IR beam for spectral recording. This technique is particularly useful for samples that are not easily pelletized.
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Pellet Preparation: Another common method for solid samples is the preparation of KBr pellets. This involves mixing the sample with potassium bromide in a 1:100 ratio, which is then compressed using a hydraulic press in a pellet die chamber. The resulting pellet, transparent to infrared light, is suitable for analysis in FTIR spectroscopy. This method is preferred when the sample can withstand high pressure and does not degrade during the pelletization process.
Liquid and Gas Samples:
For liquid and gas samples, different techniques are employed based on the nature of the sample. Liquid samples are often analyzed directly between salt plates, while gases are typically contained in gas cells made of IR transparent materials.
General Considerations:
- Particle Size: For both mull and pellet techniques, it is crucial that the average particle size of the solid is smaller than the wavelength of the IR light to be transmitted. This typically requires grinding the sample to an average size of 1 to 2 microns.
- Transparency: The prepared sample must be transparent to IR radiation. This is why salts like NaCl and KBr, which are transparent in the IR range, are commonly used.
- Avoiding Contamination: Care must be taken to avoid contamination of the sample, as impurities can interfere with the spectral results. This is particularly important when using mulling agents like Nujol, which itself has a characteristic spectrum and can affect the analysis if not used carefully.
In summary, preparing samples for IR spectroscopy involves selecting the appropriate technique based on the sample's physical state (solid, liquid, or gas) and ensuring that the sample is finely ground and transparent to IR radiation. Proper preparation is essential for obtaining accurate and meaningful spectral data.
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