In IR spectroscopy, the methods of sampling preparation vary depending on the state of the sample (solid, liquid, or gas). For solid samples, common techniques include the Mull technique, Nujol method, and KBr pellet method. Each method involves preparing the sample to ensure it is transparent to IR radiation, typically using materials like NaCl or KBr.
Mull Technique: This method involves mixing a finely crushed solid sample with Nujol, a liquid paraffin that serves as a mulling agent. The sample is pulverized in a mortar and pestle, then mixed with one or two drops of Nujol to form a thick paste. This paste is then applied onto salt plates and placed in the path of the IR beam for spectral recording. The use of Nujol is advantageous as it has low absorption in the IR region, allowing for clear transmission of the sample's IR spectrum.
Nujol Method: Similar to the Mull technique, the Nujol method also uses Nujol as a dispersing medium. The powdered sample is mixed with Nujol to create a slurry, which is then applied between two KBr crystal plates. This method is particularly useful for powder samples and requires minimal sample pretreatment. The slurry is sandwiched between the plates to ensure even distribution and minimal air gaps, which could otherwise interfere with the spectral readings.
KBr Pellet Method: In this method, the solid sample is mixed with potassium bromide (KBr) in a 1:100 ratio by weight. The mixture is then compressed using a hydraulic press to form a transparent pellet. This pellet is primarily transparent to infrared light and contains a dilute sample quantity suitable for FTIR detection. The KBr pellet method is widely used for its effectiveness in preparing samples for detailed molecular structure analysis.
Each of these methods ensures that the solid samples are prepared in a manner that optimizes their transparency to IR radiation, allowing for accurate and detailed spectral analysis. The choice of method depends on the specific characteristics of the sample and the requirements of the analysis.
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