Infrared spectroscopy can analyze solid, liquid, and gas samples. The analysis involves preparing the samples in a form that is transparent to infrared radiation, typically using salts like NaCl or KBr.
Solid Samples: Solid samples can be analyzed using various techniques:
- Mull Technique: In this method, finely crushed solid samples are mixed with Nujol (a mulling agent) to form a thick paste. This paste is then spread as a thin film onto salt plates and placed in the path of an IR beam for spectral recording.
- Solid Run in Solution: Here, the solid sample is dissolved in a non-aqueous solvent that does not interact chemically with the sample. A drop of this solution is placed on an alkali metal disc, and the solvent is evaporated, leaving a thin film of the solute for IR analysis.
- Nujol Mull Method: An alternative method involves creating Nujol mulls between KBr plates. The solid is ground to a particle size of 1 to 2 microns, mixed with Nujol, and spread between KBr plates. This method is effective if the particle size is smaller than the wavelength of the light to be transmitted.
Liquid Samples: Liquid samples are typically analyzed by placing a thin film of the liquid between salt plates or by using a liquid cell with a path length adjusted to the sample's absorbance properties.
Gas Samples: Gas samples are analyzed using gas cells with known path lengths, where the gas is sealed inside and exposed to the IR beam.
Infrared Spectrometer: The IR spectrometer detects the absorption of specific wavelengths of infrared light by the bonds in the sample. Each type of bond absorbs a characteristic wavelength, allowing the identification of the functional groups present in the molecule.
Measurement Methods: Different methods are employed based on the sample form:
- Diffuse Reflection Method: Suitable for powder samples, where the light is scattered by the sample.
- Attenuated Total Reflection (ATR): Allows direct measurement of powder samples by reflecting light internally multiple times within a crystal, interacting with the sample.
These methods ensure that the sample is prepared and analyzed in a way that maximizes the transmission and absorption of IR radiation, providing accurate spectral data for chemical analysis.
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