IR spectroscopy can be used to characterize solid, liquid, and gas samples, provided they are transparent to IR radiation. Commonly used salts for sample preparation include NaCl and KBr.
Solid Samples:
- Mull Technique: In this method, a finely crushed solid sample is 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.
- KBr Pellet Method: The sample is mixed with potassium bromide and compressed into a pellet using a hydraulic press. This method is suitable for FTIR spectroscopy and is effective for analyzing molecular structures.
- Diffuse Reflection Method: This technique has gained popularity with the advent of FTIR and allows for the direct measurement of powder samples without the need for a mulling agent.
- Attenuated Total Reflection (ATR): ATR enables direct measurement of powder samples and is particularly useful for samples that are difficult to prepare in other forms.
Liquid Samples: Liquid samples can be analyzed directly by placing a drop between two salt plates (usually NaCl or KBr) to form a thin film. Alternatively, they can be analyzed in solution form, where the liquid is dissolved in a non-aqueous solvent that does not interact chemically with the sample and does not absorb in the IR range of interest.
Gas Samples: Gas samples are typically analyzed using a gas cell with salt windows that allow IR radiation to pass through. The gas is sealed in the cell, and the spectrum is recorded as the IR light passes through the sample.
Each of these methods allows for the absorption of specific wavelengths of IR light by the different types of bonds in the molecule, which is then converted into vibrational energy. By analyzing these absorption patterns, chemists can determine the types of bonds present in an unknown molecule.
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