Infrared (IR) spectroscopy is a powerful analytical technique used to identify and analyze the chemical composition of various samples. The choice of sampling technique depends on the state of the sample—whether it is solid, liquid, or gas. Each state requires specific methods to ensure accurate and effective analysis.
1. Sampling Techniques for Solids
1.1 Mull Technique
This method involves grinding a solid sample into a fine powder and mixing it with Nujol, a special oil. The mixture forms a thick paste that is then spread thinly onto salt plates for analysis.
1.2 Solid Run in Solution
Here, the solid sample is dissolved in a solvent that does not react with the sample. A drop of this solution is placed on a metal disc, and the solvent is allowed to evaporate, leaving a thin film of the sample behind.
1.3 Cast Film Technique
This technique is used for amorphous solids. The sample is dissolved in a solvent and then deposited onto a cell made of KBr or NaCl. The solvent is evaporated, leaving a thin film that is suitable for IR analysis.
1.4 Pressed Pellet Technique
In this method, finely ground solid samples are mixed with potassium bromide and compressed into a pellet using a hydraulic press. The resulting pellet is transparent to IR radiation and ideal for analysis.
2. Sampling Techniques for Liquids
2.1 Diffuse Reflection Method
This method is suitable for powder samples and has become more popular with the advent of Fourier Transform Infrared (FTIR) spectroscopy. It involves reflecting IR radiation off the surface of the sample.
2.2 Attenuated Total Reflection (ATR)
ATR allows for the direct measurement of powder samples. IR radiation is internally reflected within a crystal, enabling analysis without the need for sample preparation.
These techniques ensure that samples are prepared in a way that allows effective interaction with IR radiation, leading to accurate analysis of the chemical bonds present in the sample.
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