Here's why this is important:
* Glass and quartz absorb IR radiation: These materials have strong vibrational modes within the IR region, which means they absorb a significant amount of the IR radiation that is being used to analyze a sample. This absorption would interfere with the measurement and make it difficult to get accurate spectra.
* Salt is transparent: Sodium chloride and potassium bromide are very transparent to IR radiation in the region typically used for analysis. This means that very little IR radiation is absorbed by the cell itself, allowing the instrument to accurately detect the absorption of the sample.
Additional advantages of using salt cells:
* Chemically inert: Salt is relatively inert and doesn't react with most organic molecules. This is important for analyzing samples without contamination.
* Easy to prepare: Salt cells can be easily prepared by pressing a powdered sample between two salt plates.
* Reusable: Salt cells can be cleaned and reused multiple times.
However, there are some disadvantages to using salt cells:
* Water soluble: Salt is water-soluble, so samples cannot be dissolved in water or any other solvent that will dissolve the salt.
* Hygroscopic: Salt absorbs moisture from the air, which can affect the quality of the spectrum.
* Brittle: Salt cells can be brittle and can break if handled carelessly.
Overall, despite their limitations, salt cells are the preferred choice for IR spectroscopy because of their excellent IR transmission properties.
