Potassium bromide infrared-transparent material

Potassium bromide (KBr) is the most commonly used infrared transparent material. It is transparent over a broad spectral range, from 400 cm" up through the visible. KBr windows are commonly used in the beamsplitters found in FTIRs, and their low wavenumber cutoff normally determines the low wavenumber cutoff of the FTIR as well. KBr is also relatively cheap and is easy to machine into windows and cells. A major drawback is that it is highly polar, which leads to two problems. First, KBr is hygroscopic, which means it absorbs water from the atmosphere. Over time a thick layer of water can build up on the surface of KBr, masking sample absorbances. Therefore, KBr in all forms—windows, cells, and powder—should... 

Infrared spectra for solid organic compounds are frequently obtained by mixing and grinding a small sample of the material with specially dry and pure potassi um bromide (the carrier), then compressing the powder in a special metal die under a pressure of 15 30 tonnes to produce a transparent potassium bromide disc. As the potassium bromide has virtually no absorption in the middle-infrared region, a very well-resolved spectrum of the organic compound is obtained when the disc is placed in the path of the infrared beam. 

The infrared spectrum of a liquid may conveniently be recorded as a thin film of the substance held in the infrared beam between two infrared-transparent discs without the need for a diluting solvent. It is customary to use polished plates of sodium chloride as the support material this material is adequately transparent in the region 2-15 /im. Spectra in the longer wavelength region (12-25 m) can be recorded using potassium bromide plates. Sealed cells (p. 267) should be used for volatile liquids. 

In a simple transmission experiment the liquid sample is examined in a cell made of a suitable infrared transparent medium. These include sodium chloride, potassium bromide, zinc selenide, cadmium telluride, and germanium. Materials like sodium chloride should not be used to study solutions in protic solvents like methanol and water. 

It is very important to make the right choice of the cuvette material for liquid and gas samples. This material must be transparent to the infrared light. Sodium chloride is the most often used material for the cuvettes and the optics of the infrared spectrometer. Other material such as special types of glass, quartz, aluminum oxide, calcium chloride, potassium bromide and so on are also used for special purposes. 

The material of the prism is important in infrared spectroscopy, since it must be transparent to infrared light. The material most frequently used for analysis in the middle wavelength region is sodium chloride. Prism materials for the analysis of short and long wave infrared light are usually potassium bromide, cesium bromide, and cesium iodide. 

Measurements in the infrared therefore must be made with the substance present in a material that does not absorb. Certain organic solvents are used frequently for this purpose. Alternatively, the solvent is eliminated completely. A common modern technique is to disperse the sample in a suitable inorganic salt, usually potassium bromide. The sample is mixed with the powdered crystalline salt, which is then pressed into a transparent disk measuring 0.5 mm in thickness and 10 mm in diameter. The disk is then mounted in a holder which is supported in the beam of the infrared instrument. There are some experimental difficulties which can be overcome by a skillful investigator. Since aqueous systems cannot be used in such experiments, infrared spectroscopy has no direct value in the study of biological systems, which are always aqueous. The usefulness of spectroscopy to the biologist is in the study of substances that have been extracted from biological systems. 

Infrared window A material that has a high transparency for IR radiation over some portion of the IR spectrum. Examples Sodium chloride silicon germanium potassium bromide (KBr) cesium iodide high density polyethylene. 

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