Infrared-transparent potassium bromide

The FTIR spectrum of a piece of paper can be measured non-destructively using an FTIR-microscope or, probably more reproducibly, by taking a sample and either placing it directly in a diamond cell or grinding it with infrared-transparent potassium bromide and pressing it into a disc to go into the spectrometer. 

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. 

Instrumental Interface. Gc/fdr instmmentation has developed around two different types of interfacing. The most common is the on-the-fly or flow cell interface in which gc effluent is dkected into a gold-coated cell or light pipe where the sample is subjected to infrared radiation (see Infrared and raman spectroscopy). Infrared transparent windows, usually made of potassium bromide, are fastened to the ends of the flow cell and the radiation is then dkected to a detector having a very fast response-time. In this light pipe type of interface, infrared spectra are generated by ratioing reference scans obtained when only carrier gas is in the cell to sample scans when a gc peak appears. 

Infrared detectors are similar in construction to those used in UV detection. The main difference is that the sample cell windows are constructed of sodium chloride, potassium bromide, or calcium fluoride. A limitation of this type of detector is caused by the low transparency of many useful solvents (Skoog et al., 1998). Recent changes to interface systems that use spraying to induce rapid evaporation of the solvent provide good sensitivity and enhanced spectral quality (LaCourse, 2000). 

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 the pressed disc technique a known weight of sample is intimately ground with pure, dry potassium bromide and the mixture inserted into a special die and subjected to pressure under vacuum. The concentration of sample in the disc is usually in the region of 1.0 per cent. The disc so produced may be mounted directly in the sample beam path of the spectrophotometer and the spectrum recorded. This method has the advantage that the spectrum so produced is entirely due to the sample since pure dry potassium bromide is infrared transparent in the 2-25 /xm region. To eliminate the possibility of impurities in the potassium bromide, however, a blank disc (no sample) can be made and mounted in the reference beam path of the spectrophotometer. Care should be taken to ensure that both discs are of equal thickness otherwise inverse peaks may occur if the potassium bromide is damp or impure, and this will be particularly noticeable if the reference disc is thicker than the sample disc. 

Dispersed in an infrared transparent powder. Pellets are made with I to 2% by weight of the sample, usually in dry potassium bromide. A pellet of approximately 300 mg is prepared in a mould called a matrix using a laboratory press. This method produces pellets 1 to 2 mm thick for a diameter of approximately 13 mm. This technique can be used for quantitative studies but the spectra may sometimes differ ftom those obtained via dispersion in a liquid. This is because, under the effect of the high pressure to which the powder is subjected, the solid may undergo certain modifications. 

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. 

This method involves mixing a solid sample (a few mg) with a dry alkali halide powder (100-200 mg). The mixture is usually ground with an agate pestle and mortar and then subjected to a pressure of 10ton in 2 (1.575 X 10 kgm ) in an evacuated die. This sinters the mixture and produces a clear transparent disc. The most commonly used alkali halide is potassium bromide (KBr), which is completely transpment in the mid-infrared region. 

The identity of pergolide mesylate is determined using the specificity of infrared spectroscopy, which differentiates it from any synthetic intermediates, process related substances or degradation products. Pergolide mesylate is triturated with potassium bromide and pressed into a transparent pellet for spectroscopic analysis. The identity is confirmed by comparison to a reference standard spectrum obtained under similar conditions. 

IR-spectra are obtained from samples in film form, provided the film is thinner than approx. 50 pm. Thicker samples or resin granules are heated above their softening temperature and then pressed to form films thin enough to be used directly for IR-spectroscopy. Films can also be obtained by casting from solutions of the plastic. A few drops of the solution are placed on a potassium bromide disc, and after evaporation of the solvent, the IR-spec-trum can be observed directly from the disc because KBr does not show any absorption in the infrared, i.e., it is completely transparent. One has to make sure that evaporation of the solvent is complete in order to avoid interference through absorption bands due to the solvent. For this reason. 

Fourier transform infrared speetroseopy (FTIR) is a modem method for last analysis of phenolic resins [204,226,227], the modilied products [33,148,183,214], the curing process [218], and the erosslinked products. Several FTIR techniques are useful in phenolic resin characterization. The simplest method is transmission spectroscopy. This method requires an optimum optical density of the sample. Soluble prepolymers and soluble products may be examined as solutions using solvents i.e. triohloromethane, that are reasonably transparent over the range of400-4000 cm. Absorbance measurements can also be made on a thin film prepared by casting from solution. Solid, insoluble products may be mixed with potassium bromide and compessed at room temperature to an optical clear disk that can measured using the transmission method. 

Sodium Chloride Cells. Single crystals of sodium chloride are cut and polished to give plates that are transparent throughout the infrared region. These plates are then used to fabricate cells that can be used to hold liquid samples. Because sodium chloride is water soluble, samples must be dry before a spectrum can be obtained. In general, sodium chloride plates are preferred for most applications involving liquid samples. Potassium bromide plates may also be used in place of sodium chloride. 

Polycrystalline or powdered samples can be prepared as a suspension in mineral oil (Nujol mull), as a potassium bromide disk (pellet), or as thin films deposited on infrared-transparent substrates. The potassium bromide pellet is the most common way of preparing powder samples in this method a small amount, usually 1 mg, of finely-ground solid sample is mixed with powdered potassium bromide, usually 300 mg. and then pressed in an evacuated die under high pressure. The resulting disks are transparent and yield excellent spectra. The only infrared absorption in the potassium bromide matrix is due to small amounts of adsorbed water, which can. however, be confused with OH-containing impurities in the sam-... 

Solids. There are several methods for determining infrared spectra for solids. One method of choice has been to mix a finely ground sample with powdered potassium bromide and press the mixture under high pressure. Under pressure, the potassium bromide melts and seals the sample into a matrix. The resulting KBr pellet is inserted in the instrument. If a good pellet is prepared, the spectrum obtained will have no interfering bands since potassium bromide is transparent down to 400 cm . ... 

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 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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