Potassium bromide disc technique

Stimson, M. M. (1962). Sampling for spectrophotometry with special reference to the use of the potassium bromide disc technique. In Progress in Infrared Spectroscopy, Voi. 1 (H. A. Szymanski, ed ). Plenum Press, New York. pp. 143-150. 

Most early work on the vibrational spectra of inorganic ions was carried out using Raman spectra, because of the problems of sample handling in the infra-red. However, a small amount of work was done using reflection spectra [ 1 ], and the development of the nujol mull technique stimulated many studies by Lecomte and his co-workers [2—5, 10]. The introduction of the pressed potassium bromide disc technique with the associated equipment for fine grinding has led to a stimulation of interest in this field. 

Transmission spectroscopy (2) is the simplest sampling technique in IR spectroscopy and is used for routine spectral measurements on diverse samples. Resin samples such as polystyrene or TentaGel (3) beads are usually prepared as a potassium bromide disc (pellet). A small amount, usually 1-3 mg, of finely ground solid sample is mixed with approximately 400 mg powdered potassium bromide and then pressed in an evacuated die under high pressure. The resulting discs are transparent and yield good spectra. 

Raynor et al separated additives in complex mixtures by capillery supercritical flow chromatography and deposited the separated products in potassium bromide discs prior to measurement of their infrared spectra by FTIR spectroscopy. Supercritical flow chromatography coupled with F.T. infrared spectroscopy (SFC-FTIR) has been used to provide quantitative information on and characterization of a range of polymeric surfactants. Bartle et al and Chester also investigated this technique. 

The production of potassium bromide discs for IR spectroscopy has never been automated, although at least one scheme has indicated the possibility. When dispersive spectrometers were used, this did not matter so much as spectrum acquisition was of the same order of duration as sample preparation, but FT-IR machines are much more rapid. Thus diffuse reflectance has recently become a very popular technique for monitoring the IR spectra of organic compounds that are soluble in volatile solvents as it only requires the placing of a drop of solution on a sm l layer of ground alkali metal halide powder. 

Procedure To the sample which contains 20-300 /xg of pertechnetate in 5-20 ml of solution, are added potassium perchlorate solution (2 ml, 1 mg KCIO per ml) and enough NaCl to make the solution approximately 1 M. The solution is heated and neutralized with ammonia. Pertechnetate is precipitated with aqueous 5 % tetraphenylarsonium chloride reagent. The precipitate is filtered, washed and dried, and a 2-mg portion is mixed with potassium bromide (300 mg). The mixture is pressed to form a clear disc by the usual technique. The infrared spectrum is recorded between 10 and 12 /x. The peak absorption is measured at 11.09 /X by the base-line technique. 

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. 

Sample handling is the same for both types of spectrometer and use is made of salts such as potassium bromide and sodium chloride which do not absorb in the IR region. Solid polymers usually are analysed in the form of either (i) discs pressed from finely powdered dilute (1-2 per cent) dispersions of polymer in potassium bromide, or (ii) melt-pressed or solution-cast thin films. Liquid polymers are analysed as thin films between the polished faces of two blocks (known as plates) of sodium chloride. Analysis of polymers in solution tends to be avoided where possible because a significant proportion of the IR spectrum of the polymer is obscured by the IR absorptions of the solvent. IR spectra of polymer surfaces can be recorded using techniques such as attenuated total reflectance and specular reflectance, and their use has grown with the increasing importance of polymer surface chemistry. 

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