Potassium bromide discs: preparation

Normal glass will only transmit radiation between about 350 nm and 3 /rm and, as a result, its use is restricted to the visible and near infrared regions of the spectrum. Materials suitable for the ultraviolet region include quartz and fused silica (Figure 2.28). The choice of materials for use in the infrared region presents some problems and most are alkali metal halides or alkaline earth metal halides, which are soft and susceptible to attack by water, e.g. rock salt and potassium bromide. Samples are often dissolved in suitable organic solvents, e.g. carbon tetrachloride or carbon disulphide, but when this is not possible or convenient, a mixture of the solid sample with potassium bromide is prepared and pressed into a disc-shaped pellet which is placed in the light path. 

The method used for sample preparation depends upon the nature of the sample. Liquids are easily examined as films formed when one drop of the liquid is squeezed between two flat sodium chloride plates, which arc transparent to IR radiation in the 4000-666 cm region. Solids can be examined as solutions, mulls in Nujol, or as potassium bromide discs. For solutions, a 5% solution of the solid is introduced into a sodium chloride cell, which is usually 1 mm thick. The solvent employed should be reasonably transparent to IR, and the background should be obtained with the cell containing the solvent only. 

Finally release the press and remove the die body (A) and the upper steel pellet from the face of the disc and then remove the disc itself with tweezers (the disc should never be handled with the fingers), and mount it in the specially designed holder. These latter operations are best conducted under a radiant heater. The disc holder is located in position in the sample beam path of the spectrophotometer if required a blank potassium bromide disc, similarly prepared, is introduced into the reference beam path. 

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. 

There are a wide variety of dies commercially available for the preparation of potassium bromide discs. In general, a die which can be evacuated prior to pressing the disc is preferable but not mandatory. Evacuation improves the lifetime of the disk clarity. Pressures of approximately 8 tons for a press-... 

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. 

To use KBr discs for quantitative measurements it is best to employ an internal standard procedure in which a substance possessing a prominent isolated infrared absorption band is mixed with the potassium bromide. The substance most commonly used is potassium thiocyanate, KSCN, which is intimately mixed and ground to give a uniform concentration, usually 0.1-0.2 per cent, in the potassium bromide. A KBr/KSCN disc will give a characteristic absorption band at 2125 cm 1. Before quantitative measurements can be carried out it is necessary to prepare a calibration curve from a series of standards made using different amounts of the pure organic compound with the KBr/KSCN. A practical application of this is given in Section 19.9. 

To obtain a calibration curve for benzoic acid, six discs should be prepared using potassium bromide containing 0.1 per cent potassium thiocyanate as described in Section 19.8 and increasing quantities of pure benzoic acid using the following quantities ... 

Test 2. According to the general method (2.2.24), examine miconazole by infrared absorption spectrophotometry, comparing with the spectrum obtained with miconazole CRS. Examine the substance as discs prepared using potassium bromide R. 

Test 2 Examine by infrared absorption spectrophotometry, according to the general procedure (2.2.24), comparing with spectrum obtained with primaquine diphosphate Chemical Reference Substance (CRS). Examine the substance as discs prepared as follows dissolve separately 0.1 g of primaquine diphosphate and the reference substance in 5 mL of water R, add 2 mL of dilute ammonia R and 5 mL of chloroform R and shake dry the chloroform layer over 0.5 g of anhydrous sodium sulfate R prepare a blank disc using about 0.3 g of potassium bromide R, apply dropwise to the disc 0.1 mL of the chloroform layer, allowing the chloroform to evaporate between applications dry the disc at 50 °C for 2 min. 

Sample preparation requires a degree of skill, partieularly when potassium bromide (KBr) discs are being prepared. 

A major difference between infrared and ultraviolet spectroscopy is in the concentrations required for assay In infrared spectroscopy as much as a 10% w/v solution of sample must be prepared. This means that the path length of the cells used in infrared must be very short, usually 0.025-0.1 mm (otherwise absorbance values would be too high). Another problem with infrared spectra is that the solvent used in the assay (usually chloroform or dichloromethane) also possesses chemical bonds that will absorb infrared radiation in some part of the spectrum, obscuring the absorption by the sample at these wavelengths. Samples are prepared in solution, in a mull or paste made with liquid paraffin (Nujol), or in a solid disc prepared by trituration with dry potassium bromide followed by compression in a hydraulic press. 

Alternatively, especially if the compound is insoluble in the usual spectroscopic solvents, the solid can be dispersed in well-ground, solid, dry, potassium bromide using a dental mixing machine and the mixture pressed into a 1 mm thick, 5 mm diameter disc. This disc can then be mounted in a cardboard or plastic holder and used to prepare a spectrum. 

IR-spectra of solutions have been little used up to now for quantitative evaluation on thin-layer chromatograms [383, 411]. The reason is that a high substance concentration is necessary for the measurements. Extraction has been performed in one case with a micro-extractor and measurement carried out in a micro-cuvette [411]. Successive drop application [640] or preparation of micro-discs with potassium bromide seems more promising. 

The silica gel corresponding to each of the six observed bands was then separately scraped off from the plate and reheated with absolute ethyl alcohol to desorb the organic matter. The six extracts thus obtained were then prepared into potassium bromide micro-discs and examined by IR spectroscopy in the 2.5-15 pm range. The results suggested that upon milling into PE at 260 °C, Santonox R degrades into at least two products, i.e., a phenolic breakdown product (band 2) and non-phenolic hydrocarbon (band 4). The phenolic breakdown product (band 2) has some resemblance to the material produced upon heating Santonox R in air for 30 minutes at 250 °C. 

Many modifications of the apparatus and method have been recommended.1 Discs of paper may be fixed across the mouth of the tube B in various ways and the stain, thus localised, is uniform in colour and sharp in outline and therefore more readily compared with standards.2 Mercuric bromide papers are satisfactory in use,3 but should be freshly prepared the stains may be developed in aqueous potassium or cadmium iodide and compared with a colour standard.4... 

---