KBr disk technique

Vibrational spectroscopies were then applied, notably infrared (IR) and Raman. The IR spectra from the spent catalysts were measured with the aid of the KBr disk technique. They showed a C-H stretch band at approximately 3050 cm-1 due to H bound to aromatic (or unsaturated) carbon (H ) and bands at 2960, 2925 and 2865 cm-1 due to H bound to saturated carbon (Hsal). The Hat/Hsat ratio was determined from the intensities of these bands. 

KBr disks here the finely ground solid compound is mixed with anhydrous KBr and squeezed under pressure. The KBr becomes fluid and forms a disk containing the solid compound dispersed evenly within it and suitable for obtaining a spectrum. The advantage of the KBr disk technique is the absence of the spectrum from the mulling liquid, but the disadvantages are the equipment required (Fig. 28.4) and the practice required to obtain suitable transparent disks, which are very delicate and rapidly absorb atmospheric moisture. 

Fourier transformed infrared (FT-IR) spectra in transmission were recorded using a Perkin-Elmer 7200 Fourier transform spectrometer and KBr disk technique. 

IR spectra were recorded using the KBr disk technique and a Perkin-Elmer 1750 FTIR spectrometer. Surface area and pore volume were determined on a C. Erba Sorptomatic model 1700 apparatus, by means of Nj adsorption. Thermogravimetric (TO) analyses were carried out using a Perkin-Elmer TGS-2 thermobalance with a He-flow of 3dm /h and a heating rate of 600K/h. 

It should be noted that the detail of spectra obtained by reflectance methods might be different from that obtained in solution or with KBr disk techniques. Modern instruments possess software to convert reflectance spectra to resemble the more usual transmission spectra. 

A pre-eminent use already described was structure determination of organic compounds. The KBr disk technique allowed complete and unmarred spectra of solids to 400 cm 1. Extensive collections of spectra (both literature and commercial) of organics, inorganics, pharmaceuticals and industrial chemicals now appeared. 

Advantages and disadvantages of the KBr disk technique are given in Table 13.5. 

A very popular technique for mnning solids is the KBr disk technique. Here a few milligrams of sample are very finely ground and then mixed with 50 to 100 parts of dry KBr powder. The mixture is placed in a special device and compressed into a disk at high pressure. If all goes well, a transparent disk results, which is put into the spectrometer and run. Commercial KBr disk makers are available in many forms. Some are activated with wrenches or levers, while others are used with a hydraulic press. Some can be evacuated, which gives the disk transparency a longer lifetime, but this is not necessary if the disk is used promptly. 

Infrared and Raman spectroscopy are nondestructive, quick and convenient techniques for monitoring the course of solid-phase reactions, and have therefore been widely used for the characterization of polymer supports and supported species [156-160]. In fact, the application of infrared spectroscopy in solid-phase synthesis has received much attention and has been the subject of several recent reviews [127, 128, 161-164]. Reactions involving either the appearance or disappearance of an IR-active functional group can be easily monitored using any of the IR techniques described in this section. Some beads are typically removed from the reaction mixture, then they are quickly washed and dried prior to IR analysis. Traditionally, polymer supports are diluted and ground with KBr, then conventional FT-IR analysis of the KBr disk is carried out Although this is a commonly used... 

The pellet (pressed-disk) technique depends on the fact that dry, powdered potassium bromide (or other alkali metal halides) can be compacted under pressure in vacuo to form transparent disks. The sample (0.5-1.0 mg) is intimately mixed with approximately 100 mg of dry, powdered KBr. Mixing can be effected by thorough grinding in a smooth agate mortar or, more efficiently, with a small vibrating ball mill, or by lyophili-... 

The use of KBr disks or pellets has often been avoided because of the demanding task of making good pellets. Such KBr techniques can be less formidable through the Mini-Press, which affords a simple procedure the KBr-sample mixture is placed in the nut portion of the assembly with one bolt in place. The second bolt is introduced, and pressure is applied by tightening the bolts. Removal of the bolts leaves a pellet in the nut that now serves as a cell. 

Fig. 28. Effect of pelleting technique on the spectra of Alon-C exposed to ammonia (A) Alon-C, (B) Alon-C after ammonia exposure, (C) ammonia-exposed Alon-C in a KBr disk.

The pellet (pressed-disk) technique depends on the fact that dry, powdered potassium bromide (or other alkali metal halides) can be compacted under pressure to form transparent disks. The sample (0.5-1.0 mg) is intimately mixed with approximately 100 mg of dry, powdered KBr. Mixing can be effected by thorough grinding in a smooth agate mortar or, more efficiently, with a small vibrating ball mill, or by lyophilization. The mixture is pressed with special dies under a pressure of 10,000-15,000 psi into a transparent disk. The quality of the spectrum depends on the intimacy of mixing and the reduction of the suspended particles to 2 gm or less. Microdisks, 0.5-1.5 mm in diameter, can be used with a beam condenser. The microdisk technique permits examination of samples as small as 1 fxg. Bands near 3448 and 1639 cm-1, resulting from moisture, frequently appear in spectra obtained by the pressed-disk technique. 

The infrared spectrum (KBr disks) was recorded on a Perkin-Elmer 683 spectrometer (range 4000-200 cm 1, calibrated with polystyrene lines) and was found to be superimposable on that given in Ref. 26. This spectrum was used by Becke-Goehring and Boppel as evidence for the ANSA structure. It is demonstrated below how careful one has to be when assigning a certain molecular configuration on the bases of indirect structural techniques such as IR spectroscopy. 

An attempt has been made to extend the intensity criterion to insoluble materials, using KBr disks. Satisfactory results have been obtained enabling N-bonded complexes to be distinguished from S-bonded [i81). This technique has been further extended by using a suitable internal standard 60) the C—0 stretching band in salicylic acid (1654 cm ) has been used, and the ratio of the intensity of vcn to the intensity of this band has been suggested as a satisfactory criterion to distinguish N- and S-bonded complexes, provided that known complexes are used for calibration. 

A number of experimental alternatives to traditional IR transmission spectroscopy are suitable for overcoming some of these complicating experimental factors. In the technique of diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) (Hartauer et al. 1992 Neville et al. 1992) the sample is dispersed in a matrix of powdered alkali halide, a procedure which is less likely to lead to polymorphic transformations or loss of solvent than the more aggressive grinding necessary for mull preparation or pressure required to make a pellet (Roston et al. 1993). For these reasons, Threlfall (1995) suggests that DRIFTS should be the method of choice for the initial IR examination of polymorphs. He has also discussed the possible use of attenuated total reflection (ATR) methods in the examination of polymorphs and provided a comparison and discussion of the results obtained on sulphathiazole polymorphs from spectra run on KBr disks, Nujol mulls and ATR. 

Bis (benzoylacetonato) diphenyltin (IV) is a white solid, which is soluble in benzene and toluene and only slightly soluble in petroleum ether. The infrared spectrum (KBr disk) has peaks centered at 1570,1550,1520, and 1374 cm.. The ultraviolet absorption spectrum (benzene) has a band centered at 308 mpi (e = 4.42 X 10 ). Attempts to effect resolution of optical isomers by a chromatographic technique (n-lactose) were unsuccessful, and it has been suggested that the phenyl groups are in trans positions. ... 

Solids are usually ground with a material such as potassium bromide and compressed into a pellet. Moisture must be absent, and the transparent disk is placed in the window of the spectrometer. In general, however, measurement of intensity of absorption in the solid phase is unreliable due to scattering and reflection losses, and a uniform distribution of sample in the pellet is difficult to achieve. One method of handling solutions is to allow them to soak into a KBr wedge, evaporate the solvent, and compress the tip into a microdisk or pellet (C26) alternatively the sample (1 /tl) may be either placed directly on the KBr disk (B13) or mixed with a little KBr powder and subsequently incorporated in the disk. These microdisk techniques can be used for the examination of gas chromatograph effluent. For multiple analyses, an enclosed turntable system loaded with the disks can be used. 

Weissman et al. (1959) have analyzed renal calculi by the KBr disk method and applied the same technique to prostatic, dental, biliary tract, and salivary gland calculi, and to the identiflcation of alcohols, barbiturates, and salicylates (Klein et al, 1960). 

This will usually be KBr disk but other techniques may be used where demanded by the nature of the sample. 

---