Alkali halide pellet

The most common way to obtain IR spectra for quantiative evaluation is to embed the sample in an alkali halide pellet. Dry, spectroscopy-grade potassium bromide or potassium chloride are suitable embedding media. (Drying procedure 120°C in oven for at least 1 h, followed by cooling in a desiccator over... 

Fig. 4.1.3A-C. Preparation of alkali halide pellets. A Press for preparation of standard (macro) pellets. B Adapter for preparation of micro pellets. C Adapter and sample holder for micro pellet...

The classic IR sampling technique is the alkali halide pellet preparation (18). This technique involves mixing the solid-state sample of interest with an alkali halide (typically KBr or KC1) at a 1-2% w/w sample/alkali halide ratio. The mixture is pulverized into a finely ground homogeneous mixture, placed into a die (typically stainless steel), and subjected to approximately 10,000 psi of pressure for a period of time to produce a glass pellet. The pellet (with the sample finely dispersed throughout the glass) may then be placed into the IR spectrophotometer for spectral data acquisition. 

With the ever-increasing need to improve quality and productivity in the analytical pharmaceutical laboratory, automation has become a key component. Automation for vibrational spectroscopy has been fairly limited. Although most software packages for vibrational spectrometers allow for the construction of macro routines for the grouping of repetitive software tasks, there is only a small number of automation routines in which sample introduction and subsequent spectral acquisition/data interpretation are available. For the routine analysis of alkali halide pellets, a number of commercially available sample wheels are used in which the wheel contains a selected number of pellets in specific locations. The wheel is then indexed to a sample disk, the IR spectrum obtained and archived, and then the wheel indexed to the next sample. This system requires that the pellets be manually pressed and placed into the wheel before automated spectral acquisition. A similar system is also available for automated liquid analysis in which samples in individual vials are pumped onto an ATR crystal and subsequently analyzed. Between samples, a cleaning solution is passed over the ATR crystal to reduce cross-contamination. Automated diffuse reflectance has also been introduced in which a tray of DR sample cups is indexed into the IR sample beam and subsequently scanned. In each of these cases, manual preparation of the sample is necessary (23). In the field of Raman spectroscopy, automation is being developed in conjunction with fiber-optic probes and accompanying... 

Developments in MIR spectrometers, particularly Fourier-transform (FT) techniques, have enabled the use of a variety of solid sampling techniques which overcome the disadvantages of classic IR-sampling techniques. Classic sampling techniques [30], such as alkali halide pellet preparation (with KBr or KCl) or mineral oil mull preparations, require a mechanical treatment of the sample and may thus induce solid-solid transformations or desolvations. 

One other technique, previously described, is the abrasion of the surface of the sample with a silicon carbide paper. The abraded material may be collected as a dust and analyzed as a compressed alkali halide pellet or by diffuse reflectance. Alternatively, the spectrum of the sample from the surface of the abrasive paper may be obtained with a diffuse reflectance accessory. 

Saiki, Y., and T. Suzuki, 1966. Anomalies in the infrared spectra of ammonium carbonate and ammonium bicarbonate prepared by alkali halide pellet technique. Japan Analyst 15 608. 

For the normal pellet technique where an alkali halide is used as pelleting material, reactions could occur between the matrix and the sample under study. Most of these reactions are found to be exchange reactions so that the sohd... 

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. 

There are two generally applicable methods of preparing dried humic samples for IR spectroscopy—the alkali halide pressed-pellet method and the r-ull technique. In the pressed-pellet method approximately 1 mg of the cned sample is thoroughly mixed with about 100 mg of dried alkali halide s.ili. usually KBr, and compressed into a pellet which is then placed in the s.i.Tiple path of an infrared spectrometer and its spectrum recorded. As KBr -s infrared-transparent over the conventional range of 4000 to 400 cm only -.e spectrum of the sample within the KBr matrix is observed. This proce-cjre. with its advantages and limitations, is discussed further in the follow-.-.g references (Stimson, 1962 Fridman, 1967 Parker, 1971 Price, 1972 Ataman and Mark, 1976). 

An advantage of IR spectroscopy in general, and as applied to humic substances in particular, is the small quantity of sample required. For example, the alkali halide pressed-pellet technique, as normally carried out, requires about 1-2 mg of sample, which is considerably less than that required for the other most useful spectroscopic method, NMR, in the study of humic substances (see Chapter 22). However, microsampling methods have been developed which can record IR spectra on less than 0.01 /xg of sample (Alpert et al., 1964 Parker, 1971 Price, 1972 Griffiths and Block, 1973). These special techniques may have advantage in the study of humic samples isolated only in minute quantities, or for investigating small samples obtained in the fractionation of humic substances. 

The infrared spectrum of the sample can be obtained either by an alkali halide pressed pellet technique or by different reflection techniques. The sample powder is ground into small particles and 3-10 mg are mixed uniformly and pressed with approximately 300 mg of alkali halide powder (such as sodium chloride (NaCl) or more commonly potassium bromide (KBr)) by means of the apparatus electrohydraulic press with an evacuable KBr die. The quantity of the leather powder used for the pellet should be determined so that the spectrum that will be obtained has an absorbance of the strongest band of around 1 unit. The transmission spectrum can be obtained by placing the thin and uniformly pressed pellet of leather powder-KBr perpendicular to the infrared radiation. 

Pellets are used for solid samples that are difficult to melt or dissolve in any suitable solvent, or which have to be measured in their native solid state. The sample is finely ground and mixed with dry potassium bromide (or other alkali halide) powder. The usual analyte/KBr ratio is ca. 1 100. Grinding and mixing can be done with an agate mortar and pestle or with a vibrating mill. The mixture is then pressed into a transparent disk in an evacuable die for 2 min at a pressure of... 

Owing to moisture in the atmosphere, pellet spectra often show weak hydroxyl bands when there is no hydroxyl group present in the sample. Care should be used when one interprets spectra of pellets in the O—H stretching region. A check on this problem is to record the spectrum of a disk of the alkali halide alone. 

Multi-component analysis can be readily apphed to the infrared spectra of minerals. The latter contain non-interacting components and so the spectrum of a mineral can be analysed in terms of a linear combination of the spectra of the individual components. However, the spectra of such solids exhibit a marked particle-size dependency. The particle size should be reduced (to 325 mesh) prior to preparation of an alkali halide disc. The pellet preparation involves separate grinding and dispersion steps because minerals tend not to be effectively ground in the presence of an excess of KBr. Figure 5.8 illustrates the analysis of a mineral containing several components. The sample spectrum (a) is shown, as well as the calculated spectrum (b) based on the reference spectra of a variety of standard mineral components. The residual difference spectrum (c) shows that the error between the two spectra is small. 

Figure 6 Example of alkali-halide disk-making apparatus (A) photograph of a manually operated hydraulic press for preparing alkali-halide disks and (B) photograph of a range of evacuable pellet dies. (Reproduced by kind permission of Specac Ltd., Orpington, Kent, UK.)...

In an attempt to test the theories of NMR shielding constants in alkali halide crystals, Baron in 1963 [367] studied the pressure dependence of the chemical shifts in different powder pellets. The... 

In the case of compressed halide pellets, the sample is first ground to the desired nominal particle size and is then mixed (with partial grinding) with the halide salt (usually KBr but sometimes KCl). The mixture is then compressed in a special high-pressure die, with a nominal pressure of approximately 10 tons (for a 13-mm-diameter pellet). It is important to ensure that there is no ionic interaction between the sample and the potassium halide. Also, be aware of materials that undergo phase changes when subjected to the extreme pressures of the compressed pellet technique. Make sure that the potassium halide is kept in a clean, dry area, and if there is any doubt about its quality, run a blank pellet without sample. For materials that react with alkali halides, or for spectra to be recorded below 400 cm , polyethylene powder may be used as an alternative compressed pellet medium. 

Less common situations to consider are when studies of surface contamination are required and when the surface cannot be separated and brought to the instrument. In the first example, a practical solution is to grind a small amount of alkali halide powder (KBr or KCl) at the point of the contamination. This will result in the transfer of some of the contaminant to the alkali halide, and this in turn may be examined either by the compressed pellet technique or by diffuse reflectance. A way to assist this surface extraction is to add a few drops of a solvent, such as methylene chloride, in the area of the contamination. In the second situation, sampling may be performed by gently abrading the surface with silicon carbide paper. This action will transfer a small amount of the surface to the abrasive paper, and this in turn may be examined with the aid of a diffuse reflectance accessory. 

KBr Pellets, In many laboratories nujol mulls have been replaced to a large extent by the KBr-pellet technique. In this technique the sample is again ground to a fine, uniform particle size (preferably under 2 pi), but is mixed with an alkali halide -usually KBr) instead of being dispersed in nujol. This sample-KBr mixture is pressed into a pellet. In this technique both the sample and the KBr are weighed, thus making the technique useful for quantitative analysis for many types of samples. Normally pellets are made to have a total weight of 300 mg of this total, 0.5-1 % is sample. 

Solid Analysis Finely divided and dispersed solids in alkali halides (KBr is the most widely used). The mixture in the proportion of 1 mg of sample 300 mg of KBr is pressed, forming a clear and transparent pellet. Material that may form films, such as polymers and resins, is directly analyzed and the film must be as thin as possible. For surface characterization the sample is prepared in a the form of a self-supported pellet, where a small quantity of solid ( 20 mg) is pressed in order to obtain a thickness of decimals of millimeters. 

The popular techniques for recording the infrared spectra of solids have been to press a KBr pellet of the sample, or grind the sample in a Nujol mull. The former technique is time consuming and the latter method suffers from spectral interferences due to the mulling material. The technique of diffuse reflectance overcomes these problems. Here, one simply mixes up the sample under consideration with an alkali halide powder such as KBr or KCl. The IR radiation is directed on to the powder matrix by a suitable optical arrangement and is diffusely scattered in all directions., Part of this diffuse radiation is collected and directed towards the detector (Figure 11)... 

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