Solid samples mulling technique

Crystalline or solid samples (ca. 10 mg) are ground with 5 drops of nujol in an agate mortar to form a paste for subsequent CD or MCD measurement. Perfluoro-butadiene can also be used instead of nujol. Otherwise, the KBr pellet technique, for which KBr and sample should be completely dried in vacuo and ground into a fine powder, can be used conveniently. On using the KBr technique, one can often observe a different CD spectrum from that of the nujol-mull method. 

For infrared measurements, cells are commonly constructed of NaCI or KBr. For the 400 to 50 cm 1 far-infrared region, polyethylene is a transparent window. Solid samples are commonly ground to a fine powder, which can be added to mineral oil (a viscous hydrocarbon also called Nujol) to give a dispersion that is called a mull and is pressed between two KBr plates. The analyte spectrum is obscured in a few regions in which the mineral oil absorbs infrared radiation. Alternatively, a 1 wt% mixture of solid sample with KBr can be ground to a fine powder and pressed into a translucent pellet at a pressure of —60 MPa (600 bar). Solids and powders can also be examined by diffuse reflectance, in which reflected infrared radiation, instead of transmitted infrared radiation, is observed. Wavelengths absorbed by the sample are not reflected as well as other wavelengths. This technique is sensitive only to the surface of the sample. 

There are several techniques to prepare solid samples for IR analysis solid KBr pellets, melts, solutions, and mulls. While analyzing chemicals related to the CWC, introduction of additional absorbance bands in the spectra should be avoided. Therefore, the last two methods should not be used if other methods are available, since they always have peaks present because of solvent or oil. 

The IR spectra of solid samples usually are recorded in transmission either by pressing samples into KBr pellets or grinding samples up as Nujol mulls. These two techniques are not considered suitable for surface analysis of paper documents. Fortunately, some information can be obtained by using internal and external reflectance techniques. Only the internal reflectance technique or the so-called attenuated total reflectance (ATR) technique will be discussed here. 

The acquisition of solid-state FTIR spectra suitable for use in the characterization of polymorphic impurities is performed using either the Nujol mull technique, diffuse reflectance (DRIFT), or attenuated total reflectance (ATR). One should avoid the use of pelleting techniques to eliminate any spurious effects associated with compaction of the KBr pellet. The simplest approach is to prepare a mull of the sample in mineral oil, sandwich this between salt plates, and measure the spectrum using ordinary transmission techniques. The main drawback of the mull technique is that regions in the IR spectrum overlapping with carbon-hydrogen vibrational modes will be obliterated because of absorbance from the oil. 

The absorption or reflection by the solid sample of photons of various energies, from the ultraviolet to the infrared regions, are probably the most accessible and widely used techniques [8]. Absorption of infrared radiation by mulls or dispersions in alkali-halide discs are standard procedures. The sample holder may be capable of being heated so that peaks of interest can be monitored during the progress of decomposition. Hisatsune et al. [9] have successfully used infrared measmements to follow the decompositions of metal carboxylates (Chapter 16) incorporated in KBr discs. Spectra of powders may differ significantly from those of the same material in the form of larger crystals. 

With a few exceptions (MacCarthy et al., 1975 MacCarthy and Mark, 1975) all infrared spctra of humic substances have been measured on dried solid samples, and the pressed-pellet method has been used almost exclusively. The mull technique has been used to a very limited extent in the study of humic substances (Ceh and Hadzi, 1956 Orlov et al., 1962 Wagner and Stevenson, 1965), and a few workers have also used the cast film method (MacCarthy and Mark, 1975 Wershaw and Pinckney, 1980). Typical infra-... 

Unlike the hexahalo complexes of the Ad and 5d series, many of which have been extensively investigated in solution, the hexafluoro anions of the first transition series are generally rather unstable towards water, this tendency increasing towards the end of the series and with increase in the oxidation number. Because of this the normal solution techniques are virtually inapplicable, although a study of NiFe in anhydrous HF has been reported (29), and recourse must be made to methods which can utilise solid samples. Such methods include measurements on dry powders between KBr discs (30), which do not appear very successful, and investigations of similarly confined fluorocarbon mulls (31), which seem to be much more satisfactory. However, the one generally available method for such materials is diffuse reflectance spectroscopy, which the authors have adopted for their own studies. 

Silver chloride windows are useful, being transparent until about 400 on, and plastic materials, such as polyethylene are used for the lower frequency region. The usual accepted techniques for obtaining spectra of solids, namely mulling with an organic oil or pressing into an alkali halide disc, are not very useful since they react with these reactive samples. 

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. 

Coarse or hard powders are not well served by either the compressed pellet or mull technique, mainly because of difficulties associated with grinding. In such situations, the best approaches require the use of an accessory, such as a diffuse reflectance or photoacoustic detector. Both diffuse reflectance and photoacoustic methods [99,100] may be applied to most forms of powdered solids. As a rule, photoacoustic measurements, which are the only form of true absorption measurement, are not significantly influenced by sample morphology. An alternative procedure for powders is ATR, especially a horizontal accessory, preferably equipped with a pressure applicator. Note that the use of pressure is recommended to ensure intimate contact between the sample and the IRE (internal reflectance element) surface. Normally, the sample must conform to the surface of the IRE, and because the strength of the IRE is typically limited, the procedure is recommended only for soft powders. However, with the introduction of diamond-based ATR accessories [101-103], it is possible to handle most types of powdered material. 

An extremely valuable technique for solid samples is the mineral oil (Nu-jol) mull method. A small amount of solid sample is mulled in a mortar with a small amount of Nujol to yield a paste which is then transferred to a rock salt plate or the sample may be mulled directly between the salt plates. The small scratches that result from this latter method (which saves time and sample) do no damage and are completely removed when the plate is resurfaced. Most beginners add too much Nujol and do not grind the sample well enough. A good mull is a paste, something like cold cream, for example. 

The mull technique is a second way of taking the spectra of solids in transmission. It works on powders or anything that can be ground into a powder, and competes directly with the KBr pellet method. Mulls are made by first grinding the sample to reduce particle size. This is to avoid light scattering and sloped... 

The compression of a solid to transparency is really not new. A standard technique in infrared spectroscopy is the use of mulls made of the heavier alkali halides. These materials are squeezed transparent at relatively low pressures. However, when these low pressures are used, evacuation of the sample is essential. In our work, evacuation is not necessary. A reasonable review of the alkali halide work is given by Ford, Wilkinson, and Price (4). 

Recently, Castiglioni, et al. have devised a new method, diffuse reflectance CD (DRCD), to measure the solid-state CD of a pure microcrystallite sample or one mixed with KBr [42,43] and checked their performance. An integrating sphere used for UV-Vis and IR spectroscopy was introduced (Fig. 6). The advantage of this technique is to remove possible artifact elements such as reactions with the KBr, the effect of pressure required for the pellet preparation, or sample dissolution in nujol. Further, samples can be recovered after the spectral measurement, in contrast to both KBr pellet and nujol mull methods. 

Another classical sampling technique for solids is the mineral oil mull preparation (18). In this technique, a small amount of sample ( 1 mg) is placed into an agate mortar. To this, a small amount of mineral oil is added and the sample and oil mixed to an even consistency. The mixture is then placed onto an IR optical window and sampled by the IR spectrophotometer. One advantage to this tech-... 

Some analytical techniques are sample specific. The group frequency bands in an infrared spectrum are largely independent of the method used to obtain the spectrum, whether it is run as a solid KBr disc, a Nujol mull or a solution and whether it is obtained by a dispersive or a Fourier... 

Infrared spectroscopy is the fastest and cheapest of the spectroscopic techniques used by organie and polymer chemists. As indicated, it is the measurement of the absorption of IR frequencies by organic compounds placed in the path of the beam of light. The samples can be solids, liquids, or gases and can be measured in solution or as neat liquid mulled with potassium bromide (KBr) or mineral oil. Recent developments in attenuated total refleetion (ATR) and diffuse reflectance techniques have made the analysis of solid adhesives possible. In fact, for bulk samples or powders, the reflectance technique is probably more suitable than transmission. 

PA spectrometry has several benefits over other sampling techniques. As noted in Chapters 13 and 16, Fresnel and diffuse reflection may contribute to the reflection spectra of solids this problem is far less important in PA spectrometry. There is no need to prepare KBr disks or mineral mulls to acquire the spectra of powders. For polymers, PA spectrometry obviates the need to prepare films from the melt or from solution. Although PA spectra measured on some of the earlier accessories were often quite noisy, contemporary accessories allow remarkably good spectra to be measured in measurement times of 5 min or less. 

Powders or solids can be examined as a thin paste or mull (mineral oil, nujol, hexachlorobutadiene, perfluorokerosene) or through the pellet technique which allows quantitative analyses to be performed since accurate measurements can be made of the ratio of weight of sample to internal standard in each disk. Modern technique using diamond ATR is removing the need for mulls and disks. 

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