Mull techniques

Salient Features The salient features of Mull Technique are as follows ... 

Chemists only began to use the technique routinely when commercial IR spectrometers had been developed.223 226 Three wartime research programmes created the initial demand, and provided the impetus for the production of commercial instruments. These were the US synthetic rubber programme,227 the British project to identify hydrocarbons in fuels from enemy aircraft,228 and the joint British-US penicillin programme. The mineral oil mull technique for obtaining the IR spectrum... 

When measuring spectra of very hard crystals, sometimes distorted bands, almost like in derivative spectra, can be seen with solid KBr or mulling techniques. This is caused by the Christiansen effect(8), which appears if too large crystals are left in the... 

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 mull technique was used to ensure compatibility with wet cake and slurry samples. A calibration curve was established for binary mixtures of the two forms using only an absorbance ratio of two characteristic absorption bands, and a limit of quantitation of 2.7% w/w was established. The method was then applied to the kinetic investigation of the transformation from the metastable form to the stable form. 

The mull technique involves thoroughly mixing the sample with a low. apor pressure, medium-molecular-weight hydrocarbon, generally known as N jiol. The mull or dispersion is then held between two infrared windows and its spectrum recorded. Since the Nujol itself absorbs radiation (absorption bands at —2900, —1460, and —1375 cm )> the observed spectrum consists of the superposition of the spectrum of the compound of interest on that... 

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-... 

Figure 1. Infrared Spectrum of Dexamethasone Split Mull Technique -...

The problems associated with water in alkali halide preparations suggest that other preparative techniques are worth pursuing. Mulling techniques were originally used in coal studies but require long grinding times. Furthermore, quantitative studies are difficult if not impossible on the resulting smears that are usually used for analysis. 

Over the last few years severd spectroscopic investigations of the NiFe system have been reported. As early as 1965 Westtand, Hoppe, and Kaseno 106) recorded, without discussion, a rough spectrum of KaNiFe, whilst in 1968 Bougon 107) prepared several salts of the NiFe anion, and reported the vz and V4 (ti ) vibrational frequencies, and noted the presence of a band at about 18.5 K. in the electronic spectrum. In 1969 two studies of the NiF spectrum were published, the first by Reisfeld, Asprey, and Penneman 31), who used a mull technique, and the other by Allen and Warren 108), who made measurements by diffuse reflectance, using samples sealed in silica cells. 

Quantitative analysis by the mull technique is difficult, but internal standards can reduce such problems. Internal standards such as calcium carbonate and lead thiocyanate have been used in quantitative analysis with mulls (Barnes et ai, 1947 Bradley and Potts, 1958). 

This problem does not arise with the Mull technique. The finely ground... 

Mull technique offers rapid preparation of the samples and moderate-quality IR spectra. 

The mull technique The basis of this approach is to fill the interstices between the particles of a finely powdered solid with an infrared transparent (or semitransparent) liquid medium. This mixture is then sandwiched into a thin film between a pair of infrared transparent windows. The mull, which should have the consistency of a paste, should be formed in such a manner as to minimize radiation scattering effects. 

The mulling technique is not recommended as a general method for presenting samples for quantitative analysis, mainly due to problems of reproducible... 

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. 

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. 

A. Crook and P. J. Taylor, Simple Mulling Technique for the Preparation of Samples for Infrared Spectroscopy, Chem. 4 Ind. (London) 95,1955. 

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... 

A major disadvantage of the mull technique is that the mulling agents have infrared peaks that can mask sample features. This is illustrated by the spectrum of mineral oil shown in Figure 4.9. Note that mineral oil has features around 3000, 1400, and 720 cm, all of which can interfere with the spectrum of your sample. In theory, one could subtract the spectrum of the mineral oil from the sample spectrum. However, the bands from the oil are usually too intense for clean subtractions to take place for reasons cited in Chapter 3. 

Note in Figure 4.10 that Fluorolube has no infrared features above 1350 cm, and note from Figure 4.9 that the mineral oil is transparent below 1350 cm- (except for the small peak near 720 cm ). In the split mull technique the spectrum of the same sample as measured using these two mulling agents, and then the 4000 to... 

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