Sample-handling techniques mulling technique

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. 

Not all materials (e.g., elastomers, leathers, plastics) can readily be ground such materials can usually be handled more easily by other techniques. However, if a mull of this type of material is required, it is frequently useful to cool the samples with liquid nitrogen until they become brittle, and then grind them at this temperature. 

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