Pellets and Mulls

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 

6 GPa (6 tons cm ). Without evacuation (e. g. when moist air is present during pressing) it is impossible to obtain highly transparent pellets. The size of the ground particles should not exceed 2 pm, otherwise scattering losses may result IR spectra obtained by the pellet technique often exhibit bands at 3450 and 1640 cm due to adsorbed moisture. Without the addition of an internal standard the pellet technique is not suitable for quantitative measurements because the thickness is not precisely reproducible and the size of the IR bands depends on the dispersion of the sample (see Fig. 6.3). 

Mulls are used as alternatives to pellets. The sample (1 to 5 mg) is carefully ground into a suspension using a couple of drops of a mulling agent. This muU is pressed between two IR transmitting windows to form a thin film. Common mulling agents are Nujol (liquid paraffin), Fluorolube (a chlorofluorocarbon poly- 

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Figure 2. Infrared Absorption Spectrum of Hydralazine Base, Potassium Bromide Pellet and Mull.

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