When measuring an infrared spectrum using the diffuse reflection method, the sample powder is normally not measured directly but diluted in an alkali halide, such as KBr. The diffuse reflection method is used to obtain spectrum using the diffuse reflected light. When light is shone onto a powder sample, some of the light undergoes specular reflection at the powder surface and other light enters the sample and reappear from the surface as diffuse reflected light after repeated transmission and reflection. However, background measurements on a pellet holder with a pellet of KBr only, that contains no sample, can correct for infrared light scattering losses in the pellet and for moisture adsorbed on the KBr. When performing measurements, the background can be measured with an empty pellet holder inserted into the sample chamber. After drying the powder, store it in a desiccator. Note that rapid heating can oxidize some of the KBr powder to KBrO 3, causing a brown discoloration. and then dry at approximately 110В вЊ for two to three hours. Before forming the KBr powder into pellets, pulverize it to 200 mesh max. Inadequate vacuum may result in easily broken pellets that scatter light. Degassing is performed to eliminate air and moisture from the KBr powder. A force of approximately 8 tons is applied under a vacuum of several mm Hg for several minutes form transparent pellets. Approximately 0.1 to 1.0 % sample is well mixed into 200 to 250 mg fine alkali halide (the example of KBr is used below) powder and then finely pulverized and put into a pellet-forming die. The preparation method for 13 mm-diameter pellets is described below. Cesium iodide (CsI) may also be used to measure the infrared spectrum in the 400 to 250В cm -1 low-wavenumber region. Potassium bromide (KBr) is the commonest alkali halide used in the pellets.

This method exploits the property that alkali halides become plastic when subjected to pressure and form a sheet that is transparent in the infrared region.

Chamber of reflection sample