Pressed halide disc method

The most common means of recording the spectrum of solid samples, the pressed halide disc method, relies on containing the material to be analysed in a self-supporting matrix of infrared transparent material. KBr is by far the most popular salt to use, although other metal halides, such as Csl or KCl, have their own particular advantages. Csl, for instance, transmits down to 180 cm whereas KBr cuts off at 350 cm  

To press a disc, the following is required infrared-quality KBr, a press (hand-held or hydraulic) capable of delivering from 15 000 to 25 000 psi pressure, a die, and a mortar and pestle (usually made of agate). The KBr should be kept in a clean, dry environment— preferably a desiccator or a drying oven maintained at 105 C. The storage unit should not be used for anything which might contaminate the KBr by sublimation or evaporation. The die is a simple infrared accessory which holds the powder between two platens while pressure is applied. 

Place 1 to 10 mg of sample in the mortar and grind to as fine a consistency as possible. Grinding reduces particle size and thereby helps prevent unnecessary scattering of the incident radiation. Add approximately 0.5 g of KBr to the sample and mix together. Further grinding is not required. Place the resulting mixture on the lower 

It is sometimes advisable to record the background through a pure KBr disc rather than an empty compartment. This may help to correct for baseline slopes or water accumulation. 

Ideally, the die should be evacuated before being compressed. This removes air from the matrix and helps prevent micro-cracks appearing in the disc during scanning. The effect is not usually too damaging, however, and this stage is frequently omitted. 

---