Sampling techniques and complex nature of IR spectra:

The IR spectra of pol yatomic molecules are complex because of a number of reasons.  First  of  all  the  infrared  spectrum  of  a  polyatomic  molecule  would consist  of an absorption  band  at  each  of the (3N -6)  or (3N -5)  fundamental frequencies for nonlinear and linear molecules, respectively. In addition the overtones, the combination bands and the difference bands of these fundamental modes are also observed.

 The sampling of the solids is done by preparing a dispersion of the solid in a liquid or a solid matrix. For that reason there are two methods. In the Nujol mull technique, the sample is finely ground to a paste in a small amount of mineral oil (nujol) the mull is then pressed between two NaCl/KBr plates to give a thin film of the sample. In KBr pelleting,  a small amount of sample (~ 1-2 mg)   is mixed and ground with about hundred times its mass of potassium bromide and is transferred  to  an  evacuuable  die  to remove  the moisture  and  then  a high pressure is applied to yield a transparent  pellet.  The film or the pellet is then placed in the path of IR radiation.