Sampling Devices
The handling of sample for the measurement of IR spectra is not simple. Firstlybecause unlike UV-VIS spectrometry, glass or quartz cannot be used as a material formaking cuvettes to be used in the infrared region. It is so because glass absorbs strongly at most of the frequencies in this region. Secondly, analysing an analyte as a  solution is also not easy because most of the solvents of the organic compounds themselves absorb in many regions of the IR spectrum. In fact there is no solvent that is transparent in the whole range. In the absence of glass, the salts like NaCl and KBr which are transparent in the IR region are used for the construction of the cuvettes. These cuvettes are called IRwindows. The NaCl windows are most commonly used for routine work in the region 4000-650 cm-1 as these are cheaper than KBr windows. However, for measurements below 650 cm-1, the KBr cells are the right choice because NaCl starts absorbing in this region.
The sampling techniques depend upon the phase (solid, liquid or gas) of the sample being analysed. Let us discuss the sample preparation procedures of samples in different physical states:
Sampling of Gases: The gaseous samples are taken in approximately 10 cm long cuvettes having sodium chloride windows at either end as shown in  Fig. (a). The cuvette is kept across the path of infrared beam. The path length of the cells can be easily varied over a long range. In these specially designed cuvettes the internal surfaces are made reflective whereby the radiation can be made to travel through the sample many a times before coming out of it.

 

Sampling of Liquids: For liquid samples, cuvettes with fixed path lengths as well as of variable path lengths are commercially available. The cuvettes of less than 0.01 mm thickness are made of sodium chloride or potassium bromide rectangular plates sealed by gaskets and clamped together as depicted in Fig.(b). A drop of liquid is squeezed between the NaCl/KBr plates which are put in a cuvettes holder and scanned.
iii) Sampling of Solids: As most of the organic compounds absorb extensively in the mid IR region, it becomes difficult to find a suitable solvent whose signals would not interfere with those of the sample. In such case it becomes almost essential to analyse the solid itself. There are two techniques for handling a solid sample. These are as follows:

• Mull technique and Pelleting with KBr

Mull technique: In this technique called Nujol mull technique, the sample is finely ground to a paste in a small amount of mineral oil hexachlorobutadiene (nujol). It forms a fine suspension or mull of the sample in the oil. The mull is pressed between two NaCl or KBr plates to give a thin film of the sample. The two plates are then clamped in a holder with open windows on both sides and the assembly is kept in the path of the infrared beam.
KBr Pelleting : in this technique, a small amount of sample (~ 1 _2 mg) is mixed and ground with about hundred times its mass of potassium bromide. The mixture is transferred to an evacuuable die (Fig. 3.9), to remove the moisture This technique is useful when the amount of the liquid sample is small or when a suitable solvent is not available In addition to these two methods, the solid samples may be analysed by using attenuated total reflectance spectrometry or with the help of photoacoustic IR spectrometry then a high pressure is applied to yield a transparent pellet. It is put in a suitable holder and the whole assembly is placed in the line of the infrared

Thin films: The IR spectrum of a thin film can be obtained by placing a sample of the
film in a suitable holder, such as a card with a slot for the sample window. This method is quite useful in the calibration of the instrument with a standard like polystyrene for which the bands are accurately known.