Spectrophotometry - IR Spectroscopy - Preparation of Sample

Earlier, we have discussed about theory and concepts, instrumentation of IR spectrophotometer. In this post, we will discuss about another important factor that needs to be considered while performing IR spectroscopy which is the preparation of sample or in other words, sampling techniques.

The sampling technique or the preparation of sample depends on whether the sample is in vapor phase, liquid or solid phase. As there is significant difference in the intermolecular forces in these three phases, hence it is better that the data obtained is specified for the sampling technique used. So, here, we will see about the preparation of samples which are in gas, liquid and solid phases.

Sampling of gases: 

The sample cell is made up of NaCl, KBr etc. and is similar to the liquid sample cell. A sample cell has a series of internal mirrors (multi-pass cells) which reflect the IR beam back and forth lengthening the path-length. A sample cell with a long path length (5 – 10 cm) is needed because the gases show relatively weak absorbance.

Sampling of liquids: 

Liquids are usually observed as a thin film between two IR-transparent windows. Liquid sample cells can be sandwiched using liquid sample cells of highly purified alkali halides, normally NaCl. However, if the sample contains water, they become useless. In such cases, CaF2 flats are used. The sample thickness should be selected so that the transmittance is between 15 – 20%. For most liquids, the sample cell thickness is 0.01 – 0.05 mm. 

Sampling of solids:

Various techniques used for preparing solid samples are as follows:

Mull technique: In this technique, the finely crushed sample is mixed with Nujol or Kaydol (which are mulling agents) in a marble or agate mortar, with a pestle to make a thick paste. A thin film is applied onto the salt plates. This is then mounted in a path of IR beam and the spectrum is recorded.

Deposited films/ Case film technique: If the solid is amorphous in nature, then the sample is deposited on the surface of a KBr or NaCl cell by evaporation of a solution of the solid and ensured that the film is not too thick to pass the radiation.

KBr Disks/ Pressed Pellet technique: In this technique, a small amount of finely ground solid sample is mixed with 100 times its weight of KBr and compressed into a thin transparent pellet using a hydraulic press. These pellets are transparent to IR radiation and it is used for analysis.

So, this was about the various sampling techniques. In the next post, we will have a look at the applications of IR spectrophotometry.