Spectrophotometry - IR Spectroscopy - Applications

After discussing about the theory and concepts, instrumentation and sampling technique for infra-red (IR) spectrophotometer, we will now discuss the last part of IR spectroscopy which are its applications.

IR spectroscopy is a simple and reliable technique. IR spectroscopy is widely used in industry as well as in research. Here are some of the applications:

Determination of functional group in an organic material: 

By IR spectroscopy, information about various functional groups of an unknown compound can be obtained by having a look at the regions in which the absorption band appears. As we have seen in earlier post that each compound has its own signature or fingerprint. Each and every peak obtained (in the IR spectrum) corresponds to different functional groups of the compound. So, as per the corresponding peak, functional group can be determined.

Determination of substances/Identification of compounds:

Another application is the determination of substances or identification of the compounds. Also, IR spectroscopy can used to establish whether two given compounds are identical or not. As we know, maximum number of absorption bands is observed in the IR spectra of organic molecules and there is almost a zero possibility that the two compounds have the identical IR spectra (as each compound has its own fingerprint or characteristic peak). If we suspect that some the compound is identical with the sample, then all we have to do is take the IR spectra of both. If the IR spectra match, then the compounds are identical.

Lets take an example of benzaldehyde. The IR spectrum of benzaldehyde will be as follows:

C-H stretching aromatic ring – 3080 cm^-1

C-H of aldehyde – 2860 cm^-1 and 2775 cm^-1

C=O stretching of aromatic aldehyde – 1700 cm^-1

C=C stretching of an aromatic ring – 1595 cm^-1

C-H bending – 745 cm^-1 and 685 cm^-1

No other compound apart from benzaldehyde will show this IR spectrum.

Assaying the progress of reaction/Determination of rate of reaction:

As we have seen that the IR spectra can used to determine the functional groups. Hence, any enzymatic reaction involving these functional groups – which are either consumed in the reaction or are produced in the reaction – can be determined with the help of IR spectra. For example if a reactant  contains carbonyl group and the product does not contain this group, so the progress of reaction can be determined by measuring the rate of disappearance of carbonyl (-C=O) stretching vibration. Thus, the rate of disappearance of a characteristic absorption band (of the reactant group) and/or the rate of appearance of the characteristic absorption band (of the product group) due to formation of the product is observed from time to time. In this way, the progress of the enzymatic reaction can be assayed.

Detection of impurities:

If we suspect that the compound is not pure, then IR spectroscopy can be used. The IR spectrum of the sample can be compared with the standard compound. If we see any changes in the spectrum like an additional band, then we can say that it is due to the impurities present in the compound.

Quantitative determination of compounds in mixtures:

The base line technique is used for quantitative determination of compound. The quantity of the compound or a mixture of two or more compounds can be determined. In this, the characteristic peak corresponding to the compound is chosen and log Io/It of peaks for the standard and the test sample is compared. 

By this, another kind of spectrophotometry, i.e.; the IR spectroscopy is completed.