Fourier Transform Infrared Spectroscopy FT-IR

Fourier Transform Infrared Spectroscopy (FT-IR): 

Principles :   It involves the absorption of electromagnetic radiation in the infrared region of 

the spectrum which results in changes in the vibrational energy of molecule. Since, usually 

all molecules will be having vibrations in the form of stretching, bending, etc., the absorbed 

energy will be utilised in changing the energy levels associated with them. It is a valuable 

and formidable tool in identifying organic compounds which have polar chemical bonds 

(such as OH, NH, CH, etc.) with good charge separation (strong dipoles).  

Instrumentation :  It was originally designed as a double beam spectrophotometer comprising 

IR source (red hot ceramic material), grating monochromator, thermocouple detector, cells 

made of either sodium chloride or potassium bromide materials, etc. In this process the light 

is dispersed by the monochromator. But, this type of basic design for IR measurements has 

been outdated. Instead a newer technique termed Fourier Transform-Infrared (FT-IR) has 

been in practice. This technique utilises a single beam of un-dispersed light and has the 

instrument components similar to the previous one. 

In FT-IR, the un-dispersed light beam is passed through the sample and the absorbances at all 

wavelengths are received at the detector simultaneously. A computerized mathematical 

manipulation (known as “Fourier Transform”) is performed on this data, to obtain absorption 

data for each and every wavelength. To perform this type of calculations interference of light 

pattern is required for which the FT-IR instrumentation contains two mirrors, one fixed and 

one moveable with a beam splitter in between them. Before scanning the sample a reference 

or a blank scanning is required. The following is the simplified design of the instrument: 

Applications :  It finds extensive use in the identification and structural analysis of organic 

compounds, natural products, polymers, etc. The presence of particular functional group in a 

given organic compound can be identified.  Since every functional group has unique 

vibrational energy, the IR spectra can be seen as their fingerprints. 

Disadvantages :  Samples containing mixture of substances can not be analysed. Since the 

sample holders and beam splitter, are made  of moisture sensitive materials like sodium 

chloride or potassium bromide (KBr), special cells are required for aqueous samples (e.g. 

KRS-5, ZnSe, etc.). Water is a bad solvent for IR spectral works.