Instrumentation

Two types of infrared spectrometers are commercially available: dispersive and Fourier Transform infrared (FTIR) spectrometers.

Dispersive spectrometer: A dispersive spectrometer is very similar in design to a UV/visible spectrophotometer. It has a radiation source, a grating monochromator, and a detector. The IR radiation generated by the source is dispersed into different frequencies by a monochromator. The selected frequencies go through sample and reference cells and the transmitted light is measured by the detector. The infrared sources are usually inert solids that are electrically heated to radiate infrared radiation. The detectors usually are either thermal sensors such as thermocouples and thermistors or the semiconductor materials that conduct following absorption of IR radiation (absorption of photon causes transition of electrons from the valence band to the conduction band).

Fourier Transform Spectrometer: A Fourier transform spectrometer uses an interferometer in place of the monochromator. An interference of polychromatic radiation is generated using an interferometer, usually a Michelson interferometer (Please see Box 10.1). Absorption of any particular wavelength will bring a change in the interferogram which gets detected. An interferogram is a time domain signal and is converted to frequency domain signal though Fourier Transformation.

Dispersive infrared spectrometers are still in use but FTIR spectrometers are slowly taking over. FTIR spectrometers have several advantages over the dispersive ones:

1. Better speed: FTIR spectrometers detect absorption of all the frequencies simultaneously; consequently, they are much faster than the dispersive spectrometers that scan the entire frequency range stepwise.
   
   
2. Better sensitivity: Their speed of data acquisition makes FTIR spectrometers more sensitive. A large number of spectra can be recorded in small time thereby giving an improved signal to noise ratio.
   
   

   .....................................................................................(10.6) where n is the number of independent measurements

   
   
3. More radiation energy: Dispersive spectrometers use slits that result in loss of radiation energy. FTIR spectrometers lack the slits as filtering of radiation is not required; this provides higher radiation energy for recording the absorbance.
   
   
4. Simple design: As dispersion of the radiation and filtering are not required in the FTIR spectrometer , the movable mirror is the only moving part in the spectrometer.
   
5. Wavelength accuracy: The FTIR spectrometers usually have a He-Ne laser emitting light of 632.8 nm. This serves as an internal calibration for the wavelength and provides an accuracy of 0.01 cm^-1 or better.