It can be distinguished among chemical elements molecular vibrations spectroscopy. Fourier Transform Infrared (FT-IR) spectrometry can be employed for a long range of frequencies “Fingerprints” which are varying over ultraviolet, visible, near-infrared, mid-infrared and even far infrared regions by selecting different beam splitters and detectors for the required ranges. Fingerprints are found to be in the range from approximately (10 to 100THz) or (400 to 4000 Cm?1). They are measured typically by direct absorption with Fourier-transform infrared (FTIR) spectrometers. No other dispersive technique can be used for such a wide range of frequencies.
Michelson invented the interferometer known as Michelson interferometer. He developed the experiment to determine the speed of light. Fig. 3.2 represents a basic Michelson interferometer.
Fig. 3.2 Schematic diagram of Michelson interferometer
Besides Michelson interferometer, different types of interferometers had developed. These interferometers are lamellar grating and Fabry-Perot interferometers. Lamellar grating spectrometers are very common with the Michelson interferometer. The optical modulation device in the lamellar grating instrument is a pair of mirrors in tongue and groove arrangement as shown in Fig. 3.3.
Fig. 3.3 Schematic diagram of a lamellar grating instrument
The surfaces of the two mirrors are fixed to be in the same plane to appear to be one large mirror. That large mirror divided into a dozen or more horizontal strips. The first mirror contains one set of alternate strips, which are fixed, while the second mirror contains strips that are connected. So they may move forward of the fixed strips to a position behind the fixed strips. In the far infrared or microwave region below (100 Cm–1), lamellar grating spectrometers are preferred to the Michelson interferometer because of their high efficiency.
The main differences between the types of FT-IR spectrometers using Michelson interferometer are the design and the manner in which it is operated. It can be operated by scanning in a discontinuous stepwise manner (step-scan interferometer), a slow continuous manner with the chopping of the infrared beam (slow scanning) or rapidly without chopping of the infrared beam. Most of the FT-IR spectrometers use of a rapidly scanning interferometer.
Fig. 3.4 Schematic diagram of rapid scanning interferometer FTIR spectrometer
The infrared source emits radiation of all wavelengths in the infrared region. It is usually selected for the desired spectral range. The collimating mirror collects the light from the infrared source. The beam splitter divided the amplitude of the beam into two parts. One part of the radiation beam goes to the moving mirror and another part of the fixed mirror. The return beams from these mirrors collected again at the beam splitter and undergo interference. The reconstructed beam is then passed through the sample and focused on to the detector. The detectors must respond quickly because intensity changes are fast because the moving mirror moves quickly.
188.8.131.52 Instrumental Process
Infrared energy is emitted from a glowing black-body source. This beam passes through an aperture which controls the amount of energy presented to the sample. The beam then enters the interferometer where the “spectral encoding” takes place. The resulting interferogram signal then exits the interferometer. The beam enters the chamber of the sample where it is transmitted through or reflected off the surface of the sample. That beam is depending on the type of analysis which performed.