basic principle of ftir

3 ANSWERS

1. amomipais82
   

   Hi,
   The application of traditional infrared spectroscopy to low concentration measurements, such as ambient air measurements, is limited by several factors. First is the significant presence of water vapour, CO 2 and methane, which strongly absorb in many regions of the infrared (IR) spectrum. Consequently, the spectral regions that can easily be used to search for pollutants are limited to 760-1300cm -1 , 2000-2230 cm -1 , and 2390-3000 cm -1 . Another problem is that the sensitivity is not enough to detect very small concentrations in the sub-ppm level. Finally, spectral analysis was difficult since subtraction of background spectra had to be carried out manually.
   
     
   
   The development of Fourier Transform InfraRed spectroscopy (FTIR) in the early 1970s provided a quantum leap in infrared analytical capabilities for monitoring trace pollutants in ambient air. This technique offered a number of advantages over conventional infrared systems, including sensitivity, speed and improved data processing.
   
     
   
   The basic components of an FTIR are shown schematically in Figure 1 . The infrared source emits a broad band of different wavelength of infrared radiation. The IR source used in the Temet GASMET FTIR CR-series is a SiC ceramic at a temperature of 1550 K. The IR radiation goes through an interferometer that modulates the infrared radiation. The interferometer performs an optical inverse Fourier transform on the entering IR radiation. The modulated IR beam passes through the gas sample where it is absorbed to various extents at different wavelengths by the various molecules present. Finally the intensity of the IR beam is detected by a detector, which is a liquid-nitrogen cooled MCT (Mercury-Cadmium-Telluride) detector in the case of the Temet GASMET FTIR CR-series. The detected signal is digitised and Fourier transformed by the computer to get the IR spectrum of the sample gas.
   The unique part of an FTIR spectrometer is the interferometer. A Michelson type plane mirror interferometer is displayed in Figure 2 . Infrared radiation from the source is collected and collimated (made parallel) before it strikes the beamsplitter. The beamsplitter ideally transmits one half of the radiation, and reflects the other half. Both transmitted and reflected beams strike mirrors, which reflect the two beams back to the beamsplitter. Thus, one half of the infrared radiation that finally goes to the sample gas has first been reflected from the beamsplitter to the moving mirror, and then back to the beamsplitter. The other half of the infrared radiation going to the sample has first gone through the beamsplitter and then reflected from the fixed mirror back to the beamsplitter. When these two optical paths are reunited, interference occurs at the beamsplitter because of the optical path difference caused by the scanning of the moving mirror.
   The optical path length difference between the two optical paths of a Michelson interferometer is two times the displacement of the moving mirror. The interference signal measured by the detector as a function of the optical path length difference is called the interferogram.

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2. Guest3916
   

    

   
   

   what is ftir

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3. Guest4346
   

    

   
   

   what is ftir

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