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2. http://cnx.org/content/m22326/latest/graphics2.jpg
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Fourier Transform Infrared Spectroscopy FT-IR
FT-IR is the prevalent method for infrared spectroscopy. It has replaced the dispersive infrared instruments.
Background
Infrared spectroscopy is used for identifying one material from another. It is based on the fact that every material has a unique combination of atoms; therefore, it is impossible to get the exact same infrared spectrum from two different compounds.
The infrared spectrum of a material consists of absorption peaks corresponding to vibration frequencies present between the bonds of its atoms. The size of a peak indicates the amount of material present.
The need for FT-IR was felt because of the limitations of dispersive-type instruments. The major problem with the older technology was that the scanning process was slow. There was a need for a process that could simultaneously measure all infrared frequencies rather than measuring one frequency at a time.
Interferometer Output Signal
The signal produced by an interferometer is unique in the sense that it contains all infrared frequencies. Measurement of the signal is quick, typically, requiring a second or so. Therefore, the time required for processing a signal is reduced to a few seconds instead of many minutes.
Fourier Transformation
In order to make identifications, the analyst will need a frequency spectrum (plotting of intensity value for each frequency value). This information is not provided by the measured interferogram signal. In order to get the individual frequencies, the measured signal needs to be “decoded.“ The decoding process is where Fourier transformation is utilized. A computer performs the transformation and presents the required spectral information for analysis.
figure 3: Conversion of measured signal into frquency spectrum by application of FFT (fast Fourier transformation)
Source: http://chemwiki.ucdavis.edu/@api/deki/files/9220/data_process.png
Relative Scale
To establish a relative scale against which all measurements can be compared, measurements are taken without any sample in the beam. The comparison with relative scale gives the “percent transmittance.” This technique produces a spectrum which eliminates all instrumental characteristics from the data. Thus, the spectrum seen is produced solely by the sample.
Sensor details
The detectors are housed in the IR-detector below the cooling unit. There are three detectors, each corresponding to three different spectral channels.
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