Infrared spectroscopy interferogram

A Fourier transform infrared spectroscopy spectrometer consists of an infrared source, an interference modulator (usually a scanning Michelson interferometer), a sample chamber and an infrared detector. Interference signals measured at the detector are usually amplified and then digitized. A digital computer initially records and then processes the interferogram and also allows the spectral data that results to be manipulated. Permanent records of spectral data are created using a plotter or other peripheral device. 

It is found that multiplication of the Fourier transform of the data by a carefully chosen window function is very effective in removing the artifacts around peaked functions. This process is called apodization. Apodization with the triangular window function is often applied to Fourier transform spectroscopy interferograms to remove the ringing around the infrared... 

The transform from the interferogram to the spectrum is carried out by the dedicated minicomputer on the instrument. The theory of Fourier-transform infrared spectroscopy has been treated, and is readily available in the literature.21,22,166 Consequently, the advantages of F.t.-i.r. dispersive spectroscopy will only be outlined in a qualitative sense (i) The Fellgett or multiplex advantage arises from the fact that the F.t.-i.r. spectrometer examines the entire spectrum in the same period of time as that required... 

During the 1960s further improvements made infrared spectroscopy a very useful tool used worldwide in the analytical routine laboratory as well as in many fields of science. Grating spectrometers replaced the prism instruments due to their larger optical conductance (which is explained in Sec. 3 of this book). The even larger optical conductance of interferometers could be employed after computers became available in the laboratory and algorithms which made Fourier transformation of interferograms into spectra a routine. The computers which became a necessary component of the spectrometers made new powerful methods of evaluation possible, such as spectral subtraction and library search. 

In infrared spectroscopy, the detector noise is usually much higher than the noise from other sources. In this case, multiplex recording provides an additional advantage. An interferogram contains the detector noise only once, independently of the number of spectral channels. Fourier transformation produces a spectrum where the SNR) of each spectral element is related to that of one line, SNR by ... 

Figure 9.17 Interferograms (a) sum of (b) and (c) (b) interferogram of light with wavelength 3X and (c) interferogram of light with wavelength X. (Reproduced with permission from B.C. Smith, Fundamentals of Fourier Transform Infrared Spectroscopy, CRC Press, Boca Raton. 1996 CRC Press LLC.)...

A method of horizontal attenuated total reflectance (HATR) by Fourier transform infrared spectroscopy (FTIR) is considered more useful for infrared spectroscopic analysis of leather samples. By this method, the sample is simply put on the flat plate of ZnSe crystal or KRS-5 crystal of the HATR accessory. The Fourier transformation of the interferogram is converted by the computer into a plot of absorption against wave number that resembles the usual IR spectrum (William and Fleming, 1998). 

Fourier transform infrared spectroscopy (FTIR) - A technique for obtaining an infrared spectrum by use of an interferometer in which the path length of one of the beams is varied. A Fourier transformation of the resulting interferogram yields the actual spectrum. The technique is also used for NMR and other types of spectroscopy. 

FTIR takes a completely different approach. The spectral data are acquired as an Interferogram (Figure 1) which must be transformed Into a plot of Intensity versus wavenumber or wavelength through the application of Fourier transform equations. Thus, the computer Is an Integral part of the system without which little useful Information could be obtained. FTIR has the following advantages over computerized dispersive Infrared spectroscopy ... 

Fourier transform infrared spectroscopy, a versatile and widely used analytical technique, relies on the creation of interference in a beam of light. A source light beam is split into two parts and a continually varying phase difference is introduced into one of the two resultant beams. The two beams are recombined and the interference signal is measured and recorded as an interferogram. A Fourier transform of the interferogram provides the spectrum of the detected light. 

---