Fourier-transform infrared spectroscopy interferometer

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. 

Fourier transform infrared spectroscopy (FT—IR) has been developing into a viable analytical technique (56). The use of an interferometer requires a computer which increases the cost of the system. The ability of IR to differentiate geometrical isomers is still an advantage of the system, and computer techniques such as signal averaging and background subtraction, improve capabilities for certain analyses. 

Between the source and the detector is put either monochromators used in dispersive instruments or interferometers used in Fourier transform infrared (FT-IR) instruments. In a dispersive instrument the intensity at each wavenumber is measured one by one in sequence and only a small spectral range falls on the detector at any one time. In a FT-IR instrument the intensities of all the wavenumbers are measured simultaneously by the detector. Fourier transform infrared spectroscopy offers some advantages compared to dispersive instruments, namely (i) higher signal-to-noise ratios for spectra obtained under conditions of equal measurement time, and (ii) higher accuracy in frequency for spectra recorded over a wide range of frequencies. Therefore we will give below a brief picture of the principle of FT-IR spectroscopy, based on a Michelson interferometer (Fig. 2). 

Figure Bl.2.6. Schematic representation of a Michelson interferometer. From Griffiths P R and de Haseth J A 1986 Fourier transform infrared spectroscopy Chemical Analysis ed P J Elving and J D Winefordner (New York Wiley). Reprinted by permission of John Wiley and Sons Inc.

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. 

Fourier transform infrared spectroscopy (FTIR) had its origins in the interferometer developed by Michelson in 1880 and experiments by astrophysicists some seventy years later. A commercial FTIR instrument required development of the laser (1960, by Theodore H. Maiman [1927- ], Hughes Aircraft), refined optics, and computer hardware and software. The Fourier transform takes data collected in time domain and converts them to frequency domain, the normal infrared (IR) spectrum. FTIR provided vasdy improved signal-to-noise ratios allowing routine analyses of microgram samples. 

In Fourier transform infrared spectroscopy the instrument consists of a Michelson interferometer, cf. Fig. 16.1. The light entering into the interferometer is split by a semi-silvered mirror into two beams. Each beam goes a different path, finally the beams being recombined again. The recombined beam is directed to a detector. In the case of monochromatic and coherent light, the intensity of the recombined beam is dependent entirely on the difference of the path length, because of interference. 

The modern spectrometers [7] came with the development of the high p>erformance Fourier Transform Infrared Spectroscopy (FT-IR) with the application of a Michelson Interferometer [10]. Both IR spectrometers classical and modern give the same information the main difference is the use of Michelson interferometer, which allows all the frequencies to reach... 

Fourier Transform Infrared Spectroscopy (FTIR) is based on the Michelson interferometer (Figure 17.87). A monochromatic source is passed through a beam splitter to give... 

The Michelson interferometer does not measure the infrared spectrum directly. Rather, an interfero-gram is measured, and converted to a single-beam spectrum via Fourier transformation. Because of the critical role of this transformation, the method is generally referred to as Fourier-transform infrared spectroscopy, or FT-IR . Instruments using this... 

Advantages of Fourier transform infrared spectrometers are so great that it is nearly impossible to purchase a dispersive infrared spectrometer. Fourier transform visible and ultraviolet spectrometers are not commercially available, because of the requirement to sample the interferometer at intervals of S = l/(2Av). For visible spectroscopy, Av could be 25 000 cm 1 (corresponding to 400 nm), giving S = 0.2 im and a mirror movement of 0.1 xm between data points. Such fine control over significant ranges of mirror motion is not feasible. 

Fourier transform infrared (FTIR) spectroscopy has been extensively developed over the past decade and provides a number of advantages. The main part of FTIR spectrophotometer is the Michelson interferometer. Radiation containing all IR wavelengths (e.g., 4000-400 cm 1) is emitted by source of infrared radiation (Globar) and is split into two beams. One beam is of fixed length, and the other is of variable length (movable mirror). 

The use of a Fourier transform infrared (FTIR) spectrometer has provided a completely different methodological approach to this spectroscopy [160,161]. The central component of any FTIR spectrometer is an interferometer. No dispersive elements are used. Based on the construction, various advantages in comparison... 

---