Interferometers infrared spectroscopy

Figure Bl.2.6. Schematic representation of a Michelson interferometer. From Griffiths P R and de Flaseth J A 1986 Fourier transfonn infrared spectroscopy Chemical Analysis ed P J Hiving and J D Winefordner (New York Wiley). Reprinted by pemiission of Jolm Wiley and Sons Inc.

Smith GD, Palmer RA (2002) Fast time-resolved mid-infrared spectroscopy using an interferometer. In Chalmers JM, Griffiths PR (eds) Handbook of vibrational spectroscopy, vol 1. Wiley, Chichester, p 625... 

In the mid-IR, routine infrared spectroscopy nowadays almost exclusively uses Fourier-transform (FT) spectrometers. This principle is a standard method in modem analytical chemistry45. Although some efforts have been made to design ultra-compact FT-IR spectrometers for use under real-world conditions, standard systems are still too bulky for many applications. A new approach is the use of micro-fabrication techniques. As an example for this technology, a miniature single-pass Fourier transform spectrometer integrated on a 10 x 5 cm optical bench has been demonstrated to be feasible. Based upon a classical Michelson interferometer design, all... 

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. 

S.2.2.2 ICLS Example 2 This example discusses the determination of sodium hydroxide (caustic) concentration in an aqueous sample containing sodium hydroxide and a salt using NIR spearoscopy. An example of this problem in a chemical process occurs in process scrubbers where CO, is converted to Na,CO and H,S is converted to Na,S in the presence of caustic. Although caustic and salts have no distinct bands in the NIR, it has been demonstrated that they perturb the shape of the water bands (Watson and Baughman, 1984 Phelan et al., 1989)-Near-infrared spectroscopy is therefore a viable measurement technique. This method also has ad tages as an analytical technique for process analysis because of the stability of the instrumentation and the ability to use fiber-optic probes to multiplex tlie interferometers and Icx ate them rcm< >tely from the processes. 

One of these devices that is typically used in infrared spectroscopy is the Michelson interferometer (Fig. 6.22). This device works by splitting the beam into two components perpendicular to each other. Then each beam gets reflected by minors in such a way that the reflected beams recombine again at the beam splitter. In one of these beams a path difference is introduced by moving the mirror on which it reflects. This... 

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. 

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. 

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). 

Smith GD, Palmer RA. (2002) Fast Time-Resolved Mid-Infrared Spectroscopy Usin an Interferometer. John Wiley Sons Ltd. 

Fast time-resolved mid-infrared spectroscopy using an interferometer, in Handbook of Vibrational Spectroscopy, Vol. 1 (eds P.R. Griffiths and J.M. Chalmers), John Wiley Sons, Ltd, Chichester, p. 525. 

Infrared spectrometers have been commercially available since the 1940s. At that time the instruments relied on prisms to act as dispersive elements, but by. the. mid 1950s, = diffraction gratings had been introduced into dispersive machines. The most significant advances in infrared spectroscopy, however, have come about with the introduction of Fourier-transform Spectrometers. This type of instrument employs an interferometer and explbits the well established mathematical process of Fourier transformation. FT-IR spectroscopy has dramatically improved the quahty of infrared spectra and has minimised the time required to obtain data. Thus j with the improvements to computers achieved in recent years, infrared spectroscopy has made great strides. 

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. 

Smith, G. D. Palmer, R A. Fast Time-resolved Mid-infrared Spectroscopy Using StepKScan Interferometer. In Handbook of Vibrational Spectroscopy, Chalmers, J. M., Griffiths, P. R., Eds. Wiley Chichester, UK, 2002 Vol. 1, pp 625-640. 

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... 

Infrared spectroscopy underwent tremendous advances after the second world war and after 1950 with improvements in instrumentation and electronics, which put the technique at the center of chemical research and later in the 80 s in the biosciences in general with new sample handling techniques, the attenuated total reflection method (ATR) and of course the interferometer [13]. The Fourier TransformIR spectrophotometry is now widely used in both research and industry as a routine method and as a reliable technique for quality control. 

---