FT-IR interferometers

Of course, even in low temperature solutions, unstable compounds may not be very long-lived. Modern fast-scanning FT-IR interferometers can produce high signal-to-noise spectra in a single scan. This means that metal carbonyl compounds with half-lives as short as 2 seconds can be easily detected using an unmodified interferometer (28,29). With improved interferometers, we anticipate that such studies will soon be extended to compounds with lifetimes —100 mseconds. However, detection of shorter lived species, such as reaction intermediates, requires much faster and more sensitive techniques. 

Figure 7.57. In situ ATR spectra of P. putida adsorbed at Ge IRE (50 x 20 x 3 mm, with 19 active reflections inside) after lOOh of adsorption (1) biofilm without toluene (2) 5 ppm toluene (3) 15 ppm toluene. Arrows increase of polysaccharide peaks at 5 ppm toluene and of carboxylic group peak at 15 ppm toluene. Spectra were obtained with multichannel ATR/FTIR spectrometer constructed on basis of RFX-30 FT IR interferometer (Laser Precision Analytical). Reprinted, by permission, from J. Schmitt, D. E. Nivens, D. C. White, and H.-C. Flemming, Water Sci. Tech. 32, 149 (1995), p. 154, Fig. 5. Copyright 1996 International Association on Water Quality (lAWQ).

The optical systems of modern FT-IR interferometers are interfaced to modern computers that have high-speed computing and data-processing capabilities, and which contain a large amount of memory and an operating system with a convenient Windows function, as well as a wide variety of application-specific software. 

Because the lower wavenumber limit covered by a conventional FT-IR spectrometer operating in the mid-infrared region is typically 400 cm it is necessary, in order to measure far-infrared spectra, to exchange a few of the main optical components of an FT-IR interferometer these are described here ... 

A multiplex/multichannel microimaging FTIR system has been developed. The strategy is to combine an IR focal plane array detector with a step-scan interferometer. A InSb focal array detector was used with a 128 x 128 pixel distribution. The detector was interfaced with a with a commercially available step-scan FT-IR interferometer. 

Ishida FI, Ishino Y, Bui]s FI, Tripp C and Dignam M J 1987 Polarization-modulation FT-IR refleotion speotrosoopy using a polarizing Miohelson interferometer App/. Spectrosc. 1288-94... 

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... 

FT-IR utilizes the Michelson interferometer rather than the grating or prism of the dispersive system. The Michelson interferometer has two mutually perpendicular arms. One arm of the interferometer contains a stationary, plane mirror the other arm contains a moveable mirror. Bisecting the two arms is a beamsplitter which splits the source beam into two equal beams. These two light beams travel their respective paths in the arms of the interferometer and are reflected back to the beam splitter and on to the detector. The two reunited beams will interfere constructively or destructively, depending on their path differences and the wavelengths of the light. When the path lengths in the two arms are the same, all of the frequencies... 

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. 

Fourier transform (FT) IR spectroscopy is one of several nondispersive optical spectroscopies based on interferometry. A two-beam interferometer first proposed by Michelson is the basis of most modern FT-IR spectrometers, as exemplified by the schematic of the Bruker Equinox 55 spectrometer (Bruker Optik, Ettlingen, Germany) in Fig. 2. Simply described, the interferometer comprises a beam splitter and two mirrors. A collimated beam of IR energy is split at the beam splitter into equal halves. Half of the energy travels through the beam splitter to one of the mirrors, which is positioned at a fixed distance away from the beam splitter. The reflected beam travels perpendicular to the incident beam to a moving mirror. IR radiation reflects off the fixed and moving mirrors and recombines at the beam splitter. The recombined IR beam projects from the interferometer towards the detector on an optical path perpendicular to the source beam. 

FT-IR spectrometers have interferometers with scanning velocities enabling the collection of tens of spectra per second at a spectral resolution of 8 cnr1 or less. With faster data collection capabilities, FT-IR spectroscopy can be used to monitor and observe dynamic gas-phase processes. To observe such a process, interferograms are sequentially collected and stored in the memory of the system. The interferograms are then processed at the end of the data acquisition. The result of this operation is a three-dimensional data cube where each vertical slice of the cube is the spectrum for a time slice in the experiment equal to the interferogram acquisition time. 

Radiation emitted from the QCL was coupled into the HWG and the radiation emitted at the exit aperture was passed through the interferometer of a FT-IR spectrometer and focused onto a MCT detector as shown in Fig. 21. The presence of ethylene gas inside the waveguide gas cell was detected by... 

Several major points should be mentioned. In FT-IR interferograms are recorded, and the infrared spectra computed from the interferograms, via a fast Fourier transform algorithm introduced relatively recently (4). It is tire replacement of the monochromator of earlier spectrometers by an interferometer which is primarily responsible for the improved performance of FT instruments. 

A Fourier transform infrared spectrometer (FT-IR) uses an interferometer,... 

Block diagram of an interferometer in an FT-IR spectrometer. The light beams reflected from the fixed and moving mirrors are combined to form an interferogram, which passes through the sample to enter the detector. 

Although standard IR spectrometers are used for studying the amide bands, FTIR spectrometers are more accurate and reliable. FT-IR spectrophotometers are based upon the Michelson interferometer. A typical instrument (Fig. 7.1) comprises an optical bench housing the interferometer, sample, infrared source and detector, coupled to a computer, which controls the spectral scanning, analysis and data processing (for review see Griffiths, 1980). 

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). 

In the late 1980s and early 1990s, interferometer-based instruments were introduced by companies already producing IR equipment Nicolet, Bomem, Perkin-Elmer, and others. Often, these FT-NIR instruments were merely adapted FT-IRs that were already in existence and were not suited for pharmaceutical samples pure raw materials, blends and granulations, tablets and capsules, and larger plastic containers. 

A schematic drawing of an FT-Raman spectrometer is shown in Figure 9.3. The wavelength analyzer is a Michelson interferometer adapted from an FT-IR spectrometer. FTIR was developed to a high level of refinement before FT-Raman was introduced in 1986, and many components were transferred from FTIR to FT-Raman with minor modification. Many vendors offer FT-Raman attachments to otherwise conventional FTIR spectrometers, so that both techniques share the same interferometer. Dedicated FT-Raman spectrometers are also available but still share many components with FTIR systems. 

The popularity of interferometers in mid-range infrared has carried over to NIR. The FT-NIR instruments are becoming quite common in the field. Speed and high resolution spectra are the strengths of FT instrumentation. Since most spec-troscopists are familiar with FT-IR instrumentation, it is logical that they would lean toward an instrument that seems familiar. 

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