FT spectrometer

The frequency of modulation is therefore proportional to the velocity of the mirror and inversely proportional to the wavelength of the incident radiation. The frequency is therefore also proportional to the wavenumber of the incident radiation, as we know from the relationship between wavelength and wavenumber. 

If the unique cosine waves can be extracted from the interferogram, the contribution from each wavelength can be obtained. These individual wavelength contributions can be reconstructed to give the spectrum in the frequency domain, that is, the usual spectrum obtained from a dispersive spectrometer. A Fourier transform is used to convert the time-domain spectrum into a frequency-domain spectrum, hence, the term FTIR spectrometer for this type of system. 

Many moving parts result in mechanical slippage. 

Calibration against reference spectra required to measure frequency. 

Stray light within instrument causes spurious readings. 

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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... 

GC-IR is becoming more widely used because FT spectrometers (p. 281) have virtually replaced the older dispersive types and even with computerized enhancements are much cheaper than mass spectrometers. 

A considerable improvement in speed and sensitivity can be achieved with a pulsed Fourier transform (FT) spectrometer. Here the sample is subjected to a series of short duration high intensity RF pulses (1-100 us)... 

Thus while one never sees commercial FT spectrometers for ultraviolet-visible (UV-VIS) absorption measurements (because photomultiplier tubes are much quieter detectors than are microphones), FT-VIS/PA spectrometers have been built that permit speedier acquisition of high S/N photoacoustic spectra (6-7). 

Equipment. All melting points were determined with a Mel-Temp apparatus from Laboratory Devices and are uncorrected. Gas chromatography was carried out on either a Varian Aerograph 700 or Hewlett-Packard 5880 chromatograph. Beckman IR-9 and Perkin Elmer FT-1800 spectrophotometers were used for the determination of mid-IR spectra. All NMR spectra were obtained using an IBM NR-80 FT spectrometer. Elemental analyses were determined for all new monomers and polymers by Micro-Tech Laboratories, Inc. of Skokie,... 

To perform chemical imaging of sample surfaces, FT spectrometers can be coupled with a microscope or macrochamber with an FPA detector. CIS are available for Raman, NIR, and MIR spectroscopy. Figure 15 illustrates an optical arrangement for chemical imaging. 

FT Spectrometers FT spectrometers (Figure 3) differ from scanning spectrometers by the fact that the recorded signal is an interferogram [14] (see Chapter 6.2). They can be coupled to a microscope or macrochamber with an FPA detector. FT chemical imaging systems (CISs) are available for Raman, NIR, and IR spectroscopy. However, they can only be considered as research instruments. For example, most IR imaging systems are FT spectrometers coupled to microscopes. This type of spectrometer allows the acquisition of spectra in reflection, attenuated total reflection (ATR), or transmission mode. 

Time resolution is a strength of modern FTIR spectrometers that makes them powerful tools for investigations of fast processes. For the investigation of slow processes, the same single-beam FT spectrometers have a disadvantage sensitive IR measurements require the acquisition of a reference spectrum, which is usually done... 

One of the main advantages of FT spectrometers is that, since the FID is in digital form, we can repeat the excitation/detection process a number of times and all the resulting scans can be added and the FT performed on the resultant FID. In this way, we can improve the signal-to-noise ratio and can detect nuclei which are not very abundant (e.g. C) or have low sensitivity to NMR (see Section 4.2). These nuclei could not have been detected on the older continuous wave instruments, as the spectrum was the result of a single scan, obtained as one of the frequency or magnetic held were varied while keeping the other constant. 

Historical Development. In this category of FT spectrometer the complete time-evolution of the IR transient is digitized whilst the interferometric mirror is held stationary at each sampling point. The transient can be initiated repeatedly and signal averaged to achieve an adequate SNR. The... 

Carli, B., F. Mencaraglia and A. Bonetti (1984) Submillimeter high resolution FT spectrometer for atmospheric studies. Applied Optics 23 2594-2603. 

The introduction of commercial Fourier transform (FT) spectrometers in the early 1960 s has made it possible, in part, to overcome the limitations associated with dispersive instruments and has helped to broaden the scope of problems amenable to investigation by infrared spectroscopy. The purpose of this review is to compare the performance of FT and dispersive spectrometers and to illustrate areas of application in which FT spectroscopy has proven advantageous for the study of adsorbed species. In view of these objectives only a limited treatment of the theory underlying FT spectroscopy will be presented here. 

To establish a quantitative basis for comparing the performance of dispersive and FT spectrometers, it is necessary to review first the optical principles governing the operation of each type of instrument. The primary purpose here will be to... 

Figure 2. Optical arrangement of a Digilab Model FTS-14 FT spectrometer...

In most instances the resolving power of an FT spectrometer is determined by the maximum retardation of the moveable mirror in the interferometer so that... 

From equations 3 and 5 it follows that Av = 1/Sj,. Consequently, the resolution of an FT spectrometer is fixed by the maximum retardation, but the resolving power increases with increasing wavenumber. 

The second fundamental advantage of an FT spectrometer over a grating spectrometer is often called Jacquinot s advantage and derives from the increased optical throughput of an interferometer compared to a grating monochrometer. 

Figure 8. (A) Theoretical advantage of an FT spectrometer with the same parame-...

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