Spectral Fourier transform spectrometer

The proton noise-decoupled 13c-nmr spectra were obtained on a Bruker WH-90 Fourier transform spectrometer operating at 22.63 MHz. The other spectrometer systems used were a Bruker Model HFX-90 and a Varian XL-100. Tetramethylsilane (TMS) was used as internal reference, and all chemical shifts are reported downfield from TMS. Field-frequency stabilization was maintained by deuterium lock on external or internal perdeuterated nitromethane. Quantitative spectral intensities were obtained by gated decoupling and a pulse delay of 10 seconds. Accumulation of 1000 pulses with phase alternating pulse sequence was generally used. For "relative" spectral intensities no pulse delay was used, and accumulation of 200 pulses was found to give adequate signal-to-noise ratios for quantitative data collection. 

Infrared analyses are conducted on dispersive (scanning) and Fourier transform spectrometers. Non-dispersive industrial infrared analysers are also available. These are used to conduct specialised analyses on predetermined compounds (e.g. gases) and also for process control allowing continuous analysis on production lines. The use of Fourier transform has significantly enhanced the possibilities of conventional infrared by allowing spectral treatment and analysis of microsamples (infrared microanalysis). Although the near infrared does not contain any specific absorption that yields structural information on the compound studied, it is an important method for quantitative applications. One of the key factors in its present use is the sensitivity of the detectors. Use of the far infrared is still confined to the research laboratory. 

Commercial Fourier transform spectrometers operating at moderate resolution (1cm-1) require fractions of seconds to complete a scan of the interferometric mirror (scans may only take tens of milliseconds if only low spectral resolution is required). A new strategy must now be used to study the... 

No discussion has been devoted to the recent use of Fourier transform spectrometers rather than dispersion instruments. The ease with which the spectral data can be manipulated and background subtracted make the FT methods particularly useful for studies of surface species, particularly during catalytic reaction. Recently there has been a surge of interest in the coupling of computer subtraction techniques to conventional grating instruments. For many IR surface studies, where only limited frequency range is required, this... 

Although there are a variety of wavelength selection methods available, the vast majority of Raman instruments utilize either dispersive or Fourier transform spectrometers. These are shown schematically in Fig. 1.6. The high throughput and spectral resolution obtainable from these instruments make them obvious choices for Raman spectroscopy however, each has specific strengths and drawbacks which make them more suitable in specific applications. 

The spectral accumulation approach is also used in Fourier Transform Spectrometers. In this case spectral data are collected far more rapidly and spectra may be obtained in a reasonable period of time fium about 20 g of sample. However, these instruments are much more expensive than continuous wave spectrometers although they may also be used to prepare C spectra. 

Fig. 8 The apodized H, emission spectrum from the southern auroral zone of Jupiter recorded by Maillard et a/. using the Canada-France-Hawaii telescope Fourier-transform spectrometer. The spectral lines match well with the laboratory spectrum at T = 1000 K given in Fig. 3. Reprinted with permission from The Astrophysical Journal (1990).

The infrared spectral range is usually served by Fourier transform spectrometers, which contain an interferometer for wavelength dispersion. Today, the spectra are immediately calculated by using Fourier... 

These instruments (Figure 10.7) can be divided into two categories the Fourier transform spectrometers, which undertake a simultaneous analysis of the whole spectral region from interferometric measurements, and numerous specialized analysers for the second category. Dispersive-type spectrometers are also used for the near-IR. 

Details of the sample preparation are given in Reference 16. The 13C NMR spectra were obtained on a Bruker HX-90E Fourier transform spectrometer at 22.6 MHz and approximately 30°C. Samples usually consisted of 0.5 g of polymer in 2 ml of CDC13 in 10 mm o.d. NMR tubes. Chemical shifts were referenced to internal (CH3)4Si. The spectral conditions were 90° radio-frequency pulses, 15 psec ... 

The use of RAIRS has recently been extended from its regular mid-IR characterization of adsorbates on metals into other exciting and promising directions. For one, changes in optics and detectors have allowed for an extension of the spectral range towards the far-IR region in order to probe substrate-adsorbate vibrations [35]. The use of intense synchrotron sources in particular looks quite promising for the detection of such weak modes [36]. Thanks to the speed with which Fourier-transform spectrometers can acquire complete IR spectra, kinetic studies of surface reactions can be carried out as well. To date this has only been done in a few cases, usually for reactions that take seconds or more to occur [37], but the advent of step scanners promises... 

In our research, we used a BOMEM DA3.002 Fourier-transform spectrometer equipped with a controlled-temperature ( 0.1 K in the range between 2 and 300 K) helium-vapor cryostat. High-resolution (up to 0.005 cm ) spectra were registered in the spectral range between 2000 and 11000 cm. ... 

Before the advent of Fourier transform spectrometers, wide-line NMR was done by sweeping the magnetic field and observing the dispersion signal, or by pulsing the radiofrequency and observing the free Induction decay without transformation. The very broad spectral widths have caused problems with baselines and faithful representations of the entire llneshapes. Various techniques, such as the quadrupole echo (lA), progressive phase alternation of the excitation pulse and detector, short spectrometer dead times, and post-acquisition spectral correction (15) have circumvented most of these. 

The main advantages of the Fourier transform spectrometer over conventional dispersive spectrometers are (i) higher energy throughput, because no slits are required, (ii) higher optical resolution, and (iii) ability for simultaneous monitoring of all spectral information for an extended period. 

Double Fourier Spatio-Spectral Interferometry is the application of a Fourier-transform spectrometer (FTS) to aperture synthesis interferometry. This technique was proposed for the near IR regime by Itoh and Ohtsuka (1986) with a single-pupil interferometry approach, and by Mariotti and Ridgway (1988) with multi-pupil interferometry for high spatial resolution. 

Ohta et al. (2006) theoretically proposed to apply a Martin-Puplett-type Fourier-transform spectrometer to the aperture synthesis system in millimeter and submil-limeter waves. They succeeded in proving that this system is capable of performing broadband imaging observations (Ohta et al. 2007). Also a laboratory prototype spectral-spatial interferometer (Chap. 3) has been constructed to demonstrate the feasibility of the double-Fourier technique at far infrared (FIR) wavelengths (0.15-1 THz) by Grainger et al. (2012). 

The optimum sensitivity of the Fourier transform spectrometer obtained by calculating the signal-to-noise ratio of a pulse Fourier transform spectrometer relative to a conventional absorption spectrometer has also been given. Suppose that a spectral range F has to be investigated in a total time, T. Both experiments will use a superheterodyne detection system with a balanced mixer. The noise is assumed to be white with a power density Pg per spectral unit. The S/N ratio is defined as the ratio of the peak signal amplitude to the rms noise amplitude. 

The Fellgett or multiplex advantage deals with the fact that a Fourier transform spectrometer records data from the entire spectral region throughout the experiment. This is quite different to the case with a dispersive spectrometer, as the grating or prism instrument only measures a narrow bandwidth at any time. The measurement bandwidth of the dispersive spectrometer is regulated by the instrument s exit slit. This difference has important effects on the acquisition of data. 

Fellgett l recognized that the detector in a Fourier transform spectrometer instrument observes all of the spectral elements in a spectrum for the entire measurement time. This differs from the operation of a dispersive instrument where the detector observes each... 

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