Fourier spectroscopy, advantages

Fourier spectroscopy, 23 137 Fourier transformation, essential equations for, 14 226-227 Fourier transform (FT) infrared (ftir) analysis, 19 563-564, 813. See also Microscopy-ftir technique dichroism, in silicone network characterization, 22 569 instruments, 23 138 advantages of, 14 228 resolution for, 14 227 microscopy, 14 232... 

Recently, first experimental results were published which have been obtained with a Michelson interferometer especially designed by J. Cast and L. Genzel > for reflection studies on small solid samples by means of asymmetric or amplitude Fourier spectroscopy. The main advantage of the optical layout is that sample and reference mirror are located at focal points which do not take part in the motion to produce the path diflerence in the interferometer. Therefore, these foci can be placed inside a cryostat that allows the sample to be cooled. Another recent development in this field is concerned with the difficulty that, when studying the reflectivity of solids, the determination of the phase depends strongly on the exact positioning of the sample mirror instead of the background mirror. This is... 

NMR equipment has also been used in kinetic examinations. The recent development of pulse Fourier spectroscopy allows the application of NMR even to solids and in flow systems. Thus, C-NMR spectroscopy can offer the advantage of monitoring chemical pathways. Even quantitative results can be achieved to determine rates of reaction [112]. 

At last, it should be mentioned that, as always applies to Fourier methods , the major advantage of FTNMR is the multiplex advantage (see also (Chapter 1). Ernst and Anderson have shown that the gain in the signal-to-noise ratio between FTNMR and conventional NMR is proportional to the square root of the number of spectral elementsS . This result is the same as that obtained for infrared Fourier spectroscopy. In this context, it should be noted that a complete analogon to infrared Fourier spectroscopy is the nuclear magnetic Fourier-transfoim resonance with an incoherent rf-field (stochastic resonance) . Of course, Fourier methods depend on electronic computers to perform the Fourier transform of the measured data and were widely used when sufficiently cheap computers became available. The use of the compute- and of the mathematical treatment of the experimental... 

The main advantage which allows Fourier spectroscopy to flourish was the fantastic progress in computational techniques, such that the majority of problems of contemporary FT-spectroscopy can be solved on the PC-type computers. 

Fourier spectroscopists are fond of reporting other advantages which their spectroscopic method can produce. In general, these advantages are not unique to Fourier spectroscopy, and spectroscopists using dispersive instruments can enjoy essentially the same advantages. 

The main advantage of Fourier spectroscopy is the fact, that all spectral intervals dftj with the intensity 7 (m) dm are measured simultaneously in contrast to classical spectroscopy with a monochromator where the different spectral intervals are measured subsequently. If a spectrum consisting of N spectral intervals Am (where Am is the spectral interval which can be resolved by the wavelength-selecting instrument) is measured in a time T with a monochromator, Fourier spectroscopy can obtain this spectrum in the shorter time T/ VA with the same signal-to-noise ratio. 

Compared with conventional spectroscopy using dispersing spectrometers, Fourier spectroscopy has some definite advantages [4.17,18] ... 

M = 1000, for example, Fourier spectroscopy gives a thirty-three-times better signal-to-noise ratio for the same total observation time, or else gives the same S/N ratio as a conventional spectrometer in a 1000 times shorter sampling time (Fellgett s advantage). 

Because of these advantages Fourier spectroscopy has rapidly developed and has become a major technique in the infrared and recently also in the visible region. The most serious disadvantage is the high price of the instrument. For a more detailed treatment the reader is referred to the literature [4.16-21]. 

As in all Fourier transform methods in spectroscopy, the FTIR spectrometer benefits greatly from the multiplex, or Fellgett, advantage of detecting a broad band of radiation (a wide wavenumber range) all the time. By comparison, a spectrometer that disperses the radiation with a prism or diffraction grating detects, at any instant, only that narrow band of radiation that the orientation of the prism or grating allows to fall on the detector, as in the type of infrared spectrometer described in Section 3.6. 

The basic methods of the identification and study of matrix-isolated intermediates are infrared (IR), ultraviolet-visible (UV-vis), Raman and electron spin resonance (esr) spectroscopy. The most widely used is IR spectroscopy, which has some significant advantages. One of them is its high information content, and the other lies in the absence of overlapping bands in matrix IR spectra because the peaks are very narrow (about 1 cm ), due to the low temperature and the absence of rotation and interaction between molecules in the matrix. This fact allows the identification of practically all the compounds present, even in multicomponent reaetion mixtures, and the determination of vibrational frequencies of molecules with high accuracy (up to 0.01 cm when Fourier transform infrared spectrometers are used). 

From Table 2 it is observed that the dispersive NIR ensembles (NIR and NIR R) result in the best cross validated models. The potential advantages of Fourier transform spectroscopy [5] are in practice outnumbered by a more reproducible setup and saimpling procedures. 

It is only since 1980 that in situ spectroscopic techniques have been developed to obtain identification of the adsorbed intermediates and hence of reliable reaction mechanisms. These new infrared spectroscopic in situ techniques, such as electrochemically modulated infrared reflectance spectroscopy (EMIRS), which uses a dispersive spectrometer, Fourier transform infrared reflectance spectroscopy, or a subtractively normalized interfacial Fourier transform infrared reflectance spectroscopy (SNIFTIRS), have provided definitive proof for the presence of strongly adsorbed species (mainly adsorbed carbon monoxide) acting as catalytic poisons. " " Even though this chapter is not devoted to the description of in situ infrared techniques, it is useful to briefly note the advantages and limitations of such spectroscopic methods. 

The photoacoustic effect was first discovered by Alexander Graham Bell in the early 1880s [18], but it was not applied to Fourier transform infrared (FTIR) spectroscopy until a century later [19,20], Significant advantages of FTIR photoacoustic spectroscopy (PAS) include the following (1) Spectra may be... 

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