Dispersive spectroscopy, advantage

The transform from the interferogram to the spectrum is carried out by the dedicated minicomputer on the instrument. The theory of Fourier-transform infrared spectroscopy has been treated, and is readily available in the literature.21,22,166 Consequently, the advantages of F.t.-i.r. dispersive spectroscopy will only be outlined in a qualitative sense (i) The Fellgett or multiplex advantage arises from the fact that the F.t.-i.r. spectrometer examines the entire spectrum in the same period of time as that required... 

These three advantages are the basis of many of the improvements of FTIR over dispersive spectroscopy. Table I lists some of the increased capabilities of FTIR. The most significant improvement is the ability to obtain an increased signal at all frequencies in reduced time. This allows the acquisition of good spectra in substantially less time, or the acquisition of low signal spectra (i.e., micro samples) in reasonable amounts of time. 

Besides XRD, other important studies are elemental analysis, either by chemical or physical methods, such as neutron activation analysis (NAA), x-ray fluorescence (XRF), or x-ray energy dispersive spectroscopy (X-EDS), for example (see Sections 7.6.1, 7.3.3, and 7.5.2, respectively) the advantage of these methods is that they are non destructive, as oppossed to wet chemical analysis. Additionally, IR spectroscopy can bring useful complementary information. Sometimes, the chemical composition is required along XRD analysis to fully identify a mineral. Also, thermal analysis (Section 7.6.5) is a useful tool in the qualitative and, sometimes, quantitative determination of clay minerals. 

The advantages and disadvantages of FTIR over dispersive spectroscopy have been discussed in general [44, 45] and in terms of the RAIRS experiment [4, 41, 40-48]. FTIR instruments hold three formal advantages over dispersive spectrometers. These are briefly outlined below. 

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. 

When the spectral characteristics of the source itself are of primary interest, dispersive or ftir spectrometers are readily adapted to emission spectroscopy. Commercial instmments usually have a port that can accept an input beam without disturbing the usual source optics. Infrared emission spectroscopy at ambient or only moderately elevated temperatures has the advantage that no sample preparation is necessary. It is particularly appHcable to opaque and highly scattering samples, anodized and painted surfaces, polymer films, and atmospheric species (135). The interferometric... 

The example is typical for many applications of Mossbauer spectroscopy in catalysis a catalyst undergoes a certain treatment, then its Mossbauer spectrum is measured in situ at room temperature. Flowever, if the catalyst contains highly dispersed particles, the measurement of spectra at cryogenic temperatures becomes advantageous as the recoil-free fraction of surface atoms increases substantially at temperatures below 300 K. Secondly, spectra of small particles that behave superparamagne-... 

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. 

Experimental limitations initially limited the types of molecular systems that could be studied by TRIR spectroscopy. The main obstacles were the lack of readily tunable intense IR sources and sensitive fast IR detectors. Early TRIR work focused on gas phase studies because long pathlengths and/or multipass cells could be used without interference from solvent IR bands. Pimentel and co-workers first developed a rapid scan dispersive IR spectrometer (using a carbon arc broadband IR source) with time and spectral resolution on the order of 10 ps and 1 cm , respectively, and reported the gas phase IR spectra of a number of fundamental organic intermediates (e.g., CH3, CD3, and Cp2). Subsequent gas phase approaches with improved time and spectral resolution took advantage of pulsed IR sources. 

The term exp(-2k2c ) in (6-9) accounts for the disorder of the solid. Static disorder arises if atoms of the same coordination shell have slightly different distances to the central atom. Amorphous solids, for instance, possess large static disorder. Dynamic disorder, on the other hand, is caused by lattice vibrations of the atoms, as explained in Appendix 1. Dynamic disorder becomes much less important at lower temperatures, and it is therefore an important advantage to measure spectra at cryogenic temperatures, especially if a sample consists of highly dispersed particles. The same argument holds in X-ray and electron diffraction, as well as in Mossbauer spectroscopy. 

The principal reasons for choosing Fourier transform infrared spectroscopy are first, that these instruments record all wavelengths simultaneously and thus operate with maximum efficiency and, second, that Fourier transform infrared spectroscopy spectrometers have a more convenient optical geometry than do dispersive infrared instruments. These two facts lead to the following advantages. 

CIR-FTIR spectroscopy provides a direct technique for studying in situ hydrous metal oxide surfaces and molecules adsorbed on these surfaces (37). By itself, FTIR spectrometry is a well established technique which offers numerous advantages over dispersive (grating) IR spectrometry (1) improved accuracy in frequency measurements through the use of a HeNe laser (2) simultaneous frequency viewing (3) rapid, repetitive scanning which allows many spectra to be collected in a small time interval (4) miriimal thermal effects from IR beam and (5) no detection of sample IR emissions (38). 

All of the usual sampling techniques used in infrared spectroscopy can be used with FT-IR instrumentation. The optics of the sampling chamber of commercial FT-IR instruments are the same as the traditional dispersive instruments so the accessories can be used without modification for the most part. To make full use of the larger aperature of the FT-IR instrument, some accessories should be modified to accomodate the larger beam. The instrumental advantages of FT-IR allow one to use a number of sampling techniques which are not effective using dispersive instrumentation. Transmission, diffuse reflectance and internal reflectance techniques are most often used in the study of epoxy resins. 

Advantages of Fourier transform infrared spectrometers are so great that it is nearly impossible to purchase a dispersive infrared spectrometer. Fourier transform visible and ultraviolet spectrometers are not commercially available, because of the requirement to sample the interferometer at intervals of S = l/(2Av). For visible spectroscopy, Av could be 25 000 cm 1 (corresponding to 400 nm), giving S = 0.2 im and a mirror movement of 0.1 xm between data points. Such fine control over significant ranges of mirror motion is not feasible. 

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