Infrared spectroscopy dispersive spectrometer

Yuzawa T, Kate C, George M W and Hamaguchi H O 1994 Nanosecond time-resolved infrared spectroscopy with a dispersive scanning spectrometer Appl. Spectrosc. 48 684-90... 

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. 

In the early work of Bewick and Robinson (1975), a simple monochromator system was used. This is called a dispersive spectrometer. In the experiment the electrode potential was modulated between two potentials, one where the adsorbed species was present and the other where it was absent. Because of the thin electrolyte layer, the modulation frequency is limited to a few hertz. This technique is referred to as electrochemically modulated infrared reflectance spectroscopy (EMIRS). The main problem with this technique is that data acquisition time is long. So it is possible for changes to occur on the electrode surface. 

In-situ Fourier transform infrared spectroscopy. The final technique in this section concerns the FTIR approach which is based quite simply on the far greater throughput and speed of an FTIR spectrometer compared to a dispersive instrument. In situ FTIR has several acronyms depending on the exact method used. In general, as in the EMIRS technique, the FTIR-... 

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. 

During the last decade infrared spectroscopy has developed into a very important tool for studies of adsorbed molecules. Very rapid FTIR spectrometers, which are easily adapted to a reflection experiment, are commercially available. Recently a dispersive, modulation spectrometer dedicated for surface studies has also come on the market. In the same way as happened... 

InfraRed Spectroscopy (IR). Infrared speetroscopy is an efifeetive method for eharaeterization of polymers as to ehemieal structure. IR speetra of the sample examined are obtained by two basie types of IR spectrometers dispersive or Fourier transform (FTIR) ones. Infrared spectra are usually presented as a dependence of absorption (in pereent transmission) on wave length or wave... 

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... 

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. 

There are several major areas of interfacial phenomena to which infrared spectroscopy has been applied that are not treated extensively in this volume. Most of these areas have established bodies of literature of their own. In many of these areas, the replacement of dispersive spectrometers by FT instruments has resulted in continued improvement in sensitivity, and in the interpretation of phenomena at the molecular level. Among these areas are the characterization of polymer surfaces with ATR (127-129) and diffuse reflectance (130) sampling techniques transmission IR studies of the surfaces of powdered samples with adsorbed gases (131-136) alumina(137.138). silica (139). and catalyst (140) surfaces diffuse reflectance studies of organo- modified mineral and glass fiber surfaces (141-143) metal overlayer enhanced ATR (144) and spectroelectrochemistry (145-149). 

Fourier transform infrared spectroscopy (FTIR) has provided support to a number of areas in Diamond Shamrock s pesticide program. Commercially available FTIR spectrometers offer a number of advantages over dispersive instruments. Although some of the advantages are related to the ability to perform computerized data manipulations, the basic design of the FTIR system does provide superior capabilities in infrared spectroscopy (1). ... 

There is a real chance of a breakthrough of Raman spectroscopy in routine analytics. Excitation of Raman spectra by near-infrared radiation and recording with interferometers, followed by the Fourier transformation of the interferogram into a spectrum -the so-called NIR-FT-Raman technique - has made it possible to obtain Raman spectra of most samples uninhibited by fluorescence. In addition, the introduction of dispersive spectrometers with multi-channel detectors and the development of several varieties of Raman spectroscopy has made it possible to combine infrared and Raman spectroscopy whenever this appears to be advantageous. 

Thus, the detection limits of infrared and Raman spectroscopy are of the same order of magnitude. In the case of dispersive spectrometers, this limit has been estimated to be about 1 pg of analyte in the sample (Schrader, 1980). It is several orders of magnitude higher than that of UV or fluorescence spectroscopy (less than 1 ag), because in this case both the number of photons equivalent to the NEP is small and the cross section is large, being nearly of the order of the gas kinetic cross section. 

Dispersive Infrared Spectroscopy The dispersive IR spectrometer generally incorporates an IR broadband source, sample cell, a diffraction grating and one or more IR detectors. Dispersive IR instruments may provide simultaneous or sequential measurements. Respectively, the instrument may have a fixed grating and many detectors, or a movable grating and a single detector. In some cases, the grating may be replaced by one or more optical filters to resolve the desired wavelengths. A reference cell and associated optics to perform simultaneous differential analysis are also incorporated to improve sensitivity or reliability of measurement. 

The technique developed by Bewick for use with a dispersive IR spectrometer is termed electrically modulated infrared spectroscopy (EMIRS) and is essentially a direct development of the UV-VIS specular reflectance technique [50], modulated specular reflectance spectroscopy (MSRS). As in MSRS, radiation is specularly reflected form a polished electrode surface while the electrode potential is modulated with a square wave between a base potential and the working potential at which the process of interest occurs the wavelength range of interest is then slowly scanned. Only that... 

Infrared spectrometers have been commercially available since the 1940s. At that time the instruments relied on prisms to act as dispersive elements, but by. the. mid 1950s, = diffraction gratings had been introduced into dispersive machines. The most significant advances in infrared spectroscopy, however, have come about with the introduction of Fourier-transform Spectrometers. This type of instrument employs an interferometer and explbits the well established mathematical process of Fourier transformation. FT-IR spectroscopy has dramatically improved the quahty of infrared spectra and has minimised the time required to obtain data. Thus j with the improvements to computers achieved in recent years, infrared spectroscopy has made great strides. 

Since the triumphant advance of interferometers in the infrared range, spectrometers are nowadays distinguished into interferometers or non-dispersive spectrometers and dispersive instruments. There are two main arguments for proving the superiority of interferometry over dispersive spectroscopy ... 

Infrared reflection absorption spectroscopy (polarization modulated spectroscopy with a dispersive spectrometer) has been used in studies of the kinetics of the formation of CuSCN multilayer Aims [248]. An advantage of this method— absolute spectra showing no differential bands, which are difficult to interpret— became obvious in a study of CO adsorption on platinum [222]. With the various neutral electrolyte solutions that are used, significant differences were observed in the results obtained with acidic solutions. The mode of CO adsorbed in the bridged position grows considerably in intensity and the position of the on-top COad is slightly shifted (see Fig. 5.56). 

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