Interferometric infrared spectroscopy

Previously, we referred to the FFT. Its development was a giant step that enabled the efficient computation of discrete Fourier transforms. The FFT algorithm and its variations have revolutionized signal analysis and made interferometric infrared spectroscopy practical. Both NMR spectra and mass spectra are also now computed from data that are acquired in their Fourier transform domain. The rediscovery of this algorithm by Cooley and Tukey (1965) is responsible for its current widespread use. Summaries of its properties and pitfalls are provided by Bergland (1969), Brigham (1974), and Bracewell (1978). 

Hitherto, in the form of reflection-absorption infrared spectroscopy (RAIRS), the infrared method had been capable of detecting single monolayers only in the exceptionally favorable (strong absorption) cases of carboxylate ions [Francis and Ellison (14)] or carbon monoxide [Chesters, Pritchard, and Sims (15)] adsorbed on flat metal surfaces. The new challenge from VEELS provided the motivation for a search for improvements in RAIRS sensitivity, and this was very successfully achieved by M. A. Chesters and his colleagues through the introduction of Fourier-transform-based interferometric infrared spectroscopy (16). 

The basic concepts of dispersive and interferometric infrared spectroscopy are dealt with in this chapter. An historical approach is taken in which many of the problems encountered in the development of both techniques are discussed along with the modifications used to solve them. The benefits, drawbacks, and limitations of both techniques are discussed. The materials and instrumentation used in infrared spectroscopy are outlined and discussed, but specific implementation is left to other sources, and sample-handling techniques should be studied in more comprehensive texts on sampling. A short bibliography is included which will allow the investigator to research selected topics more thoroughly. 

In this chapter will be presented an introduction to infrared spectroscopy. It is by no means intended to be all-inclusive whole books which have been written on highly specialized aspects of infrared spectroscopy do not even claim to be exhaustive in their own specialty. It is meant to provide a base for the beginning spectroscopist, and it is also meant to provoke a host of questions. The questions usually will not be difficult to answer it is merely beyond the scope of one chapter to provide all the answers. A novice does require at least a vague notion of where to begin the quest it is hoped that this chapter will provide stimuli and point the novice toward the most useful path. Several basic texts on both dispersive and interferometric infrared spectroscopy that have proved to be invaluable to the authors are included as references. Further chapters will also serve to explain more detailed concepts of and applications for infrared spectroscopy. 

Much of our present knowledge of lipids is based on their spectral properties. Some of the well-established methods (ultraviolet absorption or X-ray diffraction) continue giving useful information, while a host of new ones, particularly in the fields of magnetic resonance and mass spectroscopy, provide us with a variety of previously unknown insights of lipid structure and dynamics finally, the incorporation of new elements into traditional techniques (e.g. interferometric optics and computerization to infrared spectroscopy) add new dimensions to... 

The future of far-infrared spectroscopy is very promising. The far-infrared spectra of many materials will become available with the advent of commercial instrumentation. In addition, the recent revival of interferometric spectroscopy in the far-infrared is one of the most interesting developments in this field The utiliza-... 

J. A. de Haseth and T. L. Isenhour, The direct reconstruction of chromatograms from interferometric GC-IR data, presented at the 1977 International Conference on Fourier Transform Infrared Spectroscopy, Columbia, SC, June 1977, Paper MB7. 

The detectors used in infrared spectroscopy are tailored to both the instrumental technique used and the range examined. In general, dispersive instruments are radiation-limited and require sensitive detectors, while interferometric instruments require detectors with a response rate fast enough to detect and transmit rapid energy changes to a recorder. The most common detectors are summarized in Table 4, along with some of the more salient features. Array detectors are still being developed for multiplex analysis in the NIR, but thus far the cost of these detectors has proven to be prohibitive to widespread use. 

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... 

For readers interested in greater detail, Fourier transform techniques are treated in the following references (a) Marshall, A.G. Verdun, F.R. Fourier Transforms in NMR, Optical, and Mass Spectrometry Elsevier Amsterdam, 1986 (b) Griffiths, P.R., DeHaseth, J.A. Fourier Transform Infrared Spectrometry Wiley-Interseience New York, 1986 (c) Chamberlain, J. The Principles of Interferometric Spectroscopy Wiley-Interscience Chichester, 1979 (d) Bell, R. J. Introductory Fourier Transform Spectrometry Academic Press New York, 1972. 

The properties of the dual-film electrode were characterized by in situ Fourier transform infrared (FTIR) reflection absorption spectroscopy [3]. The FTIR spectrometer used was a Shimadzu FTIR-8100M equipped with a wide-band mercury cadmium teluride (MCT) detector cooled with liquid nitrogen. In situ FTIR measurements were carried out in a spectroelectro-chemical cell in which the dual-film electrode was pushed against an IR transparent silicon window to form a thin layer of solution. A total of 100 interferometric scans was accumulated with the electrode polarized at a given potential. The potential was then shifted to the cathodic side, and a new spectrum with the same number of scans was assembled. The reference electrode used in this experiment was an Ag I AgCl I saturated KCl electrode. The IR spectra are represented as AR/R in the normalized form, where AR=R-R(E ), and R and R(E ) are the reflected intensity measured at a desired potential and a base potential, respectively. 

D. Leisawitz, The Space Infrared Interferometric Telescope (SPIRIT) A Ftir-IR Observatory for High-resolution Imaging and Spectroscopy, in 37th COSPAR Scientific Assembly, volume 37 of... 

The particle concentration of the eluent is normally measured by means of infrared or ultraviolet photometers. Additionally, fluorescence photometer, interferometric measurements (for the refractive index), or mass-spectroscopic methods (e.g. induced coupled plasma mass spectroscopy—ICP-MS, Plathe et al. 2010) are employed. The combination of different detection systems offers an opportunity for a detailed characterisation of multi-component particle systems. Note that the classification by FFF is not ideal and the relevant material properties are not always known moreover, the calibration of FFF is rather difficult. The attribution of particle size to residence time, thus, bears some degree of uncertainty. Recent developments of FFF instrumentation, therefore, include a particle-sizing technique additional to the flow channel and the quantity measurement (usually static and dynamic light scattering, Wyatt 1998 Cho and Hackley 2010). 

There is no question that one of the most exciting areas of application of FT-IR spectroscopy has come about because of the time advantage that is gained in the interferometric method for obtaining an IR spectrum, i.e. -the real time coupling of an infrared spectrometer to a gas chromatograph was finally realized. Not only the packed column GC separation, but now also capillary GC separations have benefited from the identification power of the FT-IR spectrometer. An excellent review of GC/FT-IR applications is by Erickson, as well as in the special issue of the Oournal of Chromatographic Science For all these applications, special attention... 

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