Principles of FTIR

The relative absorbance changes AA/A in the infrared as a result of protein activity are on the order of 10 - to lO underneath a high background absorbance of up to 1. FT instruments have a number of distinct advantages over dispersive instruments. Dispersive instruments such as those shown in Fig. 6.6-2, which measure wavenumbers sequentially are not sufficiently stable during the entire measuring time to provide complete, high quality difference spectra. 

In principle, an FTIR instrument consists of a Michelson interferometric arrangement (see Fig. 6.6-3) with a light source G (globar), a beam splitter BS, a fixed mirror FM, a movable mirror MM, and a detector (MCT). The very sensitive MCT detectors 

The digitized (ADC) discrete interferogram is Fourier transformed (DFT) by a PC to yield a wavenumber dependent spectrum. Sampling points are determined by the interference pattern of a monochromatic HeNe laser beam which is transferred collinearly with the IR beam. The resulting high wavenumber accuracy constitutes the third advantage of FTIR. 

This section very briefly outlines some of the problems connected with the Fourier transformation. For details, textbooks such as Griffiths and de Haset (1986) can be recommended. Problems arise primarily because digital computers perform discrete rather than continuous FT of the interferogram / (,r), an approximation which requires care to avoid errors. As a result of DFT, the continuous variables, i.e., the scan length. v and the frequency T, become the discrete variables n A,v and k AT  

In practice, one uses a less redundant fast Fourier transform algorithm, e.g.. the Cooley-Tukey algorithm rather than the expression shown above. Possible problems connected with discrete Fourier transfomiation (DFT) include 

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Fourier transform methods have revolutionized many fields in physics and chemistry, and applications of the technique are to be found in such diverse areas as radio astronomy [52], nuclear magnetic resonance spectroscopy [53], mass spectroscopy [54], and optical absorption/emission spectroscopy from the far-infrared to the ultraviolet [55-57]. These applications are reviewed in several excellent sources [1, 54,58], and this section simply aims to describe the fundamental principles of FTIR spectroscopy. A more theoretical development of Fourier transform techniques is given in several texts [59-61], and the interested reader is referred to these for details. 

Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy. Attenuated total redectance (atr) ftir spectroscopy is based on the principle of total internal redection (40). Methods based on internal redection in the uv and visible regions of the spectmm are also common in addition to those in the ir region. The implementation of internal redection in the ir region of the spectmm provides a means of obtaining ir spectra of surfaces or interfaces, thus providing moleculady-specific vibrational information. 

The trends begun with the general introduction of FTIR technology will undoubtedly continue. It is safe to say that the quality of the data being produced far exceeds our ability to analyze it. In fact, for many current applications, the principle limitations are not with the equipment, but rather with the quality of the samples. Thus, the shift from qualitative to quantitative work will proceed, reaching high levels of sophistication to address the optical and matrix interference problems discussed above. 

To get over this difficulty, FTIR uses the basic principle of an interferogram. A source of IR light (containing a broad band of IR frequencies) is incident upon a fixed mirror, but passes through an optical device, the beam splitter at which about half the light is reflected and half allowed to pass through shown in Figure la ( ). 

Several methods have been developed to estimate the exposure to such emissions. Most methods are based on either ambient air quality surveys or emission modeling. Exposure to other components of diesel emissions, such as PAHs, is also higher in occupational settings than it is in ambient environments. The principles of the techniques most often used in exhaust gas analysis include infrared (NDIR and FTIR), chemiluminescence, flame ionization detector (FID and fast FID), and paramagnetic methods. 

Fourier transform methods have come into their own as a means of studying the optical spectra of gas-phase radicals. Both infrared (FTIR) and ultraviolet/visible spectroscopy (FTUV/VIS) are now used to scrutinize these reactive molecules. We discuss the underlying principles of Fourier transform spectroscopy (FTS) with particular emphasis on the advantages and drawbacks of FTIR and FTUV/VIS measurements. Extensive tables are presented of metastable molecules that have been studied by Fourier transform methods. 

Fig. 27. Schematic representation of the principle of coupled liquid chromatography and FTIR spectroscopy...

This chapter will focus on the appHcation of FTIR spectroscopy in the quantitative analysis of foods. Following a brief discussion of the fundamental principles of IR spectroscopy, we wiU describe the instrumentation, data handling techniques, and quantitative analysis methods employed in FTIR spectroscopy. We will then consider the IR sampling techniques that are most useful in FTIR analysis of foods. Finally, a survey of FTIR applications to the quantitative analysis of food will be presented. Although important, the so-called hyphenated techniques, such as GC-FTIR, will not be covered in this chapter. Similarly, near-IR (NIR) spectroscopy, which has found extensive use in food analysis, is beyond the scope of this chapter. 

The technique of choice for studying thin films on metals (or certain other substrates) directly is single reflection RAIR [47-54]. The limitation here is that the substrate must be very smooth, but this can be easily achieved by polishing the metal before deposition of the film. Characterizations of thin organic layers on metal (oxide) surfaces, such as occur in lubricants, corrosion inhibitors, adhesives, polymers, paints, and so forth, are specific applications of this rather recent form of FTIR. It should be noted that the relative band positions and shapes may be different in this technique than in conventional transmission IR. The spectrum may also change with the thickness of the organic film, which implies that polymer/metal interactions are in principle observed [47,51]. The teehnique is so surface sensitive that oxidation of metals can be determined in situ [51] and the packing... 

This exercise has introduced selected principles of molecular UV and infrared spectroscopy for qualitative chemical analysis. Interpret the significance of these spectra in terms of molecular structure. You should have obtained hardcopy printouts of UV and FTIR spectra. Please include all relevant spectra in your report. 

The use of FTIR spectrometers is now almost universal. The accuracy, sensitivity and reproducability of the new instrumentation have facilitated the development of a range of new data processing methods such as spectral subtraction, deconvolution, curve resolving etc., and new sampling techniques, but it is essential to remember that the underlying principles of infrared spectroscopy have not changed. The precautions necessary to avoid spectral artefacts are thus an important aspect of the work. 

The principles and performance of FTIR spectrometers are now sufficiently well known to need no detailed discussion. The FTIR system now allows greater information retrieval from the IR data than was possible with non-digital systems, and a much greater range of sample formats. It is therefore important to ensure that the data are all truly representative of the sample, and when comparisons are used (eg. subtraction) the samples are as far as possible in the same physical state and the same sampling method is used. 

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