Spectral subtraction technique application

Applications of the Spectral Subtraction Technique. Based on the advantage of precision wavenumber measurement provided by computerized FT-IR Instrumentation, the absorbance-subtraction technique has become a practical method in analysis of multicomponent mixtures [ ]. It was also found in this research that difference photoacoustio spectroscopy can be used to distinguish small differences between two samples. By comparing the PAS spectra of treated and untreated materials, the common spectral features can be cancelled out. The remaining bands can be interpreted in terms of the near-surface chemical species due to the treatment. 

Some of the major approaches for noise reduction from speech signals were reviewed. Emphasis of the physical circumstances where each is applicable and the theoretical assumptions upon which each is based were considered. Aural noise reduction systems have been an active area of research for many decades. The theory of Weiner and Kolmologrove was advanced in the 1940s and has been applied for many different speech models since then. However, the utility of the MSE criterion upon which these methods are based has been questioned for speech. In addition, these methods require knowledge of the spectra of speech which, due to the fact that speech is not strictly stationary, are difficult to obtain. Thus approaches, which subscribe a parametric model to the speech signal, have arisen. The MSE criterion has also been applied in the spectral domain to yield the successful spectral subtraction technique. The theoretical justification for... 

Difference spectroscopy, as the name implies, requires spectral subtraction. Two spectra are acquired. One in which the proton of interest is decoupled and a second reference spectrum in which the irradiation is appHed in an isolated region far from the nearest proton resonance. The two spectra are then subtracted from one another, the resulting difference spectrum highHghting protons that were affected by the decoupling process [34]. Several excellent examples of applications of this technique are found in the monograph by Nakanishi [24]. 

A Fourier transform infrared (FTIR) speetrometer equipped with an attenuated total refleetanee (ATR) aeeessory is used for the study of surfaees and coatings. A microscope attachment is useful for identifying particulate impurities. Through the technique of computerized spectral subtraction, many sample mixtures may be identified by FT-IR without prior physical separation and, thus, the technique lends itself to compositional analysis. Typical applications of FT-IR are the study of surfaces of polymers by ATR, identification of samples isolated by thin-layer chromatography or other preparatory chromatographic techniques, identification of impurities in polymer and polymer blends, product characterization and product formulations by spectral subtraction, as well as routine analysis of solid and liquid materials. 

Normal sample, modern FTIR spectrometer 1-4 minute scan time, with spectral resolution of 2-4 cm providing a signal-to-noise ratio (S/N) of between 1000 1 and 10,000 1. If a spectrum is to be used for qualitative purposes only, then a S/N of approximately 1000 1 to 2000 1 is normally adequate and is typically obtained in well under 1 min often within a few seconds. For quantitative apphcations, or applications involving spectral subtraction or resolution enhancement techniques, higher signal-to-noise performance is required. Note that scanning for more than 256 scans may approach a point of diminishing returns in terms of spectral quality and S/N improvement (limited by the law, and the performance of the... 

As mentioned earlier, care should be taken with background subtraction techniques, as some have been observed to leave residual background in the spectra [22]. Fortunately, there are straightforward means to calculate the applicability of the PCR model to the current spectrum based on spectral residuals [8]. 

Absorbance subtraction can be considered as a spectroscopic separation technique for some problems in polymers. An interesting application in FT-IR difference spectroscopy is the spectral separation of a composite spectrum of a heterophase system. One such example is a semicrystalline polymer which may be viewed as a composite system containing an amorphous and crystalline phase53). In general, the infrared spectrum of each of these phases will be different because in the crystalline phase one particular rotational conformation will predominate whereas in the disordered amorphous regions a different rotamer will dominate. Since the infrared spectrum is sensitive to conformations of the backbone, the spectral contributions will be different if they can be isolated. The total absorbance A, at a frequency v of a semicrystalline polymer may be decomposed into the following contributions... 

SFS technique has been explored in the last years, e.g., to classify the composition of mixtures of heterogeneous CDOM present in natural freshwaters and in their different subtractions [27,28,33,34]. Similar attempts to identify certain structural and functional constituents in natural HMs have been carried out also with the aid of total luminescence spectroscopy [35]. At present, utilizations of fluorescence techniques are increasingly related to solve certain environmental questions, e.g., how does CDOM vary spatially between different aquatic environments/seasons, what is the causative factor for the leaching of natural carbons, are there any spectral signatures that could lead to the identification of CDOM sources In connection with the heterogeneity of CDOMs between different water samples there is no single spectral parameter that could explain the variation but chemometric applications are needed for interpretation of multidimensional excitation-emission matrices [33,35-39]. 

The availability of infra-red spectrometers with Fourier-Transform capability has revitalized the field of infra-red spectroscopy and led to some exciting applications. With regard to natural products structure determination, Fourier Transform infra-red spectroscopy (FT-IR) has distinct advantages. The technique is extremely sensitive, and spectra can be recorded on submicrogram quantities, well below the sensitivity limits of NMR. Equally important is the ability to record high-resolution spectra of small amounts of samples in the solid state as well as in solution. Thus, the sample need not be soluble in a readily available deuterated solvent as is the case with NMR. The possibility of spectral storage enables the contributions from the solvent to be subtracted from the spectrum of highly dilute samples. 

Figure 14 Improvement in the Raman spectrum of peat humic acid in neutralized form by use of near-infrared excitation. The top spectrum was obtained with 514.5-nm excitation, which results in a large fluorescence background that obscures the signal. However, as shown in the bottom spectrum, with 1064-nm excitation, and after subtraction of thermal emission background, spectral features of peat humic are clearly discernible. (Reprinted with permission from YH Yang, HA Chase. Applications of Raman and surface enhanced Raman scattering techniques to humic substances. Spectr Lett 31 821-848, 1998. Copyright 1998 Marcel Dekker, Inc.)...

Absorbance subtraction can be considered a spectroscopic separation or purification technique for some problems inherent in polymers. An interesting application of FTIR difference spectroscopy is the spectral separation of a composite spectrum of a two-component system. However, complications can arise when there are inter-molecular interactions that perturb the frequencies and intensities [80]. 

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