**1. Introduction**

32 Will-be-set-by-IN-TECH

[58] W. B. Streett, L. S. Sagan, and L. A. K. Staveley. Experimental study of the equation of

[59] E. W. Grundke, D. Henderson, and R. D. Murphy. Evaluation of the percus-yevick theory for mixtures of simple liquids. *Canadian Journal of Physics*, 51:1216–1226, 1973. [60] W. H. Press, S. A. Teukolsky, W. T. Vetterling, and B. P. Flannery. *Numerical Recipes in FORTRAN: The Art of Scientific Computing*. Cambridge University Press, New York,

[61] Xianbo Shi. *Energy of the Quasi-free Electron in Atomic and Molecular Fluids*. PhD thesis, The Graduate Center of the City University of New York, New York, NY, 2010.

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1992.

Absorption and fluorescence spectroscopies in the visible and/or infrared spectrum are good options when a fast and objective analysis is required. Spectroscopy is based on light-matter interactions. This interaction occurs in different ways, and each molecule or an ensemble of molecules will show a distinctive response. The vibrational spectroscopy provides a fingerprint of the vibrational levels of a molecule usually at mid-infrared (MIR) radiation (400-4000cm-1). The optical spectroscopy uses the ultraviolet-visible (UV-VIS) region (200- 1000nm) of the electromagnetic spectrum and interrogates the electronic levels of a molecule. The instrumentation used to generate and detect this radiation is less complex and cheaper compared to other spectroscopy techniques, such as nuclear magnetic resonance, Xrays, etc. An absorption spectrum is obtained by irradiating a sample and measuring the light which is transformed into other forms of energy, e.g. molecular vibration (heat). A fluorescence spectrum is obtained only from fluorescent molecules, those that absorb and then emit radiation, acquiring the intensity of light emitted as a function of the wavelength. These spectra are characteristic for each molecule, because each one has different electronic levels and vibrational modes. These levels and modes are also influenced by the solvent of the molecule.

Vibrational spectroscopy applied to the mid portion of the infrared spectrum provides the basis to develop several of the most powerful methods of qualitative and quantitative chemical analysis. Some of the advantages to use this technique are: the information is collected on a molecular level, almost any chemical group has IR bands, it is very environment-sensitive.[1-3]

Optical fluorescence spectroscopy is highly sensitive and can provide different information about the molecules and the molecular processes such as the molecular interaction with the environment, the molecular bonding and concentration.[3, 4]

© 2012 Estracanholli et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Spectroscopic techniques in this range of the electromagnetic spectrum have shown applications in different areas, from analytical chemistry to the diagnosis of some types of cancer, detection of citrus diseases and of dental caries, with a high sensitivity and good specificity rates. This is possible because the analyzed systems are composed of different types and concentrations of molecules. Thus, the spectrum of samples obtained under different conditions will also be different. It is therefore possible to identify and also quantify different compounds. However, the spectral variation can be characterized and correlated only with difficulty. This is mainly due to the fact that other phenomena, such as scattering and/or absorption, happen with the emitted light. In some cases, there may be other molecules in the sample presenting absorption bands that overlap in the same spectral region of the compound of interest, this mainly happens for absorption spectroscopy. In other cases, as in the case of fluorescence spectroscopy, the excitation and emitted light can be absorbed by other molecules making the signal too weak to be detected. A solution to this problem may be statistical procedures applied where the spectral information is correlated with any parameter of interest. [3-5]

A new application of a statistical method to process multi-layer spectroscopy information will be presented in this chapter. A brief review of the mathematical methods to analyze these spectroscopy data will be shown here, followed by two distinct examples. The first example is UV-VIS fluorescence spectroscopy, applied to detect the postmortem interval (PMI) in an animal model. The spectroscopy and statistical methods of analysis presented can be extended to other samples, like food and beverage. Here, a MIR absorption spectroscopy of liquid samples will be presented to detect and quantify certain compounds during the production of beer. Another system to measure liquid samples, which consists of a sample holder, will also be presented. This system offers a cheaper technique with a better signal compared to techniques used to analyze liquid samples in the MIR region.
