Imaging vibrational spectroscopy

Hyperspectral imaging Vibrational spectroscopy coupled with a spatial analysis (cf. chemical imaging chapter) X Chemical compound distributions Counterfeit detection... 

Spectroscopy produces spectra which arise as a result of interaction of electromagnetic radiation with matter. The type of interaction (electronic or nuclear transition, molecular vibration or electron loss) depends upon the wavelength of the radiation (Tab. 7.1). The most widely applied techniques are infrared (IR), Mossbauer, ultraviolet-visible (UV-Vis), and in recent years, various forms ofX-ray absorption fine structure (XAFS) spectroscopy which probe the local structure of the elements. Less widely used techniques are Raman spectroscopy. X-ray photoelectron spectroscopy (XPS), secondary ion imaging mass spectroscopy (SIMS), Auger electron spectroscopy (AES), electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopy. 

SF (chiral) spectroscopy can be easily modihed for vibrational spectroscopy as shown in Figure 10.16b. This SF vibrational spectroscopy can beneht from plas-mon resonances of a metallic tip by tuning the input beam frequency and/or the output frequency tUspo- Therefore, spatial resolution of SF vibrational spectroscopy could reach to the order of several nanometers. Development of tip-enhanced SF chiral/vibrational spectroscopy is awaited for biomolecular imaging. 

The cross-section in Eq. (1 illustrates another distinguishing feature of inelastic neutron scattering for vibrational spectroscopy, i.e., the absence of dipole and polarizability selection rules. In contrast, it is believed that in optical and inelastic electron surface spectroscopies that a vibrating molecule must possess a net component of a static or induced dipole moment perpendicular to a metal surface in order for the vibrational transition to be observed ( 7,8). This is because dipole moment changes of the vibrating molecule parallel to the surface are canceled by an equal image moment induced in the metal. 

Beyond imaging, CARS microscopy offers the possibility for spatially resolved vibrational spectroscopy [16], providing a wealth of chemical and physical structure information of molecular specimens inside a sub-femtoliter probe volume. As such, multiplex CARS microspectroscopy allows the chemical identification of molecules on the basis of their characteristic Raman spectra and the extraction of their physical properties, e.g., their thermodynamic state. In the time domain, time-resolved CARS microscopy allows recording of ultrafast Raman free induction decays (RFIDs). CARS correlation spectroscopy can probe three-dimensional diffusion dynamics with chemical selectivity. We next discuss the basic principles and exemplifying applications of the different CARS microspectroscopies. 

There is substantial history regarding the application of conventional vibrational spectroscopy methods to study the intact surface of skin, the extracted stratum corneum and the ceramide-cholesterol-fatty acid mixtures that constitute the primary lipid components of the barrier. The complexity of the barrier and the multiple phases formed by the interactions of the barrier components have begun to reveal the role of each of these substances in barrier structure and stability. The use of bulk phase IR to monitor lipid phase behavior and protein secondary structures in the epidermis, as well as in stratum corneum models, is also well established 24-28 In addition, in vivo and ex vivo attenuated total reflectance (ATR) techniques have examined the outer layers of skin to probe hydration levels, drug delivery and percutaneous absorption at a macroscopic level.29-32 Both mid-IR and near-IR spectroscopy have been used to differentiate pathological skin samples.33,34 The above studies, and many others too numerous to mention, lend confidence to the fact that the extension to IR imaging will produce useful results. 

Elmore, D. L., Leverette, C., Lendon, C., Smith, S., Anderson, B. and Muroski, A. (2004) Mid-infrared spectroscopic imaging from thin film to food systems, 50th Gordon Research Conference on Vibrational Spectroscopy, Bristol, RI. 

To date, a number of chemically selective near-field imaging methods have been demonstrated. Near-field contrast mechanisms that rely on electronic spectroscopy (UV-visible absorption and fluorescence),204 vibrational spectroscopy (IR absorption and Raman spectroscopies), dielectric spectroscopy (microwave dispersion), and nonlinear spectroscopy (second harmonic generation) have all been demonstrated at length scales well below the diffraction limit of light. 

Because of the rapidly increasing availability of cryocoolers, numerous new applications have become possible many of these involve infrared imaging systems, spectroscopy, and high-temperature superconductors in the medical and communication fields. Many of these applications have required additional control of cryocooler-generated vibration and EMI susceptibility. 

Infrared spectroscopy is now nearly 100 years old, Raman spectroscopy more than 60. These methods provide us with complementary images of molecular vibrations Vibrations which modulate the molecular dipole moment are visible in the infrared spectrum, while those which modulate the polarizability appear in the Raman spectrum. Other vibrations may be forbidden, silent , in both spectra. It is therefore appropriate to evaluate infrared and Raman spectra jointly. Ideally, both techniques should be available in a well-equipped analytical laboratory. However, infrared and Raman spectroscopy have developed separately. Infrared spectroscopy became the work-horse of vibrational spectroscopy in industrial analytical laboratories as well as in research institutes, whereas Raman spectroscopy up until recently was essentially restricted to academic purposes. 

Romeo, M.J., Dukor, R.K. and Diem, M. (2008) Introduction to spectral imaging, and applications to diagnosis of lymph nodes, in Vibrational Spectroscopy for Medical Diagnosis (eds M. Diem, P.R. Griffiths and ).M. Chalmers), John Wiley Sons, Ltd, Chichester, UK, pp. 1-26. 

Wetzel, D.L. (2008) Biomedical Applications of Infrared Microspectroscopy and Imaging by Various Means, in Biomedical Vibrational Spectroscopy (eds P. Lasch and J. Kneipp), John Wiley Sons, New York, pp. 

Budevska, B.O. 2002) Vibrational spectroscopy imaging of agricultural products, in Handbook of Vibrational Spectroscopy, Vol. 5 (eds J.M. Cholmers and P.R. Griffiths), John Wiley Sons,... 

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