Spatially Offset Raman Spectroscopy SORS

The feasibility of the SORS concept as a tool for obtaining Raman signals from deeply buried layers in turbid media was first demonstrated by Matousek et al. [17]. The experiments were performed on a two-layer sample composed of a 1 mm thick PMMA powder layer placed on top of a tra s-stilbene powder 

If a higher degree of separation of the subsurface signals from the surface contribution is required, then this can be accomplished, for a two-layer system, by a simple scaled subtraction of two spectra obtained at different spatial offsets to cancel the Raman contribution from the surface layer. For a stratified sample with more than two layers, a multivariate data analysis can be applied using a 

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Spatially Offset Raman Spectroscopy (SORS) for Deep Probing of Calcifications... 

N. Stone, R. Baker, K.D. Rogers, A.W. Parker, P. Matousek, Future possibilities in the diagnosis of breast cancer by subsurface probing of calcifications with spatially offset Raman spectroscopy (SORS). Analyst 132, 899-905 (2007)... 

Spatially offset Raman spectroscopy (SORS) Conventional Raman Spectroscopy is limited to the near-surface of diffusely scattering objects and to the first few hundred micrometers depth of surface material. Spatially Offset Raman Spectroscopy (SORS) is a variant of Raman Spectroscopy that allows highly accurate chemical analysis of objects beneath obscuring surfaces. This is done by making at least two Raman measurements one at the surface and one at an offset position of t3q>ically a few millimeters away. To do this without using an offset measurement would be severely restricted by photon shot noise generated... 

Although Raman spectroscopy does not employ absorption of infrared radiation as its fundamental principle of operation, it is combined with other infrared spectroscopies into a joint section. Results obtained with various Raman spectroscopies as described below cover vibrational properties of molecules at interfaces complementing infrared spectroscopy in many cases. A general overview of applications of laser Raman spectroscopy (LRS) as applied to electrochemical interfaces has been provided [342]. Spatially offset Raman spectroscopy (SORS) enables spatially resolved Raman spectroscopic investigations of multilayered systems based on the collection of scattered light from spatial regions of the samples offset from the point of illumination [343]. So far this technique has only been applied in various fields outside electrochemistry [344]. Fourth-order coherent Raman spectroscopy has been developed and applied to solid/liquid interfaces [345] applications in electrochemical systems have not been reported so far. 

The basic concept is known as spatially offset Raman spectroscopy (SORS) [17], and it utilizes the differences in the spatial distribution of Raman photons emerging at the surface from different depths of the probed sample. In this approach, Raman spectra are collected from regions on the sample surface that is spatially offset by different amounts from the point of laser incidence (Figures 13.1 and 13.2). Such spectra contain varying relative Raman contributions from layers located at different depths within the sample. This difference is brought about by the wider lateral diffusion of photons emerging from greater depths and the effect of photon loss at sample-to-air interface [3,11,12]. 

Recently Eliasson and Matousek [26, 62] demonstrated that SORS can provide a chemical signature of the internal content of opaque plastic containers. This is demonstrated in Fig. 3.11 for aspirin tablets held inside an opaque (white) plastic pharmaceutical bottle (1.3mm thick). The conventional Raman signal is overwhelmed by the Raman component originating from the container wall and is consequently ineffective in determining the contents of the bottle. In contrast, the SORS method using a scaled subtraction of two SORS spectra measured at different spatial offsets yields a clean Raman spectrum of the tablets inside the bottle. SORS has also been used in the detection of counterfeit anti-malarial tablets by Ricci et al. [63] the chemical specificity of Raman spectroscopy readily distinguished between genuine and fake tablets and identified the content of the counterfeit tablets. 

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