Spatial Methods Spatially Offset Raman Spectroscopy SORS

Spatial Methods Spatially Offset Raman Spectroscopy (SORS) 

The zero-offset spectrum represents a conventional Raman spectrum. By increasing the spatial offset, the surface layer signal (PMMA) diminishes much more quickly that that of the sublayer (trons-stilbene) such that, at a spatial offset of 2 mm, an order of magnitude improvement in the intensity ratio is achieved. The same report also detailed improvements in the relative ratio between the Raman signals of the two layers by a factor of 19 at a 3.5 mm spatial offset. Interestingly, the quality of the spectra obtained in these measurements was substantially higher 

The application of SORS in its basic form provides information on the chemical composihon of individual layers, but no spatial information about their distribution within the sample. Additional studies performed by Schulmerich and coworkers [28] demonstrated how SORS could also be deployed as a tomographic tool. 

Another SORS variant was described [30] for which the concept relies on the separation of signals in stratified media by defocusing a conventional Raman instrument. Although less effective than a fully ophmized SORS method, the technique can be sufficiently effective with less-challenging samples, and may provide a convenient first stepping stone for those groups first encountering this area of research. 

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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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