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B. Lendl:
"Stand-off Spatial Offset Raman Scattering";
Hauptvortrag: Lasers, Sources, And Related Photonic Devices, San Diego (eingeladen); 29.01.2012 - 03.02.2012; in: "Laser Applications to Chemical, Security and Environmental Analysis (LACSEA)", (2012), ISBN: 978-1-55752-933-6; S. LT2B.4.pdf.

Stand-off Raman spectroscopy, where the instrumentation is physically separated from the sample under investigation, can be extremely advantageous for the analysis of dangerous, fragile or inaccessible samples. Using such a stand-off system usually yields Raman spectra from the surface of the object probed. If an object has transparent packaging it can also be possible to obtain stand-off Raman spectra from both the contents and the packaging. However, in many situations where stand-off systems are advantageous, such as the investigation of suspicious packages as potential bomb threats or the analysis of base materials under ancient paintings, for example, the covering layers may not always be transparent or may give rise to strong Raman signals that obscure the signals from deeper layers. Spatially offset Raman spectroscopy (SORS) is based on collecting Raman scattered light at a point laterally distant from the excitation laser spot [1]. By collecting the Raman scattered light in this manner, in conjunction with multivariate data analysis, it is possible to obtain pure Raman spectra of substances present behind a surface layer.
Here we show that it is possible to combine the depth probing advantages of SORS with the increased distance between sample and spectrometer afforded by stand-off Raman spectroscopy, with the example of NaClO3 contained in opaque plastic containers at a distance of 12 meters. One of the disadvantages of Raman spectroscopy is the potential contribution of fluorescence to the collected Raman spectra. For the examples of isopropanol and H2O2, in an opaque plastic container that gives rise to fluorescence, we show that by combining SORS with stand-off Raman it is also possible to identify substances present in such containers, despite the increased fluorescence making identification impossible in the non-offset spectra.
These results are likely to be of interest in applications where the sample of interest is present behind an opaque surface layer, such as a container or coating material, and may be dangerous, such as for chemical spills or for suspected explosive packages. In such cases, the combination of stand-off distances with SORS will allow deeper probing of the object, identifying substances within a container, whilst retaining the safety aspects of the increased distance between sample and spectrometer.



Keywords: Raman spectroscopy; spatial offset

References
[1] P. Matousek, et al. Applied Spectroscopy (2005) 59:393-400