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Transmission Raman Spectroscopy: Where Are We Looking, What Are We Seeing?
Until recently, the vast majority of Raman experiments were done in backscattering mode, with the laser focused to a point and Raman radiation collected from the same point. This meant that, particularly with opaque or turbid samples, one only probed a very small region close to the surface, and subsurface features were inaccessible. However, in recent years a whole new branch of Raman spectroscopy has appeared, where the collection and illumination zones are physically displaced. If both illumination and collection zones are on the same surface this is termed SORS (Spatially Offset Raman Spectroscopy); by varying the separation one can probe to different depths, allowing, for example, analysis of subcutaneous tissue. In the extreme variant of SORS, the collection zone is on the opposite side of the sample; this is termed transmission Raman spectroscopy. It has received a lot of interest since one can analyse surprisingly thick opaque samples; furthermore, because sampling is not limited to the surface, one can get results that are more representative of the whole sample. This can greatly improve the precision of analysis of, for example, pharma solid dosage forms. In addition, the possibilities for transmission Raman tomography are also being explored, where one is trying to get Raman spectra specifically from deeply buried inclusions such as calcifications in breast tissue. Several groups are extremely active in the fields of SORS and transmission Raman, notably those of Matousek, Morris and Stone, and instruments are now commercially available to exploit these sampling arrangements.
This webinar will concentrate on the fundamentals of transmission Raman spectroscopy from a theoretical and experimental point of view, in particular discussing where the detected Raman signals originate as a function of sampling geometry. First we will discuss a simple Monte Carlo model that predicts the volumetric sampling profile of transmission Raman as a function of sample thickness and the size of the illumination and collection zones; this tells us what fraction of a sample is really probed in a transmission experiment, and predicts the lateral and depth resolution of Raman tomography . Then we will show how a simple set of experiments have validated this model, by placing very small probe crystals at different depths with opaque matrices, and measuring their apparent size with Raman tomography . Excellent agreement was obtained between theory and experiment, suggesting that we are now better placed to interpret the data from transmission Raman experiments.
1 N Everall, P Matousek, N Macleod, K Ronayne and I P Clark, “Temporal and Spatial Resolution in Transmission Raman Spectroscopy”, Appl. Spectrosc. 64(1), 52-60(2010)
2 N Everall, I Priestnall, P Dallin, J Andrews, I Lewis, K Davis, H Owen and M George, “Measurement of Spatial Resolution and Sensitivity in Transmission and Backscattering Raman Spectroscopy of Opaque samples...” Appl. Spectrosc. 64 (5), 476-484 (2010)
AIR DATE: November 30, 2010
Name: Dr. Neil Everall
Title: Company Research Associate
Company: Intertek MSG