Transmission Raman spectroscopy

Transmission Raman spectroscopy (TRS) is a variant of Raman spectroscopy which is advantageous in probing bulk content of diffusely scattering samples. Although it was demonstrated in the early days of Raman spectroscopy it was not exploited in practical settings until much later, probably due to limitations of technology at the time. It was rediscovered in 2006, where the authors showed that it was capable of allowing Raman spectroscopy through many millimetres of tabletted or powdered samples. In addition, this research has also identified several highly beneficial analytical properties of this approach, including the ability to probe bulk content of powders and tissue in the absence of subsampling and to reject Raman and fluorescence components originating from the surface of the sample.

Theory

Transmission Raman is possible because light scatters through turbid materials that do not significantly absorb or block the light. By a similar mechanism to spatially offset Raman spectroscopy, the light in a diffusely scattering sample spreads through the object randomly (Transmission Raman can be regarded as an extreme example of SORS). As Raman photons can be created at all points that the light passes through the total scrambled Raman signal measured on the opposite face of the object is highly representative of the bulk of the material. This desirable property removes a problem with conventional, widely used back-scattering Raman spectroscopy where the signal tends to be representative of the surface and near-surface composition. Because Raman spectroscopy does not rely on absorption and the light spreads throughout the sample, a large thickness can be measured in the absence of photon absorption. This produces an analysis representative of the entire mixture and is typically insensitive to coatings, or thin containers.