We compare and contrast two Raman collection geometries, backscattering and transmission, to identify their potential for monitoring the bulk chemical composition of turbid media. The experiments performed on pharmaceutical tablets confirm the expected strong bias of the backscattering Raman collection towards surface layers of the probed sample. However, this bias is largely absent with the transmission geometry, exhibiting gross insensitivity to the depth of impurities within the sample. The results are supported by Monte-Carlo simulations. The applicability of transmission geometry to tablets without any thinning is possible because of long migration times of Raman photons in non-absorbing powder media. The absolute measured intensity of the Raman signal was only 12 times lower in transmission geometry compared with backscattering geometry for a standard paracetamol tablet with a thickness of 3.9 mm. This makes detection relatively straightforward, and detectable Raman signals were observed even after propagation through three paracetamol tablets. Given its properties and instrumental simplicity, the transmission method is particularly well suited to the on-line analysis of bulk content of tablets in pharmaceutical applications.
Vol. 2, Iss. 1 Virtual Journal for Biomedical Optics
P. Matousek and A. W. Parker, "Bulk Raman Analysis of Pharmaceutical Tablets," Appl. Spectrosc. 60, 1353-1357 (2006)