Self-absorption corrected non-invasive transmission Raman spectroscopy (of biological tissue)

Higher contrast of subsurface Raman spectra is achievable with self-absorption corrected transmission Raman spectroscopy. (Desired signal in red, interfering matrix artefacts in blue.)

Transmission Raman Spectroscopic Quantification of Active Pharmaceutical Ingredient in Coated Tablets of Hot-Melt Extruded Amorphous Solid Dispersion

Transmission Raman spectroscopy is an emerging technique, capable of quantitative analysis of drug products nondestructively using a multivariate data analysis approach. We developed and validated a chemometric method to quantify the active pharmaceutical ingredient in coated tablets of hot-melt extruded amorphous solid dispersion. A partial least squares regression (PLSR) model was developed and validated based on transmission Raman spectra data collected from coated tablet samples with variations in the content of active pharmaceutical ingredient, excipients, water content, a key oxidative degradant, milled extrudate particle size distribution, and tablet hardness. The method was proven to be accurate, linear, specific, and robust. Our work demonstrates that transmission Raman spectroscopy (TRS) is a viable, cost-effective, secondary method to high-performance liquid chromatography (HPLC) for quantitation of active pharmaceutical ingredient (API) in coated tablets of hot-melt extruded amorphous solid dispersion.

Time evolution of Symmetry-forbidden Raman lines activated by photorefractivity

Transmission Raman spectroscopy experiments were performed on iron doped congruent lithium niobate within two –in principle equivalent- configurations, namely Y(ZX)Y and Y(XZ)Y. While the former respects the Raman selection rules, the other configuration gives a time dependent spectrum that, after a transient time of several minutes, finally results in a mixture of expected and forbidden modes. This breaking of Raman selection rules is caused by the spontaneous conversion of a part of the ordinarily polarized pump beam into an extraordinarily polarized beam by photorefractive anisotropic self-scattering. A numerical modelling of the phenomenon is developed and fairly reproduces the time dependence of conversion energy.

Analytical Solution to the Depth-of-Origin Profile of Transmission Raman Spectroscopy in Turbid Media Based on the Kubelka–Munk Model

An analytical formula to the depth-of-origin profile of transmission Raman spectroscopy in turbid media was derived from the one-dimensional (1D) Kubelka–Munk model. The depth-of-origin profile of the transmitted Raman is proportional to the excitation intensity profile and the transmittance profile, which are two functions of similar forms. The effect of scattering, absorption, and signal-enhancing reflectors are incorporated into the formula. Depth-of-origin profile of a model sample was measured at better than 0.2 mm resolution and fits the formula reasonably well. Conical reflective cavities placed at the front and/or back of the sample enhanced the signal significantly; the relationship among the enhancement functions is verified by the formula. Optical parameters derived from the fitting are compared to theoretical value predicted by optical ray tracing and direct measurements; discrepancies are related to deficiency of the 1D Kubelka–Munk model.