Sensitivity-enhanced transmission Raman spectroscopy

Transmission Raman sensitivity for a representative commercial pharmaceutical tablet was increased by a factor of 40 using optics that returned lost laser and Raman photons to the tablet surface. A new achromatic one-way mirror is introduced that uses the spatial coherence of laser light to nondestructively force laser photons through the reflective tablet coating. Transmission Raman mapping and spatially offset Raman spectroscopy (SORS) mapping were developed and used to better understand the sensitivity-enhancement technology. Fundamental limitations of the sensitivity-enhancement approach are described and used to guide the design of the optics. The sensitivity-enhancement optics are compatible with commercial transmission Raman instruments.