[MOS] April 16, 2013

[Zina M Queen]

Seminar on 

Modern Optics and Spectroscopy 


Transmission Raman Spectroscopy of opaque materials 

Michael J. Pelletier, Pfizer Inc.

Tuesday, April 16, 2013 

12:00 – 1:00 p.m.

The small probe volumes typical of traditional Raman analyzers can cause substantial error in the quantitative analysis of most pharmaceutical tablets due to the inherent microscopic compositional heterogeneity of these tablets.  Transmission Raman spectroscopy can provide improved accuracy for pharmaceutical tablet analysis by probing a larger volume of the tablet.  Photon migration leads to a much longer excitation pathlength through the tablet, causing excitation of a greater tablet volume.  Ordinarily the greater pathlength through the tablet would lead to greater sensitivity, but this is not the case for transmission Raman spectroscopy.  The efficiencies for getting light into the tablet, and for collecting the Raman photons that are generated are usually poor, leading to greatly reduced sensitivity compared to traditional backscattering measurements.  An “optical diode” using a multi-layer dielectric mirror has been reported that recycles lost photons, increasing both sensitivity and volume sampling homogeneity. 

This talk will describe a different approach for increasing the sensitivity and volume sampling homogeneity of transmission Raman spectroscopy.  Detailed analysis shows the fundamental limitations of the approach and opportunities for improvement.  Initial results using a dispersive Raman instrument demonstrated a sensitivity enhancement of x40.  Transmission FT-Raman was also investigated as a way to further increase sensitivity by increasing collection etendue.  Results using commercially-available, over-the-counter pharmaceutical tablets will be presented. 

Grier Room, MIT Bldg 34-401

Refreshments served after the lecture
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