Comparison of the determination of a low-concentration active ingredient in pharmaceutical tablets by backscatter and transmission raman spectrometry

Nichola Townshend, Alison Nordon, David Littlejohn, Michael Myrick, John Andrews, Paul Dallin

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A total of 383 tablets of a pharmaceutical product were analyzed by backscatter and transmission Raman spectrometry to determine the concentration of an active pharmaceutical ingredient (API), chlorpheniramine maleate, at the 2% m/m (4 mg) level. As the exact composition of the tablets was unknown, external calibration samples were prepared from chlorpheniramine maleate and microcrystalline cellulose (Avicel) of different particle size. The API peak at 1594 cm(-1) in the second derivative Raman spectra was used to generate linear calibration models. The API concentration predicted using backscatter Raman measurements was relatively insensitive to the particle size of Avicel. With transmission, however, particle size effects were greater and accurate prediction of the API content was only possible when the photon propagation properties of the calibration and sample tablets were matched. Good agreement was obtained with HPLC analysis when matched calibration tablets were used for both modes. When the calibration and sample tablets are not chemically matched, spectral normalization based on calculation of relative intensities cannot be used to reduce the effects of differences in physical properties. The main conclusion is that although better for whole tablet analysis, transmission Raman is more sensitive to differences in the photon propagation properties of the calibration and sample tablets.

Original languageEnglish
Pages (from-to)4671-4676
Number of pages6
JournalAnalytical Chemistry
Issue number11
Publication statusPublished - 5 Jun 2012



  • Raman spectrometry
  • backscatter Raman measurement
  • chlorpheniramine maleate

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