Dependence of the Raman spectra of drug substances upon laser excitation wavelength

The laser excitation wavelength is an important parameter in obtaining definitive Raman spectra from pharmaceutical drug substances. This paper investigates the effect of changing wavelength on the Raman spectra of four different drug substances: paracetamol, paroxetine and ranitidine (polymorphic forms I and II). Excitation wavelengths in the range 244 nm to 785 nm have been used. The fluorescence intensities in the Raman spectra were found to be negligible at the two extreme laser wavelengths but tended to interfere in the visible range. Resonance Raman enhancement was observed with 244 nm excitation but there was evidence for sample degradation. Furthermore, the site selective nature of the resonance enhancement reduced the differences between the Raman spectra of the two ranitidine polymorphs.