Utility of Fourier transform-Raman and Fourier transform-infrared diffuse reflectance spectroscopy for differentiation of polymorphic spironolactone samples.

Thirteen bulk pharmaceutical preparations of spironolactone were examined by Fourier transform (FT)-Raman spectroscopy and by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) for residual solvents (including the hydrolysis product, thioacetic acid), the presence of enolic tautomeric forms, and evidence for different polymorphic forms. One sample (L) only was found to contain solvent residue (benzene). No evidence for the possible existence of enolic tautomers in the solid state was found. From these specimens, four different representative polymorphic samples (A, B, C, and D) were selected on the basis of their DRIFTS patterns in the 3600-3200-cm-1 region. Samples K and L were considered to represent mixtures of two or more of the above representative types. Similar differentiation of the samples was made on the basis of their Raman spectra over the frequency range 1800-400 cm-1. The various fundamental stretching frequencies for the C = O and C = C bonds have been assigned, and these assignments, in turn, were used to account for all the bands in the 3600-3200-cm-1 region as overtone and combination frequencies of the fundamentals. The Raman lines at 637 and 655 cm-1 were assigned to the two C-S stretching modes of the thioacetyl moiety.