Source: Spectroscopy Now

The distributions of two cocrystallising components within a pharmaceutical tablet have been chemically mapped for the first time, using terahertz spectroscopy, and their concentrations were accurately measured. THz spectroscopy, which operates at 0.1-10 THz, or 3-333 cm-1, has been used previously to distinguish between homogeneous drug tablets of different chemicals and to map the distributions of active and inactive ingredients within a tablet. However, this is the first time that cocrystals have been mapped by the technique.

The concept has been described by Katsuhiro Ajito and colleagues from NTT Corporation, Japan, in Analytical Chemistry, where they prepared cocrystals of caffeine, an active pharmaceutical ingredient, with the coformer oxalic acid. This combination was chosen for proof-of-concept experiments because the crystal structure of their cocrystals is known. The images were produced by THz-time domain spectroscopy using a near-IR pulse laser at 120 K in a vacuum chamber and images were built up by rastering across the sample surface at frequencies of 1-2 THz.

The cocrystals generated sharp peaks, allowing the distribution of the cocrystals to be mapped. For non-uniform tablets, the caffeine-oxalic acid cocrystals were successfully distinguished from the pure caffeine and oxalic acid components. The cocrystal concentrations were accurately measured from the spectra to within 0.3-1.3 % by weight of the known concentrations.

The new technique will be of value within the pharmaceutical industry, by establishing a reference THz spectral database of cocrystals and other ingredients.