TOF-SIMS analysis of curcuminoids and curcumin crystals crystallized from their pure and impure solutions

Impurities are frequently encountered during the crystallisation of active pharmaceutical compounds. Impurities can either adsorb onto active sites or replace atoms of the crystal lattice. Locating the impurities, especially structurally-similar impurities, on the crystal surface is challenging. In this work, we showed that time-of-flight secondary ion mass spectrometry (TOF-SIMS) can be successfully used to simultaneously quantify the composition of the two structurally-similar impurities, demethoxycurcumin (DMC) and bisdemethoxycurcumin (BDMC), on the surface of the curcumin (CUR) crystals obtained from crystal growth and cooling crystallisation experiments. The distribution of the three curcuminoids on the crystal surface was evaluated through imaging of the specific ions that correspond to intact molecules of the curcuminoids. In terms of location, both BDMC and DMC were evenly distributed on the surfaces of the facets of the curcumin crystals. Additionally, we found that BDMC and DMC occupy their own sites or they are located on the same site on the crystal surface providing evidence for lattice replacement of these molecules. Thus our results demonstrate the potential of TOF-SIMS to provide new information that can help to understand crystallisation.