Impact of paracetamol impurities on face properties: investigating the surface of single crystals using TOF-SIMS

Understanding the mechanism of interaction between pharmaceutical molecules (APIs) and impurities on crystal surfaces is a key concept in understanding purification and for the design of pharmaceutical crystallization processes. Several techniques may be used to study crystal surface properties, such as scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), which provide detailed imaging and elemental surface characterization. Time of flight secondary ion mass spectrometry (TOF-SIMS) is valuable in determining molecular identity and distribution. By combining TOF-SIMS, SEM, and optical (OM) and Raman microscopies, we can evaluate the usefulness of TOF-SIMS as a surface characterization technique for pharmaceutical crystals. 4-Nitrophenol has been selected as an impurity that can be incorporated during crystallization of acetaminophen (paracetamol). This study explores the distribution of impurity and its concentration on the different crystal faces of samples obtained by crystallization over a range of impurity loadings and supersaturation conditions. Raman maps of paracetamol single crystal faces were analyzed using the characteristic Raman peak intensity of 4-nitrophenol to identify regions where it accumulated; Raman maps of three single crystals produced in the presence of 4-nitrophenol using different crystallization procedures highlight how it can be difficult to detect very low concentrations of similar chemical species. In contrast, the 4-nitrophenol monoisotopic mass obtained via TOF-SIMS was shown to be detectable in all the three single crystals produced. This indicates that TOF-SIMS can be a valuable technique for single crystal impurity distribution mapping even when the impurity concentration is very low.