Application of quantitative Raman spectroscopy for the monitoring of polymorphic transformation in crystallization processes using a good calibration practice procedure

Polymorphism is the property of a substance to have more than one crystalline form. Polymorphic forms of the same chemical compound can have different physical and chemical properties that can strongly affect the manufacturing process. For this reason, determining and monitoring polymorphic transformations have become very important, especially in pharmaceutical industry. Significant work has been developed for the calibration of Raman spectroscopy to monitor the presence and amount of solid polymorphs in suspensions during crystallization, as well as the liquid concentration. Nevertheless, a clear and systematic approach to Raman calibration is missing in the literature. The present work has the aim of developing a methodical strategy for Raman calibration, taking into account the principal factors that can affect the Raman spectra of a specific compound in solution, such as solid type, solute concentration, temperature, crystal size and suspension density. Univariate and multivariate calibration techniques were investigated using pre-processing techniques to optimize the signal. The results are combined in a systematic “good calibration practice” (GCP) procedure, proposed for the first time in this work. The approach has been applied for the quantitative monitoring of the polymorphic transformation of ortho-aminobenzoic acid (OABA).