Quantitation of trace amorphous solifenacin succinate in pharmaceutical formulations by transmission Raman spectroscopy

It is important to quantify amorphous solid in active pharmaceutical ingredients (API) of formulations to guarantee high-quality pharmaceutical products. In this study, we aimed to quantify amorphous solid in API by transmission Raman spectroscopy (TRS) and transmission near-infra-red spectroscopy (NIRS). To quantify the amount of trace amorphous solids contained in the API of a formulation, calibration curves of tablets containing 40% or 3% solifenacin succinate (SLFS) in the pharmaceutical formulations were prepared. The tablets varied in the ratio of amorphous solids in the API. TRS and NIRS were carried out with the calibration tablets and partial least square regression (PLSR) analysis was performed. Good results were obtained by both TRS and NIRS with the calibration model containing 40% SLFS. On the contrary, with the calibration model containing 3% SLFS, the PLSR analysis results using the TRS data were better than those using the NIRS data. The low content calibration model based on TRS showed good results with R-2 of 0.999, RMSECV of 1.236, and LOD of 0.12. TRS can be used to specifically detect trace amorphous solids contained in the API of a formulation.