Use of high resolution LC–MS analysis in conjunction with mechanism-based stress studies: Identification of asarinin, an impurity from sesame oil in an animal health product

During analysis of certain stability batches of an animal health product, an unknown peak was found at a level above the identification thresholds set by VICH. This unknown species is extremely labile in the gas phase under normal electrospray ionization (ESI) mass spectrometric condition. Multiple ions were detected with no clear indication of which one is the molecular ion. To overcome this challenge, we utilized tandem MS/MS analysis and multiple MS instruments. The slightly different ionization processes between the two different instruments provided strong, complementary evidence leading to the identification of the correct molecular ion. Based on the formula thus determined, the unknown species was found to be related to sesame oil, which is one of the major excipients used in this drug product. The unknown species was eventually identified as asarinin using high resolution LC–MSⁿ in conjunction with mechanism-based stress studies, in which the unknown species was generated based on the degradation chemistry of sesamin as revealed by the LC–MSⁿ analysis. This overall approach in combining LC–MSⁿ analysis along with mechanism-based stress studies can be used as a general strategy for identification of unknown pharmaceutical impurities, especially the degradants related to the active pharmaceutical ingredient (API) and excipients.