Identification of a drug degradation product found in a stressed dosage form using LC/MSⁿ, LC/TOF MS and on-line H/D exchange MS

An unknown degradation product was found when a dosage form was stressed under base and heat. Comprehensive LC–MS studies have been conducted to identify this degradation product. The approach included the use of an ion-trap MS for MSⁿ ion fragmentation patterns, a time-of-flight (TOF) MS to measure the accurate mass for its potential chemical formula, and on-line hydrogen/deuterium (H/D) exchange LC–MS to determine the number of exchangeable hydrogen atoms in the gradation product. Based upon the above LC–MS results, the unknown was identified to result from the conversion of a trifluoromethyl moiety in the drug substance to a carboxylic acid under the combination of thermal and base stress. Different ion fragmentation pathways between the drug substance and its degradation product were discussed. The reaction mechanism was proposed to be nucleophilic substitution through SN2 mechanism.