Characterization and demonstration of drug compound ring-chain tautomer formation and its impacts on quality control

•Use of advanced experimental analytical toolbox and predictive science demonstrate the tautomer equilibrium.•Elucidation of three peeks being one compound, conclude that any detected tautomer should be considered as main compound.•Chromatographic conditions and sample preparation are shown to significantly impact the tautomer equilibrium.•The importance of structural information in chromatographic method development is discussed.•Almost complete loss of water from AZD5718 in the electrospray ionization is closely related to the tautomer equilibrium.