Action Spectroscopy of Mass-Selected Ions-From Principles to Applications.

Gas-phase ion spectroscopy provides direct, structure-sensitive information that complements and extends conventional mass spectrometry workflows, offering structural resolution that accurate mass and fragmentation patterns alone cannot always achieve by themselves. This review offers a practical entry point into the field by showcasing its analytical capabilities, addressed to mass spectrometrists and researchers from adjacent disciplines for whom ion spectroscopy represents an underexplored analytical resource. Four representative techniques are discussed: Electronic UV/Vis photodissociation (UVPD), infrared multiple photon dissociation spectroscopy (IRMPD), vibrational tagging photodissociation ion spectroscopy (IRPD), and helium nanodroplet isolation infrared spectroscopy (HENDI-IR). For each, the underlying physical principles are briefly introduced, and representative applications are presented with emphasis on problems of direct analytical relevance, including the structural characterization of isomeric metabolites, reactive intermediates, glycoconjugates, and drug-related compounds. Applications integrating ion spectroscopy with liquid chromatography (LC) and ion mobility spectrometry (IMS) are also discussed. The goal of this review is to convey, through carefully chosen examples, the breadth of problems that gas-phase ion spectroscopy is uniquely positioned to solve and to lower the threshold for its adoption by a wider scientific community.