Hydrogen/Deuterium Exchange To Differentiate Fragment Ions from Pseudomolecular Ions by Electrospray Tandem Mass Spectrometry

Fragmentation of protonated molecules can occur for some compounds even when using gentle electrospray ionization conditions. For many compounds, the fragment ions generated during desolvation (in-source dissociation) cannot be distinguished from protonated molecules (pseudomolecular ions), such as an impurity, decomposition product, metabolite, or reaction byproduct, which have the same structure. This application of hydrogen/deuterium (H/D) exchange allows fragment ions formed from in-source dissociation to be distinguished from pseudomolecular ions using H/D exchange. This technique is based on fragmentation processes which involve neutral losses arising from cleavage of carbon−heteroatom bonds. In these situations, the fragment ion does not have all active hydrogens exchanged and can be differentiated from a fully deuterium-exchanged molecule by 1 Da. For some analyses, the method being reported may be more suitable than chromatographic methods.