Ghost Peaks of Aromatic Metabolites Induced by Corona Discharge Artifacts in LC-ESI-MS/MS

LC-ESI-MS/MS is a preferred method for detecting and identifying metabolites, including those that are unpredictable from the genome, especially in basal metazoans like Cnidaria, which diverged earlier than bilaterians and whose metabolism is poorly understood. However, the unexpected appearance of a "ghost peak" for dopamine, which exhibited the same m/z value and MS/MS product ion spectrum during an analysis of Nematostella vectensis, a model cnidarian, complicated its accurate identification. Understanding the mechanism by which "ghost peaks" appear is crucial to accurately identify the monoamine repertoire in early animals so as to avoid misassignments. Verification experiments showed that in-source oxidation of tyramine, which produced an intense signal, was responsible for this "ghost peak." This artifact commonly occurs among aromatic compounds with high signal intensities and appears at the same m/z as their respective in vivo oxidized metabolites. In metabolomics, spectra contain diverse signals from complex biological mixtures, making it difficult to recognize artifact peaks. To prevent misassignments, despite +16 Da differences, adequate chromatographic separation of metabolites from their respective in vivo oxidation precursors is necessary. Whereas both electrolysis and gas-phase corona discharge can cause in-source oxidation in ESI, corona discharge proved to be the dominant factor. Additionally, the presence of multiple oxygen atom sources was suggested by the voltage-dependent mass shift of +16 Da to +18 Da of the "ghost peak" when using ¹⁸O-labeled water as a solvent. Accurate metabolite identification using LC-ESI-MS/MS requires accounting for in-source products that can mimic in vivo products.