Online Charge-Generation Derivatization by Electrochemical Radical Cations of Thianthrene: Mass Spectrometry Imaging of Estrogens in Biological Tissues

Estrogens play a significant role in Endocrinology and oncology. Although separation methods coupled with mass spectrometry (MS) have emerged as a powerful tool for studying estrogens, imaging the spatial distributions of estrogens is crucial but remains challenging due to its low endogenous concentration and poor ionization efficiency. Charge-generation derivatization, such as N-alkylpyridinium quaternization and S-methyl thioetherification, represents a method wherein neutral molecules involving analytes and derivatization reagents undergo chemical reactions to establish permanent charges directly onto the analytes to improve detection sensitivity. Here, we developed a novel derivatization reagent, thianthrene (TT), which enabled oxidization to radical cations ([TT]^•+) using an electrochemical method and completed the online charge-generation derivatization of estrogens on a mass spectrometry imaging platform. In this strategy, [TT]^•+ can efficiently and selectively derivatize estrogens via an electrophilic aromatic substitution reaction. Results indicated that derivatization with [TT]^•+ can significantly enhance imaging sensitivity (3 orders of magnitude), enabling the visualization of estrogen and its metabolites in ovarian and breast tissues. Furthermore, a higher mass intensity of these estrogens was captured in breast para-cancerous tissues than in cancerous tissues, which might provide estrogens spatial dimension information for further research on the initiation and progression of breast Cancer.