Detection and Characterization of Water Radical Cations by Mass Spectrometry and Electron Paramagnetic Resonance Spectroscopy

Water radical cations (H₂O^+•) serve as key reactive intermediates in fundamental chemical processes, atmospheric chemistry, and energy-related applications due to their high reactivity and ability to initiate diverse chemical reactions. In this study, we investigated the generation of water dimer radical cations (<i>m</i>/<i>z</i> 36, (H₂O)₂^+•) and their reaction products with different substrates using multiple soft ionization techniques. Our comprehensive analytical platform incorporates corona discharge ionization (CDI), electrospray ionization (ESI), dielectric barrier discharge ionization (DBDI), and microwave plasma torch (MPT), coupled with complementary mass analyzers, including ion trap, Orbitrap, and quadrupole time-of-flight (Q-TOF) mass spectrometers. To confirm the formation of (H₂O)₂^+•, we employed a multitechnique verification strategy combining high-resolution mass spectrometry (MS), isotope labeling technique, tandem MS, and electron paramagnetic resonance (EPR) spectroscopy. The stabilization and reactivity of H₂O^+• were also examined by using different substrates.