In Situ Mass Spectrometric Detection of Interfacial Intermediates in the Oxidation of RCOOH(aq) by Gas-Phase OH-Radicals

Products and intermediates of the oxidation of aqueous alkanoic acids initiated by gas-phase hydroxyl radicals, ·OH(g), at the air-water interface were detected by mass spectrometry in a novel setup under various experimental conditions. Exposure of submillimolar RCOOH (R = methyl, n-pentyl, n-heptyl) aqueous microjets to ∼10 ns ·OH(g) pulses from the 266 nm laser flash photolysis of O3(g)/O2(g)/H2O(g) mixtures yielded an array of interfacial species that were unambiguously and simultaneously identified in situ by online electrospray mass spectrometry. We found that peroxyl radicals R(-H)(COO(-))OO· react within 50 μs to produce alcohols R(-H)(COO(-))OH and carbonyls R(-2H)(COO(-))═O via competitive Russell and Bennett-Summers mechanisms. We confirmed the formation of hydroperoxides R(-H)(COO(-))OOH in experiments performed in D2O. To our knowledge, this is the first report on the prompt and simultaneous detection of products and peroxyl/peroxide intermediates in the heterogeneous oxidation of aqueous organics initiated by ·OH(g).