Mass-independent fractionation of oxygen isotopes during H2O2 formation by gas-phase discharge from water vapour

Hydrogen peroxide is an important atmospheric component involved in various gas-phase and aqueous-phase transformation processes in the Earth's atmosphere. A study of mass-independent O-17 anomalies in H2O2 can provide additional insights into the chemistry of the modern atmosphere and, possibly, of the ancient atmosphere. Here, we report the results of laboratory experiments to study the fractionation of three oxygen isotopes (O-16, O-17, and O-18) during H2O2 formation from products of water vapour dissociation. The experiments were carried out by passing an electrical discharge through a gaseous mixture of helium and water at atmospheric pressure. The effect of the presence of O-2 in the gas mixture on the isotopic composition of H2O2 was also investigated. All of the experiments showed that H2O2 produced under two different conditions (with or without O-2 added in the gas mixtures) was mass-independently fractionated (MIF). We found a positive MIF signal (similar to 1.4 parts per thousand) in the no-O-2 added experiments, and this signal increased to similar to 2.5 parts per thousand once O-2 was added (1.6% mixing ratio). We suggest that if O-2 concentrations are very low, the hydroxyl radical recombination reaction is the dominant pathway for H2O2 formation and is the source of MIF in H2O2. Although H2O2 formation via a hydroxyl radical recombination process is limited in the modern atmosphere, it would be possible in the Archean atmosphere when O-2 was a trace constituent, and H2O2 would be mass-independently fractionated. The anomalous O-17 excess, which was observed in H2O2 produced by spark discharge experiments, may provide useful information about the radical chemistry of the ancient atmosphere and the role of H2O2 in maintaining and controlling the atmospheric composition. (C) 2016 Elsevier Ltd. All rights reserved.