Analytical procedure to determine both δ 18O and δ 17O of H 2O 2 in natural water and first measurements

An experimental technique has been developed to determine both δ ¹⁸O and δ ¹⁷O of hydrogen peroxide (H ₂O ₂) in natural water. The goal of this measurement is to develop a better understanding of peroxide chemical transformations in the atmosphere. Of particular relevance is the need for these measurements to interpret mass-independent isotopic anomalies observed in atmospheric aerosol sulfate. Combined peroxide and sulfate isotopic measurements may provide a new means to resolve atmospheric oxidative pathways. The technique utilizes the well-known reaction of H ₂O ₂ with KMnO ₄ in sulfuric acid solution, which oxidizes H ₂O ₂ to O ₂. Four liters of water sample (∼4 l) are degassed by a combination of helium sparging, ultrasonic agitation and pumping to quantitatively remove air dissolved in water. KMnO ₄ is then added to the solution and O ₂ produced during the oxidation reaction is recovered. Molecular oxygen is trapped and cryogenically purified in a series of molecular sieves for isotopic measurement. Industrially manufactured H ₂O ₂, sulfite oxidation by H ₂O ₂, auto-decomposition and partial oxidation of H ₂O ₂ show that all these samples are mass dependent, and obey the relationship δ ¹⁷O=0.511δ ¹⁸O. Rainwater samples collected at La Jolla, CA were analyzed using this method. The H ₂O ₂ is mass-independently fractionated and δ ¹⁸O range from 21.9 to 52.5‰. We suggest that this range is due to the oxidation of sulfite in rain droplet and the mass-independent fractionation a result of gas-phase formation of H ₂O ₂. This is the first observation of a mass-independent isotopic composition in hydrogen peroxide.