Determining Olefinic and Phenolic Fractions in Dissolved Organic Matter by Ozonation with Stable Oxygen Isotope Analysis of H ₂ O ₂

Reactivity-related characterization of dissolved organic matter (DOM) is crucial for mitigating undesirable byproduct formation during ozonation. However, the characterization of DOM remains challenging due to its molecular complexity, especially in distinguishing olefin- and phenol-type moieties, primary O3 reactive sites responsible for generating carbonyl-containing byproducts. Here, we present a novel approach based on stable oxygen isotope analysis of H2O2, a common byproduct of ozone reactions, to differentiate between these two moieties. The natural abundance oxygen isotopic signatures of residual H2O2 (δ18OH2O2) after ozonation with 13 model compounds (6 olefins and 7 phenols) at pH 3 and 7, and their pH-dependency, Δ18OH2O2 (δ18OH2O2 (pH 7) - δ18OH2O2 (pH 3)), enabled the distinction of olefins and phenols. Olefins exhibited near-zero or positive Δ18OH2O2 (0.4-9.0‰), whereas phenols showed negative Δ18OH2O2 (-14.2 - -1.6‰). These contrasting trends allowed derivation of an empirical correlation linking Δ18OH2O2 to olefinic-phenolic fractions in mixtures, resulting in estimation of molar fractions of the two moieties in DOM isolates. This stable-isotope based approach offers unique mechanistic insights into H2O2 formation mechanisms for widespread applications in H2O2-generating reactions, as well as for synergistic use with existing DOM characterization techniques.