Molecular-level Changes in Dissolved Organic Matter Compositions of Lake Inba Water by KMnO_4 Oxidation Assessed by Orbitrap Mass Spectrometry

摘要

Chemical oxygen demand determined by KMnO_4 (COD_(Mn)) is regulated as environmental water quality standard for lakes and reservoirs in Japan. In FY 2016, only 56.7% of the nationwide lakes and reservoirs complied with the standard. Dissolved organic matter (DOM) contributing to COD_(Mn) in lakes remain mysterious due to their heterogenous nature. Moreover, KMnO_4 shows selective oxidation efficiencies depending on substances suggesting that specific portion of DOM partially contributes to COD_(Mn). Revealing DOM compositions is useful to understand the limitation of COD_(Mn). In this study, time-course changes of molecular-level compositions of DOM in Lake Inba by KMnO_4 oxidation were evaluated by Orbitrap mass spectrometry (Orbitrap MS). Water samples were collected from Lake Inba in September, 2016. After filtrating, they were oxidized with KMnO_4 at 95°C (modified JIS method K 0102) with different oxidation duration ranging from 0-120 minutes. After solid phase extraction, DOM and oxidized DOM were analyzed by Orbitrap MS. In total, 1115 molecular formulas were assigned to DOM before oxidation. Among them, 278 formulas readily disappeared during preheating step before the temperature reached 95°C. Degree of unsaturation of those formulas were higher than the others suggesting that these unsaturated compounds contribute to dissolved COD_(Mn) in Lake Inba.