Molecular-Level Changes in Dissolved Organic Matter Compositions in Lake Inba Water during KMnO₄ Oxidation: Assessment by Orbitrap Mass Spectrometry

抄録

In 2016, only 56.7% of the lakes and reservoirs in Japan complied with the standard of chemical oxygen demand determined by permanganate (COD_Mn). The dissolved organic matter (DOM) contributing to COD_Mn in lakes is heterogenous in nature; hence, DOM compositions in different lake waters are not well known. In this study, molecular components contributing to dissolved COD_Mn (DCOD_Mn) in lake water were identified via high-resolution Orbitrap mass spectrometry (Orbitrap MS). Water samples were collected from Lake Inba in September 2016. The filtered samples were oxidized by KMnO₄ according to the JIS method K0102 for different durations (ranging between 0–120 min). The DOM extracted via solid phase extraction was analyzed using Orbitrap MS, and molecular formulas were assigned. In all, 1,115 components were observed before oxidation. During time-course oxidation, components with higher unsaturation were selectively oxidized, suggesting that they contributed to the DCOD_Mn in lake water. DCOD_Mn components were screened for DOM in Lake Inba, and other DOM references were isolated from the Suwannee River and Lake Biwa. Only 14%–32% of the total components were regarded as DCOD_Mn components, indicating the limited coverage of COD_Mn. The compositions of DCOD_Mn components were highly diverse, depending on the sources.