Comparison of low molecular weight dissolved organic matter compositions in Lake Inba and Kashima river by orbitrap mass spectrometry

Abstract

Lake Inba, which serves as a drinking water source in Chiba Prefecture, Japan, has not complied with the environmental standard for COD (3 mg/L) since the 1970s. The high level of dissolved organic matter (DOM) caused by eutrophication in the lake has risen the concern of water treatment. To understand the compositions and sources of DOM in Lake Inba, this study compared molecular compositions of low molecular weight DOM (LMW-DOM, < 1 kDa) in the lake and a major river flowing to the lake. Water samples were collected from Lake Inba and Kashima River in June 2015. After solidphase extraction, LMW-DOM was analyzed by Orbitrap mass spectrometry coupled with electrospray ionization. Based on the accurate mass data, 1,263 and 1,393 molecular formulae were assigned for the lake and river samples, respectively. Among them, 1,193 formulae were shared between the lake and the river, indicating that Kashima River was the representative source of LMW-DOM in the lake at the sampling occasion. Elemental composition of molecular formula demonstrated that 61% of the shared formulae were composed of carbon, hydrogen and oxygen. Hydrogen to carbon ratio and oxygen to carbon ratio of these common molecules were similar to the fingerprints of lignin and tannin.