Dissolved Organic\nMatter Contains Ketones Across a\nWide Range of Molecular Formulas

The carbonyl functionality of natural organic matter\n(NOM) is poorly\nconstrained. Here, we treated Suwannee River NOM (SRNOM) with ammonium\nacetate and sodium cyanoborohydride to convert ketone-containing compounds\nby reductive amination to their corresponding primary amines. The\ntotal dissolved nitrogen content increased by up to 275% after amination.\nUp to 30% of the molecular formulas of SRNOM contained isomers with\nketone functionalities as detected by ultrahigh-resolution mass spectrometry.\nMost of these isomers contained one or two keto groups. At least 3.5%\nof the oxygen in SRNOM was bound in ketone moieties. The conversion\nof reacted compounds increased linearly with O/H values of molecular\nformulas and was predictable from the elemental composition. The mean\nconversion rate of reacted compounds nearly followed a log-normal\ndistribution. This distribution and the predictability of the proportion\nof ketone-containing isomers solely based on the molecular formula\nindicated a stochastic distribution of ketones across SRNOM compounds.\nWe obtained isotopically labeled amines by using <sup>15</sup>N-labeled\nammonium acetate, facilitating the identification of reaction products\nand enabling NMR spectroscopic analysis. <sup>1</sup>H,<sup>15</sup>N HSQC NMR experiments of derivatized samples containing less than\n20 μg of nitrogen confirmed the predominant formation of primary\namines, as expected from the reaction pathway.