Dissolved Organic Matter Contains Ketones Across a Wide Range of Molecular Formulas

The carbonyl functionality of natural organic matter (NOM) is poorly constrained. Here, we treated Suwannee River NOM (SRNOM) with ammonium acetate and sodium cyanoborohydride to convert ketone-containing compounds by reductive amination to their corresponding primary amines. The total dissolved nitrogen content increased by up to 275% after amination. Up to 30% of the molecular formulas of SRNOM contained isomers with ketone functionalities as detected by ultrahigh-resolution mass spectrometry. Most of these isomers contained one or two keto groups. At least 3.5% of the oxygen in SRNOM was bound in ketone moieties. The conversion of reacted compounds increased linearly with O/H values of molecular formulas and was predictable from the elemental composition. The mean conversion rate of reacted compounds nearly followed a log-normal distribution. This distribution and the predictability of the proportion of ketone-containing isomers solely based on the molecular formula indicated a stochastic distribution of ketones across SRNOM compounds. We obtained isotopically labeled amines by using ¹⁵N-labeled ammonium acetate, facilitating the identification of reaction products and enabling NMR spectroscopic analysis. ¹H,¹⁵N HSQC NMR experiments of derivatized samples containing less than 20 μg of nitrogen confirmed the predominant formation of primary amines, as expected from the reaction pathway.