New publication in Organic Geochemistry by Aristilde, Guzman, Klein, and Balkind


TITLE: Compound-Specific Short-Chain Carboxylic Acids Identified in Peat Dissolved Organic Matter using High-Resolution Liquid Chromatography-Mass Spectrometry

ABSTRACT: Dissolved natural organic matter (DNOM) is primarily composed of exuded or remnant biomolecules from plants and microorganisms. Labile amino acids and sugars have been well documented in the low-molecular-weight components of DNOM. However, little attention has been devoted to the isolation of labile short-chain carboxylic acids (SSCAs), which have demonstrated biogeochemical significance as assimilable carbons, promoters of mineral dissolution, and ligands for metal complexation. Here we present an analytical method that identifies compound-specific SSCAs in a peat-derived DNOM isolate, Pahokee peat humic acid. Using high-resolution liquid chromatography (LC) coupled with high-accurate orbitrap mass spectrometry (MS), we targeted the identification of five SSCAs of different types: gluconate (a C6 monocarboxylic acid), citrate (a branched C6 tricarboxylic acid), 2-ketoglutarate (a C5 dicarboxylic acid), and malate and fumarate (C4 dicarboxylic acids). Following LC-separation and electrospray ionization, the compounds were annotated directly by orbitrap MS using their exact mass-over-charge (m/z) ions in the negative mode and their stoichiometric composition. Validated by LC-MS metabolite annotation in a bacterial matrix, we achieved identification of all five compounds in the peat DNOM isolate. Each targeted m/z channel also captured non-targeted compounds at different retention times, which represent compound isomers or different compounds. We found that the five targeted and the two non-targeted SSCAs identified collectively accounted for high parts-per-million to low parts-per-thousand of the total carbon, oxygen, or carboxyl content. Building on these findings, an important next step is to obtain a comprehensive profiling of SSCA structures in DNOM of different origins. [Link to Article]