LC–MS analysis of low molecular weight organic acids derived from root exudation

A sensitive method for quantification of citric, fumaric, malic, malonic, oxalic, trans aconitic, and succinic acid in soil- and root-related samples is presented. The method is based on a novel, fast, and simple esterification procedure and subsequent analysis via liquid chromatography–mass spectrometry. Derivatization comprises in situ generation of HCl, which catalyzes the Fischer esterification with benzyl alcohol. As a key advance, the esterification with the aromate allows reversed-phase separation and improves electrospray ionization efficiency. The method provided procedural detection limits of 1 nM for citric, 47 nM for fumaric, 10 nM for malic, 10 nM for malonic, 16 nM for oxalic, 15 nM for succinic, and 2 nM for aconitic acid utilizing 500 μL of liquid sample. The working range was 3 nM to 10 μM for citric acid, 158 nM to 10 μM for fumaric acid, 34 nM to 10 μM for malic acid, 33 nM to 10 μM for malonic acid, 53 nM to 10 μM for oxalic acid, 48 nM to 10 μM for succinic acid, and 6 nM to 10 μM for aconitic acid. Quantification of the analytes in soil-related samples was performed via external calibration of the entire procedure utilizing ¹³C-labeled oxalic and citric acid as internal standards. The robustness of the method was tested with soil extracts and samples from hydroponic experiments. The latter concerned the regulation of phosphorus solubilization via plant root exudation of citric, malic, and oxalic acid.     

A Reactive Molecular Dynamics Study of Chlorinated Organic Compounds. Part I: Force Field Development

This work presents a novel parametrization for the ReaxFF formalism as a means to investigate reaction processes of chlorinated organic compounds. Force field parameters cover the chemical elements C, H, O, Cl and were obtained using a novel optimization approach involving relaxed potential energy surface scans as training targets. The resulting ReaxFF parametrization shows good transferability, as demonstrated on two independent ab initio validation sets. While this first part of our two-paper series focuses on force field parametrization, we apply our parameters to the simulation of chlorinated dibenzofuran formation and decomposition processes in Part II.