Whitty, S.D., D.C. Waggoner, R.M. Cory, L.A. Kaplan, and P.G. Hatcher. 2019. Magnetic Resonance in Chemistry, early online access.
NMR spectroscopy is widely used in the field of aquatic biogeochemistry to examine the chemical structure of dissolved organic matter (DOM). Most aquatic DOM analyzed by proton NMR (1H NMR) is concentrated mainly by freeze‐drying prior to analysis to combat low concentrations, frequently <100 μM C, and eliminate interference from water. This study examines stream water with low dissolved organic carbon content by 1H NMR with a direct non‐invasive analysis of whole water using a water suppression technique. Surface waters, collected from the headwaters of the Rio Tempisquito, Costa Rica, were examined directly, and the spectral characteristics were compared with those of the traditional pre‐analysis freeze‐drying approach revealing significant differences in the relative intensity of peaks between the whole water and freeze‐dried DOM. The freeze‐dried DOM required less time to obtain quality spectra, but several peaks were missing compared to the spectra of whole water DOM; notably the most dominant peak in the spectrum constituting roughly 10% of the DOM. The stream water DOM showed an increase in the relative intensity of aliphatic methyl and methylene groups and a decrease in carbonyl, carboxyl, and carbohydrate functionalities after freeze‐drying. The results of this study show that freeze‐drying alters the original composition of DOM and thus freeze‐dried DOM may not represent the original DOM. The information gained from whole water analysis of stream water DOM in a non‐invasive fashion outweighs the attraction of reduced analysis times for pre‐concentrated samples, particularly for studies interested in investigating the low molecular weight fraction of DOM.