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 (1 H 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 1 H NMR with a direct noninvasive 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 preanalysis 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 with 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 noninvasive fashion outweighs the attraction of reduced analysis times for preconcentrated samples, particularly for studies interested in investigating the low molecular weight fraction of DOM.
Keywords: dissolved organic matter; freeze-dried DOM; headwater streams; lyophilization; proton NMR; stream water DOM; whole water analysis.
© 2019 John Wiley & Sons, Ltd.