Determination of the Size Distribution of Dissolved Organic Matter by Diffusion-Ordered Nuclear Magnetic Resonance Spectroscopy (DOSY)

Marine dissolved organic matter (DOM) is a complex mixture of unique molecules with a wide range of molecular weights from <100 Da to >10⁶ Da in extremely low concentrations. This diversity has led to the development of chemical and size-dependent separation techniques to isolate and concentrate specific fractions of DOM. Separations based on size often utilize ultrafiltration (UF), electrodialysis, or size-exclusion chromatography (SEC). In this study, we reexamined the size distribution of DOM separated by UF, SEC, and solid phase extraction (SPE) using pulsed field gradient spin echo nuclear magnetic resonance (NMR) spectroscopy. Representative DOM samples from the North Pacific (National Energy Laboratory of Hawai’i and at Station ALOHA) and North Atlantic (Sargasso Sea) Oceans were analyzed using diffusion-ordered NMR spectroscopy (DOSY). Optimizations for sample concentration and gradient strength for each sample type were performed. This technique can be utilized in conjunction with size-dependent separations to better characterize the organic matter being studied. DOSY can also be coupled with heteronuclear correlation spectroscopy for functional group analysis.