ULTRA-HIGH RESOLUTION FTICR MASS SPECTROMETRY FOR THE MOLECULAR-LEVEL CHARACTERIZATION OF DISSOLVED ORGANIC NITROGEN IN HUMAN-DOMINATED WATERSHEDS

Dissolved organic nitrogen (DON) is an important fraction of the total nitrogen pool in aquatic systems and can be a significant source of nitrogen to the phytoplankton and bacteria that cause problems such as eutrophication and algal blooms. DON is a complex mixture of thousands of potential compounds that are to date largely uncharacterized, especially in urban landscapes. Fourier transform ion cyclotron resonance (FTICR) mass spectrometry with high magnetic fields is an advanced analytical tool appropriate for characterizing complex mixtures such as DON. We used ultra-high resolution FTICR mass spectrometry to characterize the molecular composition of DON from urban stormwater runoff, an urban stormwater treatment pond, an urban stream, and an agricultural stream. We also conducted a 5-day bioassay of water from the streams to compare DON formulas before and after biodegradation. We then used van Krevelen diagrams to plot H:C vs. O:C of identified compounds and to visually divide their molecular formulas into biomolecules such as lipids, proteins, amino sugars, and lignins. The urban runoff and pond samples had 88% overlap in identified molecular formulas. This indicates that runoff DON moves through the treatment pond largely unchanged. Both the runoff and the pond, however, are carrying compounds characteristic of the highly bioavailable lipids, proteins, and amino sugars, suggesting that urban runoff and water from the pond are sources of reactive DON to downstream waters. Both the urban and the agricultural stream contained formulas expected of bioavailable DON as well as numerous low H:C formulas that are typically considered characteristic of more refractory lignin-like material. The bioassay showed that many of the lipids, proteins, and amino sugars were consumed, as well as lignin-like compounds with O:C ratios less than 0.5. Thus, even some lignin-like formulas often thought of as refractory were reactive over the 5-day bioassay.