Marine dissolved organic nitrogen (DON) represents one of the least understood nitrogen pools in global nitrogen cycles, comprising thousands of nitrogen-containing molecules. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been employed to identify the molecular composition of DON across various environments. Coupling FT-ICR MS with high-performance liquid chromatography (HPLC) enables a more detailed analysis of DON's molecular compositions. This study utilizes both direct injection (DI-) and liquid chromatography (LC-) FT-ICR MS to investigate DON cycling along a river-to-ocean continuum in the Pearl River Estuary. HPLC separates DON based on polarity, providing a more comprehensive molecular analysis. The molecular composition identified by LC-FT-ICR MS was categorized into nine segments according to polarity. LC-FT-ICR MS detected a greater number of nitrogen-containing formulas compared to DI-FT-ICR MS, with many low-polarity formulas undetected by DI-FT-ICR MS. Additionally, nitrogen-containing formulas exhibited higher polarity than other formulas, suggesting that polar sorbents may more effectively in enriching DON. This study highligts the advancement of LC-FT-ICR MS in DON studes and enhance our understanding of its dynamics in marine environments.
 

Dissolved organic nitrogen,LC-FT-ICR MS,Pearl River Estuary