Selective Solid Phase Extraction Using a Mixed-Mode Cation Exchange Cartridge Facilitates the Mass Spectrometry Analysis on Dissolved Organic Nitrogen

Rationale: Dissolved organic nitrogen (DON) is a crucial component in environment, which acts as the largest reservoir of nitrogen and plays a significant role in the nitrogen cycling, pollutant transport, and substrate utilization among various environmental systems. DON exhibits a relatively low concentration in environment, which presents a great challenge for DON detection, and an efficient separation and enrichment lab protocol is required to fully understand its structural and compositional characteristics. However, there is no standard method to extract DON from complex environmental samples efficiently. Methods: The DON was extracted utilizing solid-phase extraction (SPE), with a one-step elution by the Bond Elut PPL cartridge and the Waters HLB cartridge, and a three-step elution by the Waters MCX cartridge. UV-Vis, fluorescence, mass spectrometry (MS), and gas chromatography-nitrogen chemiluminescence detector (GC-NCD) techniques were utilized to investigate and compare the characteristics of DON from the three different SPE strategies. Results: Combined the fluorescence and MS results, it is found that the stepwise extraction using the MCX cartridge exhibits the best recovery of DON for both standard and environmental samples, and its performance is less affected by the chemical interferences such as surfactants during MS analysis. Furthermore, the MCX fraction exhibits the highest number of DON molecular formulas with a low O/C ratio and a high H/C ratio in environmental samples, and such a fraction also shows an enrichment of nitrosamine-type substances. Conclusions: This work establishes an efficient MCX-SPE protocol to extract and analyze DON, which can be applied to environmental samples straightforward. The presented work provides a theoretical support for the analysis of DON, which facilitates a comprehensive understanding of the chemical composition and environmental effect of nitrogen-containing substances.