Simultaneous determination of dissolved organic nitrogen, nitrite, nitrate and ammonia using size exclusion chromatography coupled with nitrogen detector

Accurate quantification of dissolved organic nitrogen(DON) has been a challenge due to the cumulative analytical errors in the conventional method via subtracting dissolved inorganic nitrogen species(DIN) from total dissolved nitrogen(TDN). Size exclusion chromatography coupled with an organic nitrogen detector(SEC-OND) has been developed as a direct method for quantification and characterization of DON. However, the applications of SECOND method still subject to poor separations between DON and DIN species and unsatisfied N recoveries of macromolecules. In this study, we packed a series of SEC columns with different lengths and resin materials for separation of different N species and designed an independent vacuum ultraviolet(VUV) oxidation device for complete oxidation converting N species to nitrate. To guarantee sufficient N recoveries, the operation conditions were optimized as oxidation time ≥ 30 min, injection mass(sample concentration × injection volume) < 1000 μL × mg-N/L for macromolecular proteins, and neutral p H mobile eluent. The dissolved O2concentration in SEC mobile phase determined the upper limit of VUV oxidation at a specific oxidation time. Compared to conventional HW50S column(20 × 250 mm),HW40S column(20 × 350 mm) with mobile phase comprising of 1.5 g/L Na2HPO4·2H2O + 2.5g/L KH2PO4(p H = 6.85) could achieve a better separation of DON, nitrite, nitrate, and ammonia. When applied to river water, lake water, wastewater effluent, groundwater, and landfill leachate, the SEC-OND method could quantify DON as well as DIN species accurately and conveniently even the DIN/TDN ratio reached 0.98.