Deep UV-LED induced nitrate-to-nitrite conversion for total dissolved nitrogen determination in water samples through persulfate digestion and capillary electrophoresis

Background Capillary electrophoresis (CE) with capacitively coupled contactless conductivity detection (C 4 D) is widely used for water quality monitoring. However, there is currently no reported CE method for detecting total dissolved nitrogen (TDN), a crucial parameter for assessing water eutrophication . One challenge is the high sulfate concentration (100 mM) introduced during persulfate digestion, leading to overlap of nitrate (from TDN) and poor electrical stacking of nitrate in CE-C 4 D analysis. Results We introduced an in-capillary UV-LED induced photoreaction to convert nitrate to nitrite, which can be baseline-separated from sulfate via the CE method, enabling accurate quantification of nitrate concentration derived from nitrite. A 2 nL post-persulfate digested sample solution within a fused silica capillary was exposed to UV-LED irradiation at the capillary tip. Subsequently, photoreduction-produced nitrite was electrophoretically separated from sulfate in an acidic buffer (pH = 3.7) within the same capillary, followed by contactless conductivity detection. The nitrate-to-nitrite conversion efficiency was influenced by irradiation wavelength, power, and duration, reaching a maximum efficiency of 77.4% when employing two 230 nm LEDs for 5 min. For more general applications, two 255 nm LEDs were used, providing a conversion efficiency of (66.4 ± 3.3)% ( n  = 11) for 5 min of irradiation. The proposed CE-C 4 D method exhibits a detection limit of 13 μM (0.18 mg N/L) and has been successfully employed for TDN determination in lake water samples. Significance This innovative approach not only enhances the attractiveness of the CE-C4D method for the determination of water quality indicators but also highlights the potential for integrating deep-UV LEDs into environmental analysis .