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 (CD) 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-CD 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-CD 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.