A portable analytical system for rapid on-site determination of total nitrogen in water

Excessive total nitrogen (TN) in the aqueous environment causes a notable negative impact on agriculture, human health, and the economy on a global scale. Conventional analytical techniques for determining TN in water usually involve long and tedious procedures with extensive sample preparation for digestion and titration. In recent years, lab-on-a-chip platforms have enabled in-situ measurements of water pollutants such as nitrate, nitrite, and ammonium. However, the digestion of organic nitrogen compounds in aqueous solutions still remains to be a challenge for portable analytical systems. In this paper, a portable TN analytical system is developed for on-site measurement of TN in a short assay time. It contains a TN reaction chamber for nitrogen digestion and reduction, and an optical measurement chamber for colorimetric determination of total nitrite. The ultraviolet-C (UVC)-thermal digestion method and the United States Environmental Protection Agency (EPA)-standard nitrate-nitrite determination method are implemented on the TN analytical system. Thorough investigations are performed to explore the optimum reaction conditions and reagent volumes in the process of TN oxidation, nitrate reduction, and nitrite detection, including oxidation time, temperature and substrate, oxidizer concentrations, nitrate reduction time, nitrite colorimetric reaction time, and reagents stability over time. Our system can complete fast oxidation and colorimetric determination of TN within 36 min, with a measurement range of 1 μg/L to 10 g/L and a limit of detection of 1.2 mg/L (lower than the World Health Organization standard of 10 mg/L). This portable TN analytical system enables the digestion and measurement of TN in a quick, portable, and low-cost manner.