Measuring Nitrite and Nitrate in Rain and River Water Samples Using a Portable Ion Chromatograph in Step-gradient Mode and High Sensitivity Detection Flow Cell

Abstract

With the increasing environmental pollution issues, there is a growing need for sensitive and real-time monitoring of pollutants. Nitrite and nitrate are common nutrients that are related to water quality. This study aims to enhance nitrate and nitrite detection capabilities using a portable ion chromatography-based nutrient analyzer, Aquamonitrix. By optimizing it for ultra-low detection limits (LODs), we address challenges in environmental water quality assessment in Tasmania, Australia. Using step-gradient mode with a stereolithography three-dimensional printed flow cell with a 5 cm optical path length, a 300 µL injection loop, and 60 mM KOH as eluent, LODs of 0.004 µg/mL for nitrite and 0.023 µg/mL for nitrate were achieved. Further improving to 0.008 µg/mL for nitrate with a 10 cm optical path length flow cell and 120 mM NaCl as eluent. A repeatability assessment over 84 automatic runs showed a relative standard deviation under 1.42% for peak area and 0.49% for retention time. The system demonstrated tolerance to salinity, handling up to 5 parts per thousand in artificial seawater. Comparative analysis of environmental samples revealed that nitrate levels in Tasmanian rainwater were five times lower than in Ireland. An average concentration of 2.08 µg/mL nitrate was found in Tamar River samples, aligning with local commercial lab data. Real-time, on-site analysis along the Derwent River detected an average nitrate concentration of 0.17 µg/mL. Validation against conventional standard ion chromatography showed no significant differences (p > 0.05), underscoring Aquamonitrix's robustness for field-based water quality monitoring. A 5-day deployment of Aquamonitrix further demonstrated the system's reliability under significant temperature fluctuations between day and night.