High-Frequency Ammonium Monitoring in Coastal Seawater via a Flow-Batch System Featuring a Heating Reaction Coil

Abstract

Ammonium (NH 4 + ) is a critical nutrient in marine ecosystems, and its high-frequency monitoring is essential for understanding dynamic biogeochemical processes. This study presents an optimized flow-batch analysis (FBA) system for the rapid and reliable spectrophotometric determination of NH 4 + in coastal and estuarine waters based on the indophenol blue (IPB) reaction using o-phenylphenol (OPP) as a substitute for toxic phenol. The key innovation is the integration of a thermostatic heating reaction coil, which significantly accelerates reaction kinetics, reducing the incubation time to 25 s. A layered injection protocol and a novel mixing coil with periodic indentations ensure efficient homogenization of sample and reagents. Under optimized conditions-including reagent concentrations carefully tuned to minimize salinity effects-the system achieves a sample throughput of 17 h -1 , representing a 42% improvement over previous methods. The method exhibits excellent linearity (R 2 = 0.9999) over a range of 0-8 μmo•L -1 , a low detection limit of 0.06 μmol•L -1 , and high precision (RSD = 1.7% at 2 μmol•L -1 ). Crucially, it demonstrates robust tolerance to salinity variations (slope RSD = 2.7% over salinity 0-35) and common seawater matrix ions. Recovery rates in environmental samples ranged from 92.5% to 106.0%, and results correlated strongly (R 2 = 0.9912) with a reference method. The proposed system provides a practical, accurate, and high-throughput tool for high-frequency NH 4 + monitoring in dynamic marine environments.