High-frequency ammonium monitoring in coastal seawater via a flow-batch system featuring a heating reaction coil

Ammonium (NH₄⁺) 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 NH₄⁺ 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 samples 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 (R² = 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 a salinity range of 0-35) and common seawater matrix ions. Recovery rates in environmental samples ranged from 92.5% to 106.0%, and results correlated strongly (R² = 0.9912) with a reference method. The proposed system provides a practical, accurate, and high-throughput tool for high-frequency NH₄⁺ monitoring in dynamic marine environments.