An in-situ analyzer based on CCD spectral integration for enhanced dynamic range fluorometric detection of ammonium in seawater

Accurate measurements of ammonium (NH₄⁺) are essential for elucidating the nitrogen cycle, assessing eutrophication and supporting sustainable marine ecosystem management. In this study, we present a fully automated in-situ ammonium analyzer based on fluorometric detection combined with reverse flow injection analysis (rFIA). The system integrates a CCD detector with digitally optimized spectral-band integration to improve signal extraction and achieve a wide detection range. The analyzer achieves a broad analytical range from 0.13 μmol/L to 50 μmol/L, with a calculated detection limit of 0.043 μmol/L and precision better than 5% RSD above the quantification limit, enabling reliable quantification of both background concentrations in open oceans and elevated levels in eutrophic coastal waters. Designed for long-term deployments, the sensor features a waterproof housing and optimized reagent preservation to maintain analytical stability over extended periods. Laboratory and field deployments demonstrate high sensitivity, accuracy and stability, enabling autonomous ammonium monitoring in seawaters with varying salinities at an analytical frequency of approximately one measurement per hour. This work provides a reliable analytical tool for real-time in-situ measurements of ammonium and contributes to improved understanding of nitrogen biogeochemistry and ecosystem responses to environmental change.