Abstract: As a key component of the marine nitrogen cycle, ammonium is regulated by complex physical, biological, and chemical processes in estuarine and coastal waters. This results in dynamic ammonium distributions in coastal waters with the concentrations typically at trace(sub-micromolar to nanomolar)levels, which largely challenges the analytical technique of ammonium in seawaters. In this study, we developed and validated a segmented continuous flow analysis method with fluorometric detection to determine ammonium concentrations in seawater based on the reactions of o-phthaldialdehyde and sulfite. The quality of this method has been tested: The linearity of it has been observed in the range of 0.025~10.0μmol·L^-1, with a detection limit of up to 0.024μmol·L^-1; The relative standard deviation ranged from 0.3% to 2.1% in the concentrations of 0.050~2.00μmol·L^-1; Spike recoveries at the level of 0.100~0.500μmol·L^-1 were between 91.0% and 99.8%. A deviation of less than 4.0% was detected due to the interference by salt effect. This method was further applied to determine ammonium concentrations in the Pearl River Estuary. A comparison with the established flow injection analysis coupled with solid-phase extraction and fluorometric detection proved that the proposed method offers comparable sensitivity and accuracy, along with the advantage in simplified operation and more rapid analysis. Therefore, this new method is more suited for high-throughput, in-situ determination of trace level ammonium, representing a promising technique for the analysis of ammonium in estuary, coastal oceans, and open oceans.

Key words: Ammonium, Segmented continuous flow analysis, Fluorometric detection, OPA method, Nitrogen biogeochemical cycling