Real-time underway measurement of ammonium in coastal and estuarine waters using an automated flow analyzer with hollow fiber membrane contactor

Abstract

Ammonium (NH4+) is an important parameter for aquatic ecosystems. To date, continuous and underway acquisition of NH4+ in coastal and estuarine waters has been challenged by the strongly varying salinity and complex matrices in these waters. To address these issues, a hollow fiber membrane contactor (HFMC) was constructed and incorporated in flow injection analysis (FIA) to achieve online separation/preconcentration of NH4+ in water. In the FIA-HFMC system, NH4+ in the water sample was converted into NH3 under alkaline conditions in the donor channel. The generated NH3 diffused across the membrane and was absorbed in an acid solution in the acceptor channel. The resultant NH4+ in the acceptor was then quantified based on a modified indophenol blue (IPB) method. Parameters affecting the performance of the FIA-HFMC-IPB system were evaluated and optimized. Under the optimized conditions, the proposed system exhibited a limit of detection of 0.11 μmol L−1, with relative standard deviations of 1.0–1.9 % (n = 7), and a good linear response (R2 = 0.9989) for the calibration in the field with NH4+ standards in the range of 0.40–80 μmol L−1. The proposed system was applied to a shipboard underway measurement of NH4+ in a two-day cruise in the Jiulong River Estuary–Xiamen Bay, China. A good agreement was observed between measurements from the proposed system and those from manual sampling and laboratory analysis. Both laboratory and field results demonstrated that the system was free of salinity effect and interference from organic nitrogen compounds. The system also showed excellent stability and reliability during a 16-day observation. This work suggests that the proposed FIA-HFMC-IPB system is applicable for the underway measurement of NH4+ in water, especially for estuarine and coastal waters with varying salinity and complex matrices.