Ammonium is an essential nutrient in the marine environment. Phytoplankton generally prefer ammonium over nitrate, which requires additional energy to reduce to ammonium during assimilation. However, high concentrations of ammonia are toxic to marine organisms such as fish, shrimp, abalone, and sea urchin, especially larvae or juveniles of these species. Water quality monitoring requires accurate measurements of the concentration of ammonium in the coastal environment; however, both physical and biological processes, such as biological uptake and release, atmospheric deposition, air-sea gas exchange, and wastewater discharge, rapidly change the ammonia concentrations in both space and time.
A shipboard fluorometric flow analyzer has been developed for near-real-time, high-resolution underway measurement of ammonium in seawater. The fluorometric method is based on the reaction of ammonium with o-phthaldialdehyde (OPA) and sulfite. The reagents used in this method have been modified to suit seawater analysis. This method shows no refractive index and salinity effect from seawater samples. The potential interferences in seawater have been studied, and their effects have been reduced. The instrument response is linear over a wide range of ammonium concentration. The limit of detection of 1.1 nM was estimated in laboratory using ammonium standards prepared in distilled water. It should be noted that application of this method to low-level ammonium measurement requires a correction of interference species, such as amino acids. The system can be used for both freshwater and seawater samples and has been used to monitor the distribution of ammonium in Florida coastal waters around an oceanic wastewater outfall.
(1) Amornthammarong, N.; Zhang, J.-Z. Shipboard Fluorometric Flow Analyzer for High-Resolution Underway Measurement of Ammonium in Seawater. Click here to download.
(2) Amornthammarong, N.; Zhang, J.-Z. Anal. Chem. 2008, 80, 1019-1026.