Holmes, R. M. Ecosystems Center, Marine Biological Laboratory, rholmes@mbl.edu
Aminot, A. M. IFREMER-Brest, alain.aminot@ifremer.fr
Kérouel, R. IFREMER-Brest,
Hooker, B. A. Ecosystems Center, Marine Biological Laboratory, bhooker@mbl.edu
Peterson, B. J. Ecosystems Center, Marine Biological Laboratory, peterson@mbl.edu

Accurate determination of ammonium concentration is fundamental to understanding nitrogen biogeochemistry in aquatic ecosystems. Unfortunately, the indophenol blue method that is commonly used often yields inconsistent results, particularly when ammonium concentrations are low. Here we present a manual fluorometric method designed to give precise measurements of ammonium at sub-micromolar concentrations in marine and fresh waters. The procedure not only addresses analytical issues but also substantially overcomes problems in sample collection and preservation. It uses a single mixed reagent (consisting of orthophthaldialdehyde "OPA", sodium sulfite, and sodium borate) that is non-toxic and is stable for months when stored in the dark at room temperature. The mixed reagent is added to the sample immediately after collection and the reaction proceeds to completion within 4 hours at room temperature. Salt effects and background fluorescence are relatively small and can be corrected without introducing substantial error. We find the method to be simple and quick and yet to yield highly reproducible results even at ammonium concentrations less than 1 micromolar.
Day: Wednesday, Feb. 3
Time: Poster
Location: Sweeney Center
Code: CS69WE0381S