AN IMPROVED FLUORESCENCE METHOD FOR THE DETERMINATION OF NANOMOLAR CONCENTRATIONS OF AMMONIUM IN NATURAL-WATERS

An improved fluorescence method is described for measuring nanomolar concentrations of NH4+ in natural waters. This method is based on the conversion of NH4+ to NH3 and subsequent diffusion of NH3 across a microporous hydrophobic Teflon membrane into a flowing stream of omicron-phthaldialdehyde reagent to produce a fluorescent adduct. The product is detected fluorometrically with a lower detection limit of better than 1.5 nM. Up to 30 determinations h-1 can be made. The method works well in fresh-water or salt water. Field tests of the method in the Dry Tortugas and Gulf Stream gave NH4+ concentrations that ranged from 18.0 nM in Gulf Stream waters to 2,254.7 nM in interstitial waters from coralline reef sands. The method can be used to measure near real-time NH4+ concentrations in situations where it was previously difficult or impossible.