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 NH 4 + in natural waters. This method is based on the conversion of NH 4 + to NH 3 and subsequent diffusion of NH 3 across a microporous hydrophobic Teflon membrane into a flowing stream of o ‐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 freshwater or salt water. Field tests of the method in the Dry Tortugas and Gulf Stream gave NH 4 + 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 NH 4 + concentrations in situations where it was previously difficult or impossible.