Fluorometric determination of ammonia in sea and estuarine waters by direct segmented flow analysis

A direct automated method for routine determination of ammonia in sea and estuarine waters has been developed using segmented flow analysis. The method, based on the reaction of ammonia with orthophtaldialdehyde (OPA) and sulfite, is easy to use, sensitive and highly reproducible. The combination of reagents makes it highly selective, thus eliminating the need for the gas diffusion cell commonly used to remove amino acid interference. Results on kinetics, on reagent composition and light effects and on pH and salinity changes are presented. The salt-effect does not exceed 3% in the 0–35 salinity range. Interference from primary amines is < 0.5%. Among the naturally occurring inorganic species tested, only sulfide presents a significant, depressive effect. The method can be used in oligotrophic areas (detection limit 1.5 nmol l−1); specific recommendations are provided accordingly. Repeatability of 0.01 μmol l−1 was achieved in marine samples with ammonia concentrations ranging from 0.5 to 5 μmol l−1. Using on-line dilution, ammonia determination can be performed up to 250 μmol l−1 and salt effect becomes negligible. Comparison of the present OPA-method with the indophenol blue method yielded fully comparable results.