Abstract
Abstract A new flow injection analysis (FIA) method using spectrophotometric detection has been developed for the simultaneous determination of ammonium, nitrite and nitrate in marine waters. The method is based on absorbance measurement of azo dye obtained by the reaction of nitrite with N -(1-naphthyl)ethylenediamine dihydrochloride and sulfanilamide. Ammonium was previously oxidized to nitrite by hypochlorite in the presence of large amount of potassium bromide, while nitrate was reduced to nitrite in a copperized cadmium column. The conditions of the former reaction have been studied and its continuous application in seawater has been optimized. With a single sample injection three signals were obtained, corresponding to the concentration of nitrite (N-NO 2 − ), nitrite plus ammonium (N-NO 2 − + N-NH 4 + ) and nitrite plus nitrate (N-NO 2 − + N-NO 3 − ). The detection limit for each species was 21 μg l −1 N-NO 2 − , 26 μg l −1 N-NH 4 + and 32 μg l −1 N-NO 3 − . This method was applied to both synthetic and real samples of seawater, giving average relative errors of 3.2, 4.9 and 4.0% for NO 2 − , N-NH 4 + , and N-NO 3 − , respectively. The present method allows for the simultaneous determination and speciation of nitrogenous with a single injection, in a simple way and a high sampling rate (10 samples h −1 ). Besides, the method enables the monitoring of nitrogen species in marine ecosystems.
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