ION-CHROMATOGRAPHIC DETERMINATION OF NUTRIENTS IN SEA-WATER

The application of ion chromatography, with eluent suppression and conductometric detection, to the determination of nitrate and phosphate concentrations in sea water was investigated. The aim was to evaluate the concentration of both species simultaneously and without the need to pretreat the sample, thus avoiding the disadvantages inherent in the usual analytical methods (molecular spectroscopy of suitable derivatives). The determination of nitrate was also accomplished using UV detection at 210 nm by means of a diode-array detector connected in series with the conductivity detector. In this way, by comparing the absorption spectra of nitrate in the standard solution and in the samples and by checking the purity index of the related peaks, the effectiveness of the chromatographic separation and the lack of interferences in the analysis were monitored. About 40 samples of sea water, taken over several months in four different locations, were analysed after filtration and dilution. In all the samples nitrate was detectable and the separation between the peaks of nitrate and bromide was complete, even when the bromide concentration was as high as 70 ppm. Only some of the samples, owing also to the nearly always necessity for dilution, showed a concentration of phosphate higher than the limit of detection.