ISOTOPE-DILUTION ANALYSIS OF COMBINED NITROGEN IN NATURAL-WATERS - III - NITRATE AND NITRITE

The measurement of nitrate uptake is of major importance to our invest igations of primary production and nitrogen regeneration in the photic zone. Endogenous nitrate production (nitrification), by diluting the enrichment of added N-15-nitrate, can lead to underestimation of nitra te uptake. Although nitrification is assumed to he minimal in the phot ic zone, this needs to be confirmed by experiment. We have developed a specific isotope dilution method based on the analysis of a volatile derivative of the azo dye, 1-phenylazo-2-naphthol (Sudan-l), Sea water is spiked to approximately 10% of the nitrate pool with a known quant ity of N-15-nitrate, An aliquot is filtered immediately. a second aliq uot is incubated in situ for a fixed period and is then filtered. Nitr ate concentration is estimated by isotope dilution analysis of the fir st filtrate. Nitrification rate is estimated from the change in N-15 e nrichment between the first and second filtrate. Nitrate from a 10 mt aliquot is first reduced to nitrite, Sudan-1 is formed by diazotizatio n with aniline and coupling to 2-naphthol. It is extracted by solid ph ase extraction using C-18 cartridges, with ethyl acetate as eluant, 4- Phenylazophenol is added as internal standard and the samples are drie d. They are analysed as tert-butyldimethylsilyl derivatives, using gas chromatography/mass spectrometry in selected ion recording mode. The intense M-57 fragment ion formed under electron impact ionization is u sed for N-15 analysis, The procedure is precise, sensitive and, unlike most previous methods, it is free from interference by other forms of nitrogen. This method compliments our existing isotope dilution metho ds for the analysis of ammonium and dissolved free amino acids. Here, the new method has been used to confirm that our estimates of nitrate uptake in the photic zone of the Southern Ocean are not compromised by high nitrification rates. (C) 1998 John Wiley & Sons, Ltd.