In our investigations of nitrogen regeneration in the photic zone of open seawater, we need to measure ammonium concentration and flux. 15N-uptake measurements conventionally rely on ammonium concentration measurement by spectrophotometry. However, ammonium concentrations in natural waters are frequently at the limit of conventional colourimetric analysis (<0.05 μM). Existing 15N-dilution methods are also insensitive. We have developed the phenol/hypochlorite reaction to permit isotope dilution analysis of indophenol formed from ammonium. Seawater is spiked to approximately 10% of the ammonium pool with a known quantity of 15N-ammonium. An aliquot is filtered immediately. A second aliquot is incubated in situ for 24 hours. This is then filtered and the ammonium flux is estimated by isotope dilution analysis of each filtrate (analysis of 15N in particulates provides an assay of ammonium assimilation). Indophenol is formed in 50 mL aliquots, extracted by solid-phase extraction, dried, derivatized and analysed by GC/MS as bis(trimethylsilyl)indophenol. 15N analysis is performed using EI ionization and monitoring the intense molecular ion. The procedure is precise and sensitive. Unlike previous methods, it is free from interference by non-specific nitrogen sources. The new method provides accurate ammonium concentration and flux data and should have wide application.
Preston T, S Bury, M Présing et al.: Isotope dilution analysis ... (1996)
Preston T, Bury S, Présing M, Moncoiffe G, Slater C
Isotope dilution analysis of combined nitrogen in natural waters: I. Ammonium
Rapid communications in mass spectrometry 10(8): pp. 959-964.
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