Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i
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LOW LEVEL NITRATE AND NITRATE BY CHEMILUMINESCENCE
SUMMARY: Seawater is collected from known depths using CTD-rosette sampling protocols. Subsamples are carefully drawn and stored in acid-washed polyethylene bottles. Nitrate/nitrite is converted to nitric oxide by wet chemical reduction in a highly acidic solution. The nitric oxide produced is measured by a chemiluminescent detector.
Surface water samples (<100m) in oligotrophic waters usually have nitrate concentrations below the 0.03 µM detection limit of the Technicon Autoanalyzer. Except for a narrow band of elevated nitrite around 100 m (the primary nitrite maximum), nitrite concentrations are also below standard detection limits. To achieve high-precision high-accuracy measurements at these low levels, we employ the chemiluminescent method of Cox (1980) and Garside (1982). In this method nitrite and/or nitrate are chemically reduced to gaseous nitric oxide by an acidic solution of glacial acetic acid and sodium iodide (nitrite only) or concentrated sulfuric acid, ferrous ammonium sulfate and ammonium molybdate (nitrate plus nitrite). The reduced nitric oxide is carried by an inert carrier gas (argon) through a cold finger filled with 6M sodium hydroxide, a drying tube filled with anhydrous sodium carbonate, and a membrane drier in order to remove acid and water vapors. The gas stream is then routed into the chemiluminescent analyzer, where the nitric oxide is combined with ozone to produce a metastable nitrogen dioxide. The nitrogen dioxide subsequently emits a photon as it returns to ground state, and the emitted light is detected by a photomultiplier tube. The integrated electrical signal produced by the photomultiplier is proportional to the content of nitrite (or nitrate plus nitrite) in the sample.
As with the autoanalysis of these nutrients, contamination is the primary concern. Because nanomolar concentrations are to be measured, all sample bottles and laboratory glassware must be meticulously cleaned with dilute HCl (10%) and distilled deionized water (DDW) before use, and both the argon carrier gas and the oxygen supplied to the ozone- generator must be of the highest purity available. An additional concern with the chemiluminescent technique is the quenching effect that water vapor has on the signal. All drying and scrubbing tubes must be well maintained to ensure that no water enters the detector.
3. Sample Collection and Storage
Concerns about the potential "freezing effect" on these samples are heightened due to the low levels of the nutrients and the high precision of the measurements. However, results from experiments conducted by Constantinou (unpubl.) indicate no evidence for any discrepancy between replicate samples analyzed immediately on ship and those frozen and analyzed later in the laboratory. Samples are therefore collected and stored exactly as described for autoanalysis of these nutrients.
Nitrate and nitrite analyses are performed on an Antek model 720 nitrogen oxide analyzer. The reaction apparatus is a slight variation of that described by Garside (1982).
5. Calibration, Data Reduction and Calculations
6. Precision and Accuracy
The detection limit for nitrate plus nitrite is approximately 1-2 nM. The nitrite-only analysis produces less noisy signals; standards with as little as 0.4 nM nitrite and samples with as little as 0.1 nM nitrite have been detected. Precision and accuracy of the nitrate plus nitrite analysis are approximately 2-3 nM while for nitrite-only they are generally <2 nM.