<h2>Garside's Methodology for Low Level Nitrate Measurements</h2>
<pre>
In many oceanic systems subject to seasonal nutrient supply, nutrient
concentrations in the euphotic zone periodically fall below the
detection limit of colorimetric methods, and low level detection
methods are required for their analysis.
During Process Cruise 1 and 6 samples were taken for nitrate analysis
at the nano-molar concentration level as part of the 15N isotope
studies done by McCarthy and Nevins. McCarthy and Nevins provided the
samples for shore based analysis. Samples were taken from the TM/CTD
into c.a. 25 cc scintillation vials pre-rinsed several times with
sample. Vials were 2/3 filled and 1cc of concentrated sulfuric acid was
added to stop biological activity. Filtration was not employed because
the potential for contamination at low concentrations generally exceeds
the likelihood of particulate contamination problems. Samples were
stored refrigerated in the dark, except during transit from Oman to
BLOS. Acid blanks in DIW and DIW blanks were taken to check
preservative contamination.
Samples were analyzed with the method described by Garside (1982) with
the following modifications. The reagents were degassed on a separate
helium impinger allowing a higher analytical rate; the three way
stopcock was replaced with solenoid valves controlled by a computer
program; and the integration was achieved with an A:D converter signal
supplied to the computer program.
Calibration was based on the addition of ul quantities of a 1 uM
standard to an ASW reagent matrix mimicking the sample matrix, and a
linear response (0 -100 nM) was established relative to calibration
blanks prior to sample analysis. Additional standards were run
periodically during each run to check that the analytical response was
not deteriorating. Data reduction involved computing the linear
regression of integrator signal versus standard concentration, and
employing this regression with the sample integrator resposnse to
compute sample concentrations. The standard error of the estimate for
these regressions was 1 - 2 nM using 10ml samples (appropriate for the
0-100nM range) in a total of 14 runs. Higher concentrations required
lesser sample volumes and the reagent matrix was compensated with ASW
to maintain a final SW/ASW volume of 10ml. For these samples the
standard error is proportionately larger, but still at c.a. 1-2% of
their concentration.
A total of 659 samples in duplicate, DIW and DIW plus acid blanks were
analyzed. Generally blanks were satisfactory, with few exceptions, but
several samples contained contaminants, which often were absent on
re-analysis suggesting that they were particulate, or otherwise
non-homogeneous. The signal from these contaminants provided a broad
signal peak lasting for 10 - 20 minutes, completely obscuring the NOx
signal which they in no way resembled, and cost many hours of lost
time. Had the analyses been run on board ship, the problem would have
been identified and filtration employed, demonstrating one of the
perils of sample preservation and later analysis ashore. Nevertheless,
no sample was entirely lost, and the lower of very disparate duplicates
was taken to be correct. Thus, single (lower) values are archived for
each sample.
In the 150 - 1500 nM range we have compared NOx analysis with the CFA
colorimetric analyses, and on both cruises the CFA concentrations fall
in the range of 0 - 0.2 uM higher than the NOx data. Since the
difference is within the precision of the CFA, and the data are
obviously skewed because negative concentrations are precluded, we
conclude that the two data sets are consistent.
Garside C., 1982. A chemiluminescent technique for the determination of
nanomolar concentrations of nitrate and nitrite in seawater. Marine
Chemistry 11. 159-167.
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