PARTICULATE CARBON AND NITROGEN
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SUMMARY: Seawater samples are collected at discrete depths
in 12-liter Niskin bottles. The water samples are prefiltered
(202 µm) and transferred to specially designed, precalibrated
filtration bottles, pressure filtered through combusted GF/F
filters and stored frozen for subsequent analysis. In the
laboratory, the filters are dried and analyzed for C and N
using a Perkin-Elmer model 2400 analyzer.
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1. Principle
Particulate carbon (PC) and particulate nitrogen (PN), including
both inorganic and organic forms, are present in seawater primarily
as by-products of biological activity. Living organisms also contribute
a variable amount (<1% in deep waters to >50% in the euphotic zone)
to the total particulate carbon and nitrogen content of seawater.
Both PC and PN can be measured using commercially-available
instruments which detect the gaseous by-products of high-temperature
combustion. The Perkin-Elmer model 2400 CHN analyzer used in the
HOT-GOFS project, combines the classical Pregal and Dumas methods
for the determination of carbon and nitrogen, respectively. The
samples are combusted in a pure oxygen environment, the gases mixed
and separated by frontal chromatography. The separated gases are
determined stepwise by a thermal conductivity detector.
2. Precautions
Care must be taken to ensure that contamination with PC and PN
is avoided. Samples should, therefore, be kept away from paper,
wood, food or other carbon-containing compounds. The filtration
system that is set up to collect these samples is constructed of
plastic. Plastic contains carbon and, therefore, is a potential
source of contamination. The carboys and tubing used in this system
must be kept clean and free of abrading surfaces.
3. Sampling, Filtration and Storage
3.1. Seawater samples are collected in 12-liter Niskin bottles and
transferred directly to acid-cleaned filtration bottles. The
samples are transferred via Tygon tubing which incorporates an
in-line 202 µm Nitex screen prefilter to remove zooplankton or
any other rare particles which might otherwise affect the
precision of the estimate. The filtration bottles are 4- and
12-liter polyethylene aspirator bottles fitted with a valve
assembly and tubing connection.
Once the rosette sampler is on deck, the vent valve of each
Niskin bottle is opened and one end of the drawing tube is
attached to the outflow spigot of the sampling bottle and the
other end to the tubing connector on the cap of the filtration
bottle. Particular attention is paid to the orientation of
the in-line screened drawing tube (the shorter, larger bore
section is attached to the Niskin bottle). The filtration
bottle valve is opened and 100-200 ml is run through the
transfer tube and valve assembly to rinse the sampling bottle.
The bottle and cap are rinsed 3 times in this fashion. During
the rinsing and filling operation, the filtration bottle cap
valve is used to control and direct the sample flow. After
rinsing,the cap is placed on the bottle mouth (without
tightening), the valve is opened and the polyethylene bottled
is filled to the calibration mark.
3.2. After filling, the filtration bottles are inverted and placed
into the filtration rack. The contents are then pressure
filtered (4-7 psi nitrogen gas) through combusted in-line 25
mm GF/F filters.
3.3. Following filtration, clean forceps are used to transfer each
filter to a clean plastic petri dish containing a 3 cm square
piece of combusted foil. The sample is labeled and stored
frozen (-20°C). Usually only a few ml or less of water remains
in the carboy after filtration; this residual water is ignored
in assessing the volume filtered unless it appears to be
significant compared to the volume of the carboy. All appropriate
data are entered on the data sheet.
4. Analysis
Standard procedures for instrument warm-up are followed. Primary
PC/PN standards are prepared using acetanilide (C8H9NO; mol. wt. =
135.16). Standards are made to encompass the range 20-500 ug/sample.
Blanks are prepared by analyzing combusted GF/F "field filters" and
the mean value subtracted from the sample value. Typical ranges in
analytical blanks, expressed as percentages of the total signal
produced by a sample in the course of a given analysis, are 5-17%
for both PC and PN.
5. Data Reduction and Calculations
External standard data are used to prepare a standard curve of C
(or N) versus corrected signal counts, and linear regression statistics
are calculated. An EXCEL spreadsheet is used to calculate PC and PN
(µg l-1) for each sample based on standard curve, corrected signal
counts and volume of seawater filtered.
6. Precision and Accuracy
Replicate samples are routinely analyzed to estimate the precision
of our PC and PN analyses. The average coefficients of variation for
such replicate analyses are 12% and 10%, respectively. Accuracy is
estimated from determinations of the C and N contents of reference
standards analyzed along with samples during each analytical run.
This accuracy, expressed as the mean percent difference between the
analytical determination and the known C and N contents of the
standards, is found to be 6% for PC and 13% for PN. Note that the
estimates of accuracy apply only to standards containing >10 µg N
and >70 µg C. Below these levels, we have found the performance of
the PE-2400 to be suspect. Therefore sample volumes large enough
to keep above this lower limit are used.
7. Equipment/Supplies
PE-2400 (or equivalent) Carbon/Nitrogen analyzer with integrator
Cahn (or equivalent) electronic balance
combusted 2.5 cm GF/F filters (450°C, 4 hours)
ethanol-cleaned forceps for handling filters
combusted foil (450°C, 4 hours)
aluminum foil for covering work area
clean petri dishes
low pressure filtration apparatus (4-7 psi)
valved polyethylene aspirator bottles
8. Reagents
acetanilide standard
9. References
Sharp, J. H. 1974. Improved analysis for particulate organic
carbon and nitrogen from seawater. Limnology and Oceanography, 19,
984-989.
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