PARTICULATE CARBON AND NITROGEN


SUMMARY: Seawater is collected from known depths using CTD-rosette sampling procedures. Subsamples are prescreened and transferred to specially designed, calibrated filtration bottles, pressure-filtered through combusted GF/F filters and stored frozen for subsequent analysis. In the laboratory, the filters are dried, combusted and analyzed for C and N using a commercially available instrument.


1. Rationale and Assay Principle

The particulate carbon and nitrogen pools in the ocean include both living and nonliving components and both organic and inorganic fractions. However, most of the particulate matter in the ocean is of biogenic origin. Accurate measurement of these bioelements is especially important in assessing the chemistry and dynamics of particulate matter in the ocean. Organic matter in the ocean exists along a size continuum and operational divided between "dissolved" and "particulate" pools using either 0.2, 0.45, 0.7 or 1.2 µm pore size filters, depending upon the specific analytical measurements. It is important to note that by this operational definition, colloids and small particles including most bacteria and all viruses are sometimes included in the "dissolved' matter pool.

Both PC and PN can be measured using commercially available instruments that detect the gaseous products of high-temperature combustion oxidation. The Perkin-Elmer model 2400 CHN analyzer was used initially then the Europa ANCA system for HOT cruises 53-128. The Carlo Erba NA 2500 elemental analyzer coupled with a Finnigan Delta S mass spectrometer was used to analyze HOT cruises 129-165. Presently the CE-440 CHN elemental analyzer by Exeter Analytical is used by the HOT program. These instruments combine the classical Pregal and Dumas methods (Kudzin and Waskowski, 2004) for the determination of PC and PN, respectively. The samples are combusted in pure O2 under static conditions and the by-products are measured by a series of high precision thermal conductivity detectors each containing a pair of thermal conductivity cells (P.E. Hemming, Exeter Analytical (UK) Ltd).

2. Precautions

Because suspended particle concentrations and particle fluxes in oligotrophic habitats are expected to be low, special attention must be paid to the preparation and processing of water samples and sediment trap collections to reduce potentially contaminating particles (see Sediment traps chapter). The time interval between trap retrieval and subsample filtration should be minimized in order to limit the inclusion of extraneous abiotic particles and the post-collection solubilization of particles. For watersample analysis, it is necessary to process large volumes of water to increase the signal-to-noise ratio and precision estimates of the determinations. The carboys and tubing used in this system must be kept clean and free of abrading surfaces to minimize unintended C contamination.

3. Sampling, Filtration and Storage

3.1. Seawater samples are collected in 12 L PVC water bottles and transferred directly to acid-cleaned filtration bottles. The samples are transferred via Tygon tubing with an in-line 202 µm Nitex screen to remove zooplankton or any other large particles that might otherwise affect the precision of the estimate. The filtration bottles are 4 and 10 L HDPE aspirator bottles fitted with a valve assembly and tubing connection.

Once the CTD-rosette sampler is on deck, the vent valve of each 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 given to the orientation of the in-line screened drawing tube. The filtration bottle valve is opened and 100-200 ml is passed through the transfer tube and valve assembly to rinse the sampling bottle. The bottle and cap are rinsed 3 times with sample water. After rinsing, the cap is placed on the bottle mouth (without tightening), the valve is opened and the HDPE bottle 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 N2) through combusted in-line 25 mm GF/F filters.
3.3. Following filtration, clean forceps are used to transfer each filter to a plastic Petri dish containing a piece of combusted tin. The sample is labeled and stored frozen (-20°C). All appropriate data are entered on the data sheet. Combusted GF/F filters are included for filter blank corrections and are treated as the samples.
3.4. Prior to analysis the samples are removed from the freezer and placed in a 60°C drying oven for at least 12 hours. After drying the samples stored in a desiccator.
3.5. The dried samples are pelletized using the tin capsules and packed into nickel sleeves. The samples are placed in a 96 well plate and all appropriate data are recorded on the data sheet. The 96 well plate is stored in a desiccator until samples are analyzed.

4. Analysis

Standard procedures for instrument warm-up are followed. Primary PC/PN standards are prepared using acetanilide (C8H9NO; mol. wt. = 135.16). Standards typically range from 20-500 µg acetanilide per sample. The mean value of the blank filters is subtracted from the sample value.

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 the PC and PN analyses. The average % difference for field replicates collected at 350 m (HOT 166-319) are 25% and 21%, respectively. The average % difference for bottle replicates collected at 125 m and 25 m (HOT 166-319) are 11% and 9%, 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 deviation from the certified value and mean percent error, is 0.67 wt %C, 0.95% for PC and 0.21 wt %N, 2.02% for PN.

7. Equipment/Supplies

8. Reagents

9. References

10. HOT Program Analytical Summary

11. HOT Program Analytical Summary