Chapter 15. Determination of Particulate Organic Carbon and Particulate Nitrogen

1.0 Scope and field of application
This procedure describes a method for the determination of particulate organic carbon and particulate nitrogen in seawater. The assay is appropriate for measuring oceanic levels of particulate organic carbon (5.0 - 500.0 mg C/kg) and particulate nitrogen (0.5 - 100.0 mg C/kg).The principles for this method were first described by Gordon (1969) and Kerambrun and Szekielda (1969). Sharp (1974) describes a number of useful modifications to the existing method applied here. Detailed description of the analytical procedure is given by the manufacturer (Control Equipment Corporation 1988). Some of the details of the actual  measurement of carbon and nitrogen in this method are specific to the Control Equipment Corporation (CEC) 240-XA Elemental Analyzer hardware used at the Bermuda Atlantic Time-series Study. Scientists who employ this or other methods to measure POC and PN should make themselves aware of the current and historical issues that surround these techniques and make appropriate decisions about specific methodologies for their application based on the scientific requirements and constraints of their individual programs.

2.0 Definition
2.1 The concentration of particulate organic carbon is given in mg C/kg seawater.

2.2 The concentration of particulate nitrogen is given in mg N/kg seawater.

3.0 Principle of Analysis
A dried, acidified sample of particulate matter is combusted at 960°C. The organic carbon is converted to CO2 and the nitrogen oxides are subsequently reduced to N2 gas. Both gases are measured by thermal conductivity.

4.0 Apparatus
4.1 Control Equipment Corporation (CEC) 240-XA Elemental Analyzer (Leeman Labs, Inc.)

4.2 CAHN Model 4400 Electrobalance

4.3 Hewlett Packard (HP-150) Analytical Software

5.0 Reagents
5.1 Hydrochloric acid (concentrated HCl: reagent grade)

5.2 Acetanilide (reagent grade): Acetanilide has 0.7109 g C and 0.1036 g N per g total mass.

6.0 Sampling
The POC/PN samples are taken after oxygen, CO2 , salinity and nutrient samples have been removed, approximately 30–60 minutes after the CTD/rosette reaches the surface. Settling of large particles in the Niskin bottles will create a non-uniform distribution of the particles within this period of time. For best results, the Niskin bottle should therefore be shaken before sampling or the entire volume filtered (including the volume below the spigot).

Samples are collected in 4 liter polypropylene bottles equipped with a 1/4” outlet at the base. The filtration is “in-line” with the filter mounted in a Delrin filter holder. The holder is connected to the outlet at the bottom of the 4 liter bottle on one end and a vacuum system (liquid container and pump) on the other. Two liters are normally filtered at all depths (although this volume may not be adequate for all systems) from surface to 1000 m
onto precombusted (450°C, 5 hours) 25 mm Whatman GF/F filters (nominal pore size 0.7 mm). The filter is removed, wrapped in precombusted aluminum foil and stored frozen in a deep freezer (-20°C) until processed.

7.0 Procedures
7.1 Sample Analysis

7.1.1 Prior to analysis, the filters are thawed, allowed to dry overnight at 65°C in acid washed and precombusted (450°C, 2 hours) scintillation vials and then placed overnight in a desiccator saturated with HCl fumes. The air in the desiccator is kept saturated by leaving concentrated HCl in an open container in
the lower compartment of the desiccator. Thereafter, the filters are dried again at 65°C and packed in precombusted (850°C, 1 hour) nickel sleeves.

7.1.2 The samples are analyzed on a Control Equipment Corporation (CEC) 240- XA Elemental Analyzer following the guidelines given by the manufacturer. Sixty-four samples are run at a time on the auto-sampler, of which one is a standard (see below) and approximately nine are Ni sleeve blanks. The machine operator checks on the machine regularly to ensure that problems have not developed. Data are collected and stored by a microcomputer automatically.

7.2 Standardization and blank determination: Acetanilide standard and blanks (empty Ni sleeves) are measured prior to each batch run of samples (64 samples). A minimum of three empty filters are processed as an ordinary sample and analysed for each cruise as filter blanks. The acetanilide standard is weighed in acetone washed tin capsules on a CAHN Electrobalance. Standard weights are usually between 0 and
2.0 mg. The tin capsule with the standard is put into a nickel sleeve and run on the Elemental Analyzer. The empty filter blanks should be treated exactly like sample filters except that no sample water is passed through them.

8.0 Calculation and expression of results
The POC and PN weights of each of the samples are integrated and estimated automatically by the Hewlett Packard (HP-150) Analytical Software, supplied with the CEC instrument. The program automatically includes the latest Ni sleeve blank into its calculations. The in-situ concentration is estimated:
        mg/kg = ( S - B) Vr
Where:
        S = the result for the filtered sample
        B = the measured filter blank
        V = volume filtered (liters)
        r = density (a function of T, S and P, where T = model temperature at filtration, S = salinity of the sample,  and P = atmospheric pressure)

9.0 References
Control Equipment Corporation. (1988). The automated and advanced Model 240-XA  Elemental Analyzer. Lowell, MA.
Gordon, Jr. D.C. (1969). Examination of methods of particulate organic carbon analysis.
Deep-Sea Research 16:661-665.
Kerambrun, P. and K.H. Szekielda. (1969). Note technique. Tethys 1:581-584.  Sharp, J.H. (1974). Improved analysis for “particulate” organic carbon and nitrogen from  seawater. Limnology and Oceanography 19:984-989.