[*]1. Scope and field of application
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[FONT=Times New Roman,Times New Roman]This procedure describes a method for the determination of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) in sea water, expressed as micromoles of carbon (nitrogen) per liter of sea water. The method is suitable for the assay of oceanic levels of dissolved organic carbon (< 400 [/FONT][/FONT]
[FONT=Times New Roman,Times New Roman][FONT=Times New Roman,Times New Roman]μ[/FONT][/FONT][FONT=Times New Roman,Times New Roman]
[FONT=Times New Roman,Times New Roman]mol L–1) and total dissolved nitrogen (< 50 [/FONT][/FONT]
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[FONT=Times New Roman,Times New Roman]mol L–1). The instrument discussed and procedure described are those specific to the instrument employed in the Hansell Laboratory at the University of Miami. Instruments produced by other manufacturers should be evaluated for suitability.
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[*]2. Definition
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[FONT=Times New Roman,Times New Roman]The dissolved organic carbon content of sea water is defined as:
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[FONT=Times New Roman,Times New Roman][FONT=Times New Roman,Times New Roman]The concentration of carbon remaining in a sea water sample after all particulate carbon has been removed by filtration and all inorganic carbon has been removed by acidification and sparging.
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[FONT=Times New Roman,Times New Roman]The total dissolved nitrogen content of sea water is defined as:
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[FONT=Times New Roman,Times New Roman][FONT=Times New Roman,Times New Roman]The concentration of nitrogen remaining in a sea water sample after all particulate nitrogen has been removed by filtration.
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[*]3. Principle
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[FONT=Times New Roman,Times New Roman]A filtered and acidified water sample is sparged with oxygen to remove inorganic carbon. The water is then injected onto a combustion column packed with platinum-coated alumina beads held at 680°C. Non-purgeable organic carbon compounds are combusted and converted to CO2, which is detected by a non-dispersive infrared detector (NDIR). Non-purgeable dissolved nitrogen compounds are combusted and converted to NO which, when mixed with ozone, chemiluminesces for detection by a photomultiplier.
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[FONT=Arial,Arial]October 12, 2007 [/FONT][/FONT]
SOP 7 [FONT=Arial,Arial]
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Dissolved organic carbon [FONT=Arial,Arial]
[FONT=Arial,Arial]Version 3.0
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[*]4. Apparatus
[*]• [FONT=Times New Roman,Times New Roman][FONT=Times New Roman,Times New Roman]Shimadzu TOC-VCSH with ASI-V auto sampler and TNM-1 Total Nitrogen detector (or equivalent).
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[*]5. Reagents
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[*]5.1 Compressed gas
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[FONT=Times New Roman,Times New Roman]Ultra High Purity (UHP 99.995%) oxygen is used as the carrier gas for the Shimadzu TOC-VCSH. High quality carrier gas is required to obtain low background levels in the detector. Oxygen is used to ensure complete combustion of all organic material.
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[*]5.2 Combustion column catalyst
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[FONT=Times New Roman,Times New Roman]The carrier gas passes through a column packed with 2 mm platinum-coated alumina beads (Shimadzu P/N 017-42801-01) held at 680°C.
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[*]5.3 Platinum gauze
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[FONT=Times New Roman,Times New Roman]Pure platinum wire gauze (52 mesh woven from 0.1 mm diameter wire) is roughly formed into cubes ([/FONT][/FONT][FONT=Times New Roman,Times New Roman]
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[FONT=Times New Roman,Times New Roman]0.5 cm to a side) and several (3–5) are placed on top of the combustion column bed. The platinum gauze improves analytical reproducibility and retains injected salt.
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[*]5.4 Acidification of sample
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[FONT=Times New Roman,Times New Roman]Trace-impurity analyzed concentrated hydrochloric acid is used to acidify samples prior to analysis. Approximately 0.1% by volume of the concentrated acid is added to each sample prior to analysis to lower the pH of the sample to pH < 2. At this pH and with sparging, all inorganic carbon species are converted to CO2 and removed from the sample. Automated acidification by the TOC-VCSH is not used, as with time the blank using this acid solution increases. When the sample is acidified manually with acid freshly taken from a sealed bottle, the increase of the blank has not been observed.
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[*]6. Sampling
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[FONT=Times New Roman,Times New Roman]Proper sampling techniques and handling are essential to good quality data. Care must be taken to minimize contamination of the sample. Sampling from the rosette should be done using clean silicone tubing. Gloves should be worn during sampling. It is recommended that anyone sampling from the rosette prior to collection of the samples ([/FONT][/FONT]
[FONT=Times New Roman,Times New Roman][FONT=Times New Roman,Times New Roman]e.g.[/FONT][/FONT][FONT=Times New Roman,Times New Roman]
[FONT=Times New Roman,Times New Roman], gases) also wear gloves. If that it not possible, every effort must be made not to touch the sample nipple (the path of the water stream, from Niskin to sample bottle, must be kept very clean). Grease (whether mechanical grease from ship operations or sealing grease, as employed for some gas sampling) should never be allowed to come in contact with the sample nipple.
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