Dissolved Organic Carbon Measurements

Section P16S_2005

The DOC samples on Section P16S_2005 cruise were collected and analyzed by Dr. Craig Carlson's group from University of California, Santa Barbara (UCSB). All samples were collected directly from the Niskin bottles. Because particulate organic carbon concentrations in the surface waters can be elevated all samples collected from the upper 500 m were filtered. Water was filtered through a combusted GF/F housed in an acid-washed polycarbonate filter cartridge attached directly the Niskin bottle spigot. All samples were collected directly into an acid washed and Nanopure flushed high density polyethylene (HDPE) bottles (60 mL). Samples were immediately placed upright in a -20°C freezer and later were packed in dry ice and shipped to a shore laboratory. All samples were kept frozen at -20°C in an organic (volatile-free) environment for shore side analysis.

All DOC samples were analyzed via high temperature combustion using a Shimadzu TOC-V series TOC analyzer in a shore based laboratory at UCSB. The operating conditions of the Shimadzu TOC-V were slightly modified from the manufacturer's model system. The condensation coil was removed and the head space of an internal water trap was reduced to minimize the system's dead space. The combustion tube contained 0.5 cm Pt pillows placed on top of Pt alumina beads to improve peak shape and to reduce alteration of combustion matrix throughout the run. CO₂ free carrier gas was produced with a Whatman gas generator (Carlson et al. 2004). Samples were drawn into 5 mL injection syringe and acidified with 2 M HCl (1.5%) and spared for 1.5 min with CO₂-free gas. Three to five replicate 100 µL of sample were injected into combustion tube heated to 680°C. The resulting gas stream was passed though several water, including an added magnesium perchlorate trap followed by a halide trap. The CO₂ in the carrier gas was analyzed with a non-dispersive infrared detector, and the resulting peak area was integrated with Shimadzu chromatographic software. Injections continued until at least three injections met the system-specified range of an SD of 0.1 area counts, CV < or = 2%, or best 3 of 5 injections.

Extensive conditioning of the combustion tube with repeated injections of low-carbon water and deep seawater was essential to minimize the machine blanks. After conditioning, the system blank was assessed with ultra-violet-oxidized, low-carbon water. The system response was standardized with a four-point calibration curve of potassium hydrogen phthalate solution in low-carbon water. All samples were systematically referenced against low-carbon water, deep Sargasso Sea reference waters (2600 m) and Sargasso Sea surface water every 6 to 8 analyses (Hansell and Carlson 1998). The standard deviation of the deep and surface references analyzed throughout a run generally have a coefficient of variation ranging between 1-3% over the 3 to 7 independent analyses (number of references depends on size of the run) (see Hansell 2005). Daily reference waters were calibrated with DOC CRM provided by D. Hansell (RSMAS). The UCSB DOC laboratory exchanges references and samples with the Hansell DOC laboratory to ensure similar performance of DOC systems and comparability of data.

DOC calculation:

µMC = (average sample area - average machine blank area) / (slope of std curve)

Section P16N_2006

The DOC samples on Section P16N_2006 cruise were collected and analyzed by Dr. Dennis Hansell's group from RSMAS/UM. Water samples were collected from the rosette. Samples collected from the surface to 250 meters were filtered using precombusted (500°C) GF/F inline filters as they were being collected from the Niskin bottle. At depths > 250 meters, the samples were collected without filtration. After collection, samples were frozen upright in 60 mL acid-cleaned HDPE bottles and remained cold until analysis. Prior to analysis, samples were returned to room temperature, then acidified to pH < 2 with concentrated hydrochloric acid. Analysis was performed on shore using a Shimadzu TOC-VCSH TOC analyzer with the TNM-1 total nitrogen (TN) detector attached. Instrument conditions were as follows:

The TOC system was calibrated using potassium hydrogen phthalate in Milli-Q water and the TN system was calibrated using potassium nitrate in Milli-Q water. System performance was verified daily using consensus reference water distributed by Dr. Hansell's laboratory at RSMAS/UM. This reference water is deep Sargasso Seawater that has been acidified and sealed in 10 mL ampoules, the concentration of which (~44 µM C) has been determined by the consensus of up to six expert and independent laboratories. After verifying proper operation of the TOC/TN instrument, samples were set up on an auto sampler for analysis. The run started with a QW (Q Water) blank and a reference seawater analysis. Then six samples were analyzed followed by another QW blank and reference seawater. This sequence was repeated until all samples for that run were analyzed. The run ended with a QW blank, reference water, and a non-acidified QW blank. This was done to verify that the hydrochloric acid used to acidify the samples was not contaminated. QW blanks and reference water samples were used to evaluate system performance during the analytical run. If a problem was detected with the blanks or reference waters, the samples were reanalyzed.