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:
combustion temperature 680 °C carrier gas UHP oxygen carrier flow rate 150 mL/min ozone generation gas zero air from Whatman TOC gas generator ozone flow rate 500 mL/min sample sparge time 2.0 min minimum number of injections 3 maximum number of injections 5 number of washes 2 standard deviation maximum 0.1000 CV maximum 2.00% injection volume 100 µL
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.