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Automated determination of the stable carbon isotopic composition (δ 13 C) of total dissolved inorganic carbon (DIC) and total nonpurgeable dissolved organic carbon (DOC) in aqueous samples: RSIL lab codes 1851 and 1852

Kinga M. Revesz·Daniel H. Doctor
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Abstract

The purposes of the Reston Stable Isotope Laboratory (RSIL) lab codes 1851 and 1852 are to determine the total carbon mass and the ratio of the stable isotopes of carbon (δ 13 C) for total dissolved inorganic carbon (DIC, lab code 1851) and total nonpurgeable dissolved organic carbon (DOC, lab code 1852) in aqueous samples. The analysis procedure is automated according to a method that utilizes a total carbon analyzer as a peripheral sample preparation device for analysis of carbon dioxide (CO 2 ) gas by a continuous-flow isotope ratio mass spectrometer (CF-IRMS). The carbon analyzer produces CO 2 and determines the carbon mass in parts per million (ppm) of DIC and DOC in each sample separately, and the CF-IRMS determines the carbon isotope ratio of the produced CO 2 . This configuration provides a fully automated analysis of total carbon mass and δ 13 C with no operator intervention, additional sample preparation, or other manual analysis. To determine the DIC, the carbon analyzer transfers a specified sample volume to a heated (70 °C) reaction vessel with a preprogrammed volume of 10% phosphoric acid (H 3 PO 4 ), which allows the carbonate and bicarbonate species in the sample to dissociate to CO 2 . The CO 2 from the reacted sample is subsequently purged with a flow of helium gas that sweeps the CO 2 through an infrared CO 2 detector and quantifies the CO 2 . The CO 2 is then carried through a high-temperature (650 °C) scrubber reactor, a series of water traps, and ultimately to the inlet of the mass spectrometer. For the analysis of total dissolved organic carbon, the carbon analyzer performs a second step on the sample in the heated reaction vessel during which a preprogrammed volume of sodium persulfate (Na 2 S 2 O 8 ) is added, and the hydroxyl radicals oxidize the organics to CO 2 . Samples containing 2 ppm to 30,000 ppm of carbon are analyzed. The precision of the carbon isotope analysis is within 0.3 per mill for DIC, and within 0.5 per mill for DOC.

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Kinga M. Revesz, Daniel H. Doctor. 2014. Automated determination of the stable carbon isotopic composition (δ 13 C) of total dissolved inorganic carbon (DIC) and total nonpurgeable dissolved organic carbon (DOC) in aqueous samples: RSIL lab codes 1851 and 1852. https://doi.org/10.3133/tm10c20

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