δ13CDOC in water samples is also measured using a Thermo GasBench II interfaced with a Thermo Finnigan DELTAplus XP IRMS.
Proper sample collection and storage is critical for accurate δ13CDOC measurements. See the liquid sample submission page for more details.
In addition to normal cleaning, Labco Exetainers used for δ13CDOC measurements are baked at 500 °C for at least five hours to remove trace organic contamination. The baked Exetainer is capped and flushed with ultra high purity (99.999% pure) helium to remove atmospheric CO2 and 1 to 5 mL of water sample is injected from a headspace-free, septum-sealed vial and a further 0.1 mL of 100% H3PO4 is injected into the Exetainer.
CO2 produced from the reaction of inorganic carbon with H3PO4 is then flushed from the vial, again using ultra high purity (99.999% pure) helium. An oxidant (Na2S2O8 + H2O) is then injected, and the Exetainers are placed into a heating block at 99.6 °C for one hour.
After cooling, the Exetainers are agitated using a vortex mixer to ensure that evolved CO2 moves into the headspace, then centrifuged at low RPM to remove water droplets from the bottom of the septa. The Exetainers are loaded into the autosampler tray and seven individual injections of headspace CO2 are transferred by autosampler on a stream of UHP helium to the IRMS for measurement.
δ13CDIC and δ13CDOC analysis on the same sample is possible but are charged as separate analyses.
Raw data are currently normalized using two in-house standards (UK-ALA, UK-GLY) that have been normalized to the VPDB scale via EA-IRMS using USGS40 and USGS41. During normalization, the first injection is ignored while the last 6 injections are averaged to produce a raw carbon isotope value. Long-term standard deviation of δ13C measurements is still under assessment, but is generally better than 0.4 permil.
Sample size: waters containing between 0.003 to 0.05 mg of dissolved organic carbon can be analyzed. Contact KSIGL staff prior to sample submission if concentrations outside this range are expected. We strongly recommend a 10% duplication of samples for accurate precision determination.
Gandhi H, Wiegner TN, Ostrom PH, Kaplan LA, Ostrom NE. 2004. Isotopic (13C) analysis of dissolved organic carbon in stream water using an elemental analyzer coupled to a stable isotope ratio mass spectrometer. Rapid Communications in Mass Spectrometry 18: 903-906.
Lang SQ, Bernasconi SM, Früh-Green GL. 2012. Stable isotope analysis of organic carbon in small (μg C) samples and dissolved organic matter using a GasBench preparation device. Rapid Communications in Mass Spectrometry 26: 9-16.
Zhou Y, Guo H, Lu H, Mao R, Zheng H, Wang J. 2015. Analytical methods and application of stable isotopes in dissolved organic carbon and inorganic carbon in groundwater. Rapid Communications in Mass Spectrometry 29: 1827-1835.