Improved method for isotopic and quantitative analysis of dissolved inorganic carbon in natural water samples.

We present here an improved and reliable method for measuring the concentration of dissolved inorganic carbon (DIC) and its isotope composition (delta(13)C(DIC)) in natural water samples. Our apparatus, a gas chromatograph coupled to an isotope ratio mass spectrometer (GCIRMS), runs in a quasi-automated mode and is able to analyze about 50 water samples per day. The whole procedure (sample preparation, CO(2(g))-CO(2(aq)) equilibration time and GCIRMS analysis) requires 2 days. It consists of injecting an aliquot of water into a H(3)PO(4)-loaded and He-flushed 12 mL glass tube. The H(3)PO(4) reacts with the water and converts the DIC into aqueous and gaseous CO(2). After a CO(2(g))-CO(2(aq)) equilibration time of between 15 and 24 h, a portion of the headspace gas (mainly CO(2)+He) is introduced into the GCIRMS, to measure the carbon isotope ratio of the released CO(2(g)), from which the delta(13)C(DIC) is determined via a calibration procedure. For standard solutions with DIC concentrations ranging from 1 to 25 mmol . L(-1) and solution volume of 1 mL (high DIC concentration samples) or 5 mL (low DIC concentration samples), delta(13)C(DIC) values are determined with a precision (1sigma) better than 0.1 per thousand. Compared with previously published headspace equilibration methods, the major improvement presented here is the development of a calibration procedure which takes the carbon isotope fractionation associated with the CO(2(g))-CO(2(aq)) partition into account: the set of standard solutions and samples has to be prepared and analyzed with the same 'gas/liquid' and 'H(3)PO(4)/water' volume ratios. A set of natural water samples (lake, river and hydrothermal springs) was analyzed to demonstrate the utility of this new method.

[1]  H. Craig.  Isotopic standards for carbon and oxygen and correction factors for mass-spectrometric analysis of carbon dioxide , 1957 .

[2]  I. Barnes,et al.  Warm springs, South Island, New Zealand, and their potentials to yield laumontite , 1978 .

[3]  A. Aucour,et al.  Use of 13C to trace origin and cycling of inorganic carbon in the Rhône river system , 1999 .

[4]  R. Krishnamurthy,et al.  Isotopic and geochemical evolution of ground and surface waters in a karst dominated geological setting: a case study from Belize, Central America , 2004 .

[5]  S. Caliro,et al.  Variation in the total dissolved carbon isotope composition of thermal waters of the Island of Ischia (Italy) and its implications for volcanic surveillance , 1999 .

[6]  S. Schiff,et al.  Evaluating Dissolved Inorganic Carbon Cycling in a Forested Lake Watershed Using Carbon Isotopes , 1992, Radiocarbon.

[7]  L. Cifuentes,et al.  A rapid and precise method for measuring stable carbon isotope ratios of dissolved inorganic carbon , 2000 .

[8]  K. Telmer,et al.  Carbon fluxes, pCO2 and substrate weathering in a large northern river basin, Canada: carbon isotope perspectives , 1999 .

[9]  Gilles St-Jean,et al.  Automated quantitative and isotopic (13C) analysis of dissolved inorganic carbon and dissolved organic carbon in continuous-flow using a total organic carbon analyser. , 2003, Rapid communications in mass spectrometry : RCM.

[10]  J. Szaran.  Carbon isotope fractionation between dissolved and gaseous carbon dioxide , 1998 .

[11]  D. Solomon,et al.  On the isotopic composition of carbon in soil carbon dioxide , 1991 .

[12]  A. Mix,et al.  Precise δ13C analysis of dissolved inorganic carbon in natural waters using automated headspace sampling and continuous‐flow mass spectrometry. , 2005 .

[13]  W. G. Mook,et al.  CARBON ISOTOPE FRACTIONATION BETWEEN DISSOLVED BICARBONATE AND GASEOUS CARBON-DIOXIDE , 1974 .

[14]  D. Wilbur,et al.  CARBON ISOTOPE FRACTIONATION DURING GAS-WATER EXCHANGE AND DISSOLUTION OF CO2 , 1995 .

[15]  C. Hillaire‐Marcel,et al.  Seasonal changes in the sources and fluxes of dissolved inorganic carbon through the St. Lawrence River—isotopic and chemical constraint , 2002 .

[16]  R. Harmon,et al.  Stable carbon isotope ratios and the existence of a gas phase in the evolution of carbonate ground waters , 1974 .

[17]  P. Wachniew,et al.  Carbon budget of a mid-latitude, groundwater-controlled lake: Isotopic evidence for the importance of dissolved inorganic carbon recycling , 1997 .

[18]  R. Weiss.  Carbon dioxide in water and seawater: the solubility of a non-ideal gas , 1974 .

[19]  K. Telmer,et al.  Chemical dynamics of the “St. Lawrence” riverine system: δDH2O, δ18OH2O, δ13CDIC, δ34Ssulfate, and dissolved 87Sr/86Sr , 1996 .

[20]  R. Wieler,et al.  Accumulation of mantle gases in a permanently stratified volcanic lake (Lac Pavin, France) , 1999 .

[21]  J. McCrea.  On the Isotopic Chemistry of Carbonates and a Paleotemperature Scale , 1950 .

[22]  Henry Lin,et al.  Surface and Ground Water, Weathering, and Soils , 2006 .

[23]  T. Miyajima,et al.  Determining the stable isotope ratio of total dissolved inorganic carbon in lake water by GC/C/IIRMS , 1995 .

[24]  C. Spötl.  A robust and fast method of sampling and analysis of δ13C of dissolved inorganic carbon in ground waters , 2005, Isotopes in environmental and health studies.

[25]  M. Shaw,et al.  Automated analysis of 13C/12C ratios in CO2 and dissolved inorganic carbon for ecological and environmental applications. , 2003, Rapid communications in mass spectrometry : RCM.

[26]  M. Meybeck,et al.  Risques d'eruption gazeuse carbonique en Auvergne , 1993 .

[27]  W. G. Deuser,et al.  Carbon Isotope Fractionation in the System CO2(gas)—CO2(aqueous)—HCO3−(aqueous) , 1967, Nature.

[28]  M. O'Leary.  Measurement of the isotope fractionation associated with diffusion of carbon dioxide in aqueous solution , 1984 .

[29]  S. Inguaggiato,et al.  A simple method to determine the δ13C content of total dissolved inorganic carbon , 2002 .

[30]  J. Veizer,et al.  Carbon cycle in St. Lawrence aquatic ecosystems at Cornwall (Ontario), Canada: seasonal and spatial variations , 1999 .

[31]  R. Krishnamurthy,et al.  Seasonal variations of dissolved inorganic carbon and δ13C of surface waters : application of a modified gas evolution technique , 1998 .

[32]  J. Hayes,et al.  Isotopic and quantitative analysis of the major carbon fractions in natural water samples , 1976 .

[33]  W. G. Mook,et al.  Isotopic fractionation between gaseous and dissolved carbon dioxide , 1970 .