title: Improved method for isotopic and quantitative analysis of dissolved inorganic carbon in natural water samples.
abstract: 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.
authors.affiliation: Laboratoire de Physico-chimie des Fluides Géologiques, Institut de Physique du Globe de Paris & Université Paris 7 - UMR CNRS 7154, 2 Place Jussieu, 75251 Paris Cedex 05, France. assayag@ipgp.jussieu.fr
authors.forename: Nelly
authors.initials: N
authors.lastname: Assayag
authors.forename: Karine
authors.initials: K
authors.lastname: Rivé
authors.forename: Magali
authors.initials: M
authors.lastname: Ader
authors.forename: Didier
authors.initials: D
authors.lastname: Jézéquel
authors.forename: Pierre
authors.initials: P
authors.lastname: Agrinier
mesh_terms: D002244:Carbon / Q000032:analysis*; D002247:Carbon Isotopes / Q000032:analysis; D005618:Fresh Water / Q000032:analysis* / Q000737:chemistry*; D008401:Gas Chromatography-Mass Spectrometry / Q000379:methods*; D007287:Inorganic Chemicals / Q000032:analysis*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012995:Solubility; D012996:Solutions; D013048:Specimen Handling / Q000379:methods*
chemical_list: D002247:Carbon Isotopes; D007287:Inorganic Chemicals; D012996:Solutions; D002244:Carbon