A method for isotope ratio analysis of water samples is described comprising an on-line high-temperature reduction technique in a helium carrier gas. Using a gas-tight syringe, injection of 0.5 to 1 microL sample is made through a heated septum into a glassy carbon reactor at temperatures in excess of 1300 degrees C. More than 150 injections can be made per day and both isotope ratios of interest, delta2H and delta18O, can be measured with the same setup. The technique has the capability to transfer high-precision stable isotope ratio analysis of water samples from a specialized to a routine laboratory task compatible with other common techniques (automated injection for GC, LC, etc.). Experiments with an emphasis on the reactor design were made in two different laboratories using two different commercially available high-temperature elemental analyser (EA) systems. In the Jena TC/EA unit, sample-to-sample memory (single injection) has been reduced to approximately 1% and high precision of about 0.1 per thousand for delta18O and < 1 per thousand for delta2H has been achieved by a redesign of the glassy carbon reactor and by redirecting the gas flow of the commercial TC/EA unit. With the modified reactor, the contact of water vapour with surfaces other than glassy carbon is avoided completely. The carrier gas is introduced at the bottom of the reactor thereby flushing the outer tube compartment of the tube-in-tube assembly before entering the active heart of the reactor.With the Leipzig high-temperature reactor (HTP) similar precision was obtained with a minor modification (electropolishing) of the injector metal sleeve. With this system, the temperature dependence of the reaction has been studied between 1100 and 1450 degrees C. Complete yield and constant isotope ratio information has been observed only for temperatures above 1325 degrees C. For temperatures above 1300 degrees C the reactor produces an increasing amount of CO background from reaction of glass carbon with the ceramic tube. This limits the usable temperature to a maximum of 1450 degrees C. Relevant gas permeation through the Al2O3 walls has not been detected up to 1600 degrees C.

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In order to generate a local daughter scale from the material defining the international delta13C and delta18O stable isotope ratio scales (NBS19-calcite),1,2 the carbon and oxygen must be liberated to the gas phase, usually as CO2, using acid digestion of the calcite with H3PO4. It is during this conversion step that systematic errors can occur, giving rise to commonly observed discrepancies in isotopic measurements between different stable isotope laboratories. Scale consistency is of particular importance for air-CO2 isotope records where very small differences in isotopic composition have to be reliably compared between different laboratories and quantified over long time periods.3 The information is vital for estimating carbon budgets on regional and global scales and for understanding their variability under the conditions of climate change. Starting from this requirement a number of CO2 preparations from NBS19 were made at Environment Canada (EC) and analyzed in our laboratories together with Narcis II, a set of well-characterized CO2 samples in sealed tubes available from the National Institute for Environmental Studies (NIES).4,5 Narcis II is very homogeneous in delta13C and delta18O with the isotopic composition close to NBS19-CO2. Among our laboratories the results for delta13C agreed to within +/-0.004 per thousand. The same level of agreement in delta13C was obtained when CO2 was generated from NBS19-calcite using different experimental procedures and conditions in the other two laboratories. For delta18O, the corresponding data were +/-0.011 per thousand when using NBS19-CO2 produced at EC, but discrepancies were enhanced by almost one order of magnitude when NBS19-CO2 was prepared by the other laboratories using slightly different reaction conditions (range=0.13 per thousand).In a second series of experiments, larger amounts of CO2 prepared from NBS19 at the Max-Planck-Institut für Biogeochemie (MPI-BGC) were analyzed together with Narcis II and then mixed into CO2-free air. The resulting artificial air samples then were measured by the same three laboratories for the stable isotopic composition of CO2 using locally established extraction and evaluation procedures. Comparison of the results with the prior CO2 values and between the laboratories revealed additional systematic differences owing to the local CO2 extraction processes and standardization procedures. For delta13C the results showed a narrow range of discrepancies of about 0.02 per thousand; for delta18O cumulative disagreements in the range of 0.1 per thousand were observed. From these results the following conclusions are inferred: NBS19-CO2 is a reliable primary anchor to the VPDB delta13C scale. Although prepared by different methods an accuracy of better than +/-0.003 per thousand has been reached. This applies to sample amounts of 5 mg calcite or more.NBS19-CO2 can be used as a general anchor to the VPDB delta18O scale only for accuracy requirements of +/-0.1 per thousand. For a higher scale resolution additional agreements regardin...

The stable carbon isotopic (δ 13 C) reference material (RM) LSVEC Li 2 CO 3 has been found to be unsuitable for δ 13 C standardization work because its δ 13 C value increases with exposure to atmospheric CO 2. A new CaCO 3 RM, USGS44, has been prepared to alleviate this situation. Methods: USGS44 was prepared from 8 kg of Merck high-purity CaCO 3. Two sets of δ 13 C values of USGS44 were determined. The first set of values was determined by online combustion, continuous-flow (CF) isotope-ratio mass spectrometry (IRMS) of NBS 19 CaCO 3 (δ 13 C VPDB = +1.95 milliurey (mUr) exactly, where mUr = 0.001 = 1‰), and LSVEC Li 2 CO 3 (δ 13 C VPDB = −46.6 mUr exactly), and normalized to the two-anchor δ 13 C VPDB-LSVEC isotope-delta scale. The second set of values was obtained by dual-inlet (DI)-IRMS of CO 2 evolved by reaction of H 3 PO 4 with carbonates, corrected for cross contamination, and normalized to the singleanchor δ 13 C VPDB scale. Results: USGS44 is stable and isotopically homogeneous to within 0.02 mUr in 100-μg amounts. It has a δ 13 C VPDB-LSVEC value of −42.21 ± 0.05 mUr. Single-anchor δ 13 C VPDB values of −42.08 ± 0.01 and −41.99 ± 0.02 mUr were determined by DI-IRMS with corrections for cross contamination. Conclusions: The new high-purity, well-homogenized calcium carbonate isotopic reference material USGS44 is stable and has a δ 13 C VPDB-LSVEC value of −42.21 ± 0.05 mUr for both EA/IRMS and DI-IRMS measurements. As a carbonate relatively depleted in 13 C, it is intended for daily use as a secondary isotopic reference material to normalize stable carbon isotope delta measurements to the δ 13 C VPDB-LSVEC scale. It is useful in quantifying drift with time, determining massdependent isotopic fractionation (linearity correction), and adjusting isotope-ratioscale contraction. Due to its fine grain size (smaller than 63 μm), it is not suitable as a δ 18 O reference material. A δ 13 C VPDB-LSVEC value of −29.99 ± 0.05 mUr was determined for NBS 22 oil.

For anchoring CO(2) isotopic measurements on the δ(18)O(VPD-CO2) scale, the primary reference material (NBS 19 calcite) needs to be digested using concentrated ortho-phosphoric acid. During this procedure, great care must be taken to ensure that the isotopic composition of the liberated gas is accurate. Apart from controlling the reaction temperature to ±0.1 °C, the potential for oxygen isotope exchange between the produced CO(2) and water must be kept to a minimum. The water is usually assumed to reside on the walls in the headspace of the reaction vessel. We demonstrate here that a large fraction of the exchange may also occur with water inside the acid. Our results indicate that both exchange reactions have a significant impact on the results and may have largely been responsible for scale inconsistencies between laboratories in the past. The extent of CO(2)/H(2)O oxygen exchange depends on the concentration (amount of free water) in the acid. For acids with a nominal H(3)PO(4) mass fraction of less than 102%, oxygen isotope exchange can create a substantial isotopic bias during high-precision measurements with the degree of the alteration being proportional to the effective isotopic contrast between the acid and the CO(2) released from the calcite. Water evaporating from the acid at 25 °C has a δ(18)O value of -34.5‰ relative to the isotopic composition of the whole acid. This large fractionation is likely to occur in two steps; by exchange with phosphate, water inside the acid is decreased in oxygen-18 relative to the bulk acid by ∼ -22‰. This water is then fractionated further during evaporation. Oxygen exchange with both water inside the acid and water condensate in the headspace can contribute to the measured isotopic signature depending on the experimental parameters. The system employed for this study has been specifically designed to minimize oxygen exchange with water. However, the amount of altered CO(2) for a 95% H(3)PO(4) at 25 °C still accounts for about 3% of the total CO(2) produced from a 40 mg calcite sample, resulting in a δ(18) O range of about 0.8‰ when varying the δ(18)O value of the acid by 25‰. Least biased results for NBS19-CO(2) were obtained for an acid with a δ(18)O value close to +23‰ vs. VSMOW. In contrast, commercial acids from several sources had an average δ(18)O value of +13‰, amounting to a 10‰ offset from the optimal value. This observation suggests that the well-known scale incompatibilities between laboratories could arise from this difference with measurements that may have suffered systematically from non-optimal acid-δ(18)O values, thus producing variable offsets, depending on the experimental details. As a remedy, we suggest that the δ(18)O of phosphoric acid reacted with calcites for establishing a δ(18)O scale anchor be adjusted, and this should reduce the variability of the δ(18)O of CO(2) evolved in acid digestion to less than ±0.05‰. The adjustment should be made by taking into account the difference in δ(18)O between the calcite-CO(2) and the acid, with a target d...