Calcite‐CO2 mixed into CO2‐free air: a new CO2‐in‐air stable isotope reference material for the VPDB scale

Abstract

AbstractIn order to generate a reliable and long‐lasting stable isotope ratio standard for CO2 in samples of clean air, CO2 is liberated from well‐characterized carbonate material and mixed with CO2‐free air. For this purpose a dedicated acid reaction and air mixing system (ARAMIS) was designed. In the system, CO2 is generated by a conventional acid digestion of powdered carbonate. Evolved CO2 gas is mixed and equilibrated with a prefabricated gas comprised of N2, O2, Ar, and N2O at close to ambient air concentrations. Distribution into glass flasks is made stepwise in a highly controlled fashion. The isotopic composition, established on automated extraction/measurement systems, varied within very small margins of error appropriate for high‐precision air‐CO2 work (about ±0.015‰ for δ13C and ±0.025‰ for δ18O). To establish a valid δ18O relation to the VPDB scale, the temperature dependence of the reaction between 25 and 47°C has been determined with a high level of precision. Using identical procedures, CO2‐in‐air mixtures were generated from a selection of reference materials; (1) the material defining the VPDB isotope scale (NBS 19, δ13C = +1.95‰ and δ18O = −2.2‰ exactly); (2) a local calcite similar in isotopic composition to NBS 19 (‘MAR‐J1’, δ13C = +1.97‰ and δ18O = −2.02‰), and (3) a natural calcite with isotopic compositions closer to atmospheric values (‘OMC‐J1’, δ13C = −4.24‰ and δ18O = −8.71‰). To quantitatively control the extent of isotope‐scale contraction in the system during mass spectrometric measurement other available international and local carbonate reference materials (L‐SVEC, IAEA‐CO‐1, IAEA‐CO‐8, CAL‐1 and CAL‐2) were also processed. As a further control pure CO2 reference gases (Narcis I and II, NIST‐RM 8563, GS19 and GS20) were mixed with CO2‐free synthetic air. Independently, the pure CO2 gases were measured on the dual inlet systems of the same mass spectrometers. The isotopic record of a large number of independent batches prepared over the course of several months is presented. In addition, the relationship with other implementations of the VPDB‐scale for CO2‐in‐air (e.g. CG‐99, based on calibration of pure CO2 gas) has been carefully established. The systematic high‐precision comparison of secondary carbonate and CO2 reference materials covering a wide range in isotopic composition revealed that assigned δ‐values may be (slightly) in error. Measurements in this work deviate systematically from assigned values, roughly scaling with isotopic distance from NBS 19. This finding indicates that a scale contraction effect could have biased the consensus results. The observation also underlines the importance of cross‐contamination errors for high‐precision isotope ratio measurements.As a result of the experiments, a new standard reference material (SRM), which consists of two 5‐L glass flasks containing air at 1.6 bar and the CO2 evolved from two different carbonate materials, is available for distribution. These ‘J‐RAS’ SRM flasks (‘Jena‐Reference Air Set’) are designed to serve as a high‐precision link to VPDB for improving inter‐laboratory comparability.a Copyright © 2005 John Wiley & Sons, Ltd.