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Calcite‐CO 2 mixed into CO 2 ‐free air: a new CO 2 ‐in‐air stable isotope reference material for the VPDB scale
Author(s) -
Ghosh Prosenjit,
Patecki Michael,
Rothe Michael,
Brand Willi A.
Publication year - 2005
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.1886
Subject(s) - calcite , chemistry , carbonate , stable isotope ratio , analytical chemistry (journal) , isotope , isotope ratio mass spectrometry , laboratory flask , mineralogy , isotopes of carbon , isotope analysis , environmental chemistry , mass spectrometry , geology , chromatography , total organic carbon , physics , oceanography , organic chemistry , quantum mechanics
In order to generate a reliable and long‐lasting stable isotope ratio standard for CO 2 in samples of clean air, CO 2 is liberated from well‐characterized carbonate material and mixed with CO 2 ‐free air. For this purpose a dedicated a cid r eaction and a ir mi xing s ystem (ARAMIS) was designed. In the system, CO 2 is generated by a conventional acid digestion of powdered carbonate. Evolved CO 2 gas is mixed and equilibrated with a prefabricated gas comprised of N 2 , O 2 , Ar, and N 2 O 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‐CO 2 work (about ±0.015‰ for δ 13 C and ±0.025‰ for δ 18 O). To establish a valid δ 18 O 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, CO 2 ‐in‐air mixtures were generated from a selection of reference materials; (1) the material defining the VPDB isotope scale (NBS 19, δ 13 C = +1.95‰ and δ 18 O = −2.2‰ exactly); (2) a local calcite similar in isotopic composition to NBS 19 (‘MAR‐J1’, δ 13 C = +1.97‰ and δ 18 O = −2.02‰), and (3) a natural calcite with isotopic compositions closer to atmospheric values (‘OMC‐J1’, δ 13 C = −4.24‰ and δ 18 O = −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 CO 2 reference gases (Narcis I and II, NIST‐RM 8563, GS19 and GS20) were mixed with CO 2 ‐free synthetic air. Independently, the pure CO 2 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 CO 2 ‐in‐air (e.g. CG‐99, based on calibration of pure CO 2 gas) has been carefully established. The systematic high‐precision comparison of secondary carbonate and CO 2 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 CO 2 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.