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Methods and limitations of 'clumped' CO_2 isotope (Δ_(47)) analysis by gas-source isotope ratio mass spectrometry

Huntington, K. W. and Eiler, J. M. and Affek, H. P. and Guo, W. and Bonifacie, M. and Yeung, L. Y. and Thiagarajan, N. and Passey, B. and Tripati, A. and Daëron, M. and Came, R. (2009) Methods and limitations of 'clumped' CO_2 isotope (Δ_(47)) analysis by gas-source isotope ratio mass spectrometry. Journal of Mass Spectrometry, 44 (9). pp. 1318-1329. ISSN 1096-9888.

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The geochemistry of multiply substituted isotopologues (clumped-isotope geochemistry) examines the abundances in natural materials of molecules, formula units or moieties that contain more than one rare isotope (e.g. ^(13)C^(18)O^(16)O, ^(18)O^(18)O, ^(15)N_2, ^(13)C^(18)O^(16)O_2^(2-). Such species form the basis of carbonate clumped-isotope thermometry and undergo distinctive fractionations during a variety of natural processes, but initial reports have provided few details of their analysis. In this study, we present detailed data and arguments regarding the theoretical and practical limits of precision, methods of standardization, instrument linearity and related issues for clumped-isotope analysis by dual-inlet gas-source isotope ratio mass spectrometry (IRMS). We demonstrate long-term stability and subtenth per mil precision in 47/44 ratios for counting systems consisting of a Faraday cup registered through a 10^(12)Ω resistor on three Thermo-Finnigan 253 IRMS systems. Based on the analyses of heated CO_2 gases, which have a stochastic distribution of isotopes among possible isotopologues, we document and correct for (1) isotopic exchange among analyte CO_2 molecules and (2) subtle nonlinearity in the relationship between actual and measured 47/44 ratios. External precisions of ~0.01 are routinely achieved for measurements of the mass-47 anomaly (a measure mostly of the abundance anomaly of ^(13)C-^(18)O bonds) and follow counting statistics. The present technical limit to precision intrinsic to our methods and instrumentation is ~5 parts per million (ppm), whereas precisions of measurements of heterogeneous natural materials are more typically ~10 ppm (both 1 s.e.). These correspond to errors in carbonate clumped-isotope thermometry of ±1.2 °C and ±2.4 °C, respectively.

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Additional Information:© 2009 John Wiley & Sons, Ltd. Received: 20 January 2009; Accepted: 4 June 2009. Published Online: 20 Jul 2009. This work was supported by the National Science Foundation and by the Division of Geological and Planetary Sciences and the Davidow Fund at the California Institute of Technology. H.P.A. thanks the Earth System Center for Stable Isotope Studies of the Yale Institute for Biospheric Studies. The authors also thank John Hayes and an anonymous reviewer for insightful comments that improved this article.
Funding AgencyGrant Number
Caltech Division of Geological and Planetary ScienceUNSPECIFIED
Caltech Davidow FundUNSPECIFIED
Subject Keywords:carbonate thermometry, isotopologues, mass 47, clumped isotopes, precision
Issue or Number:9
Record Number:CaltechAUTHORS:20100108-201754636
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:17124
Deposited By: Joy Painter
Deposited On:11 Jan 2010 17:42
Last Modified:03 Oct 2019 01:23

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