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Value Assignment and Uncertainty Estimation of Selected Light Stable Isotope Reference Materials: RMs 8543-8545, RMs 8562-8564, and RM 8566

Published

Author(s)

R M. Verkouteren, D Klinedinst

Abstract

We report the technical basis for value assignments of several carbon and oxygen stable isotope reference materials distributed by NIST and the International Atomic Energy Agency (IAEA), and make recommendations regarding laboratory practices and future intercomparison exercises. Assignments and estimates of uncertainty are based on measurement data using one method at NIST and another method at outside collaborating laboratories. All methods are based on isotope ratio mass spectrometry (IRMS) of carbon dioxide: outside collaborators measured three isobaric forms of CO2, whereas NIST measured four forms. This additional capability allowed NIST to assess measurement accuracy, and design an IRMS method that minimized bias from cross-contamination.The international comparison exercise was designed, involving standard chemical preparation and isotopic measurement - under strict requirements designed by the IAEA-Working Group for Reference Materials - of CO2 derived from carbonates, waters, and pure CO2 reference materials. Our goal was to determine precise {delta-C-13} and {delta-O-18} value assignments that were consistent across the materials and corrected for cross-contamination, and to relate variations in results with specific laboratory practices to guide future intercomparisons. Measurement data from all participants were compiled in a spreadsheet and inspected for consistency. Those data meeting the performance requirements were corrected for cross-contamination through direct measurement of this effect, then processed using internationally accepted procedures to determine standardized {delta-C-13} and {delta-O-18} values. For the carbonate and CO2 materials, the results of this intercomparison provide value assignments shifted slighted towards more depleted compositions with uncertainties improved by factors up to two over the previous assignments. For the water materials, results were more variable yet suggested that the {delta-O-18} value of SLAP on the VSMOW isotope ratio scale is between -55.7 and -56.2 . Statistical approaches were used to discern relationships between results and discretionary factors; these relationships are described.To investigate the effects of oxygen isotope compositional assumptions used in data processing, several sets of assumptions were applied. Results are compared and recommendations made regarding the use of these assumptions and the effect on intercomparability of data.
Citation
Special Publication (NIST SP) - 260-149
Report Number
260-149

Keywords

carbon, cross-contamination, intercomparison, isotope, mass spectrometry, multivariate analysis, oxygen, reference material, standard
Created December 1, 2003, Updated February 19, 2017