D.B. Klinedinst, J. Lee, and R.M. Verkouteren

Objective: To enable U.S. industry to realize high-accuracy isotope ratio measurements by developing a relevant suite of isotopic reference materials and associated Web-based interactive data tools to be used for calibration and normalization procedures.

Problem: The use of 13C and 18O high precision isotope ratio mass spectrometry (IRMS) has successfully been applied to high dollar impact industrial and environmental problems such as product authentication and groundwater pollution remediation. However, even with state-of-the-art measurement precision of about 0.03 ‰, achieving high accuracy d13C and d18O measurements has been elusive, which has limited many industrial and environmental applications of this technique. One of the primary factors contributing to this situation is the methodology employed to prepare the primary standard. By international consensus, d13C measurements are reported relative to the VPDB scale that has assigned values based on measurements of the primary standard artifact RM 8544, a carbonate material. d13C IRMS require generation and measurement of CO2 gas. Despite years of debate within the working community, an accepted standardized procedure for the conversion of the RM carbonate to CO2 is lacking, and methods for the conversion, standardization and normalization of CO2 isotopic measurement data to d13C values are inconsistent across laboratories.

Approach: We are developing the infrastructural tools (RMs and Web-based data routines) for utilizing CO2 proxies rather than carbonate standards to realize the international isotopic scales for carbon and oxygen. Three CO2 isotopic RMs that span the natural range of d13C compositions were prepared and characterized in our laboratory,1 the value and uncertainty assignments of which were determined by international consensus and unique measurement capabilities at NIST. Since wide compositional (13C and 18O abundance) gaps existed between these materials, we contacted several large providers and industrial manufacturers of CO2 to help fill these gaps. Each manufacturer sent a cylinder of their liquefied product, and we analyzed samples for purity using self-consistent high accuracy measurement procedures developed in our laboratory, and provided feedback regarding the isotopic composition. We are also developing a Web-based interactive standard reference data reduction algorithm for use with the RMs. This algorithm (at http://www.acg.nist.gov/outputs/algorithm.html) permits an internet user to input measurement data of samples and RMs in a particular format that allows consistent data reduction, standardization, and normalization. The isotopic results can be calculated and returned to the user using a well-tested routine.

Results and Future Plans: We have completed our initial characterization of a suite of gases consisting of CO2 derived from: 1) a polymer fiber manufacturing plant, where byproduct acid is neutralized, 2) a subterranean CO2 archive, used by dry ice manufacturers and oil companies for petroleum recovery, 3) C4 plants used for fermentation in a distillery, 4) C3 plants used for fermentation in a brewery, and 5) the co-generation of H2 and CO2 associated with steam refining of petrochemicals. All but one of these CO2 sources were of adequate purity to be considered candidate RMs. The standard reference data reduction algorithm was written in C++ programming code, compiled onto a Unix workstation, tested, and integrated into our Website. This algorithm will continually be improved as Web tools and internet standards improve. The combination of standard materials and Web-based data routines provides a powerful infrastructural tool for assuring data quality. Through this mechanism, traceability of CO2 measurements to the international isotopic scales is improved by a factor of four, as measured by an international comparison exercise.

Publications: Verkouteren, R.M., Preparation, Characterization, and Value Assignment of Carbon Dioxide Isotopic Reference Materials: RMs 8562, 8563 and 8564, Analytical Chemistry, 71, (1999) 4740-4746.

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