CSTL Program: International Measurement Standards

Improvement of Value Assignments for NIST and IAEA Stable Isotope Reference Materials

R.M. Verkouteren, D.B. Klinedinst, M. Groening (IAEA)

Abstract: Operational artifacts were recently discovered in isotope ratio mass spectrometers (IRMS). This prompted the International Atomic Energy Agency (IAEA) to convene a panel of experts to inspect the susceptible isotope metrology. One recommendation was a comparison exercise, led by NIST, of key isotope Reference Materials (RMs) designed to overcome the limitations of IRMS.

The intercomparison exercise was completed this year and data were processed through a standard algorithm (http://www.nist.gov/widps-co2). Results show significant shifts in most value assignments with up to 3-fold improvements in uncertainties over prior exercises.

International utilization of the NIST isotope RMs includes the Institute for Reference Materials and Measurements (IRMM) in Belgium, where the SI has been realized through NIST RMs 8562, 8563, and 8564. This traceability will give modelers of climate change more accurate information regarding the global carbon cycle. The National Institute for Environmental Studies (NIES) in Japan has used NIST RMs 8562 and 8564 to calibrate a specialized CO₂ standard for detailed studies of ocean-atmosphere exchange. Researchers at the Max-Planck Institute in Germany, under the European Network for Research in Global Climate Change, have utilized the NIST Web-based standard algorithm to verify the accuracy of isotopic measurements used for extremely precise analysis of atmospheric species.

Purpose: Subtle yet significant operational and data evaluation artifacts were recently discovered in modern isotope ratio mass spectrometers (IRMS). This prompted the International Atomic Energy Agency (IAEA) to convene a panel of technical experts to inspect the susceptible isotope metrology frameworks and to assess the impacts to the international isotope measurement communities and IAEA Member States. A recommendation from the panel was the reassessment of value assignments of ¹³C and ¹⁸O isotope Reference Materials (RMs) distributed by the IAEA and NIST. A comparison exercise, led by NIST, of key RMs was designed to overcome the inherent limitations of IRMS in order to substantially improve the accuracy and consistency of the RM value assignments.

Major Accomplishments. The intercomparison exercise was completed this year with the reporting of measurement data from Poland, Canada, and China. These data were merged with data from The Netherlands, Germany, Austria, and the United States, and were processed through a standard algorithm now available on a NIST web site (http://www.nist.gov/widps-co2). Results show significant shifts in most values and up to 3-fold improvements in uncertainties over prior exercises.

Impact. Interest from several international and U.S. industrial groups has been evident this year:

·       Researchers at the Institute for Reference Materials and Measurements (IRMM) in Belgium have realized the SI through absolute IRMS of the three NIST CO₂ isotope RMs (RM 8562, RM 8563, RM 8564). These determinations of the absolute ratios of CO₂ isotopomers now give modelers of climate change more accurate information regarding the global carbon cycle.

·       The National Institute for Environmental Studies (NIES) in Japan has used RM 8562 and RM 8564 to value assign a CO₂ Reference Material produced especially for ocean studies. This material has an oxygen isotopic signature very close to ocean water, and will be useful for detailed studies of ocean-atmosphere chemical exchange.

·       Researchers at the Max-Planck Institute in Germany, under the European Network for Research in Global Climate Change (ENRICH, DG XII), have utilized the NIST Web-based data processing system to verify the accuracy of isotopic measurements used for extremely precise analysis of carbonaceous species in the atmosphere.

Future Plans. The task is completed, and our activities are being redirected towards problem-driven R&D although the core program will also evolve with new international needs. U.S. industry representatives at the NIST-NOAA Workshop on Atmospheric Measures and Standards identified the need to develop more precise methods for the accurate apportionment of carbonaceous source emissions. These methods included the development of SRMs/RMs that are representative (chemically and isotopically) of atmospheric PM samples. The ¹³C isotope RMs developed over the last several years will be a foundation for this application.

We anticipate certifying the ¹³C and ¹⁸O value assignments of key RMs, as well as developing natural gas standards and isotope dilution mass spectrometry (IDMS) for carbon certification in soils and PM. The natural gas standards and the IDMS development are now funded as SRM tasks.

Several international atmospheric monitoring networks and organizations (Asia-Pacific Network, CSIRO; INSTAAR/NOAA; AEROCARB; WMO) are searching for a solution to a long-standing measurement traceability problem. Biases in high-precision measurements of atmospheric CO₂, including isotopic composition, exist across these networks. While the NIST isotope standards have been helpful to researchers in these networks, there are further needs to improve the commutivity of these standards and the reproducibility of the CO₂-in-real air measurements. We are actively involved in international discussions to address these issues, which include the proposed GlobalHubs and BIPM/CCQM mechanisms.