NIST-led international collaboration results in improved measurement technology for Light Stable Isotopes relevant to climate change research. (General Developments).

Two manuscripts recently have been published in Rapid Communications in Mass Spectrometry [RCM 17, 771-776 (2003) and RCM 17, 777-782 (2003)] that detail an international collaboration to identify, understand at a fundamental level, and correct measurement biases manifested in gas isotope ratio mass spectroeters. These biases arise from memory effects in ion sources and affect the reliability and comparability of carbon-13 and oxygen-18 measurements used in studies requiring high interlaboratory reproducibility, including research relevant to climate change. In this regard, measurements of light stable isotopes in carbon dioxide, methane, and other atmospheric trace gases provide a unique means to better understand their sources, fates, and contributions in biogeochemical cycles. This collaboration already has resulted in the tangible improvement of the measurement tools used by hundreds of international research groups, and improved the value assignments of isotope reference materials distributed by NIST and the International Atomic Energy Agency (IAEA).

The first manuscript details an intercomparison exercise that was successful in providing extensive measurements of sufficient quality needed to construct mechanistic models of memory effects in the mass spectrometers. The models were consistent with the presence of two instrumental memory sources--one short-lived (10 s to 20 s) and the other long-lived (6 min to 10 min)--that could not be compensated accurately by normal background corrections. Observed biases were substantial and dependent upon several operational parameters. The results, originally presented in December 1995 to the IAEA Consultants' Meeting for Light Stable Isotope Reference Materials in Vienna, Austria, initiated research and development activities to address these effects at several other organizations in collaboration with NIST.

The second manuscript describes the results of a collaboration with an instrument manufacturer, to test new focusing plates and slits in the NIST ion source and in an identical instrument at the Max-Planck-Institute for Chemistry in Mainz, Germany. The new ion source materials were designed and engineered to minimize the specific memory effects previously identified, and historical data from the NIST instrument provided the benchmarks needed to compare precisely and document the change in observed performance. Biases in carbon-13 and oxygen-18 measurements were decreased by up to a factor of five, which enabled up to a 50 % reduction in the uncertainty of reference material value assignments. The newly engineered ion source is incorporated in the isotope ratio instruments, which has benefited the research of greater than 550 research groups worldwide. This manuscript reveals the technology to a wider audience of instrument manufacturers and offers several practical recommendations to the measurement communiti es regarding the minimization of the biasing effects.

CONTACT: Michael Verkouteren, [phone]; [e-mail].