Interactive Internet Site Established to Improve Traceability of Carbon and Oxygen Isotope Measurements

Common Gateway Interface (CGI) form used for the input of carbon dioxide isotope measurements through the Web-based data-processing system

To address a fundamental and long-standing measurement problem in stable isotope metrology, CSTL’s Atmospheric Chemistry Group has established a Web-based tool at the site http://www.nist.gov/widps-co2. This tool, a Common Gateway Interface (CGI) program, allows raw isotopic measurements of carbon dioxide from field laboratories to be centrally processed through standard data algorithms. This is the first use of the Internet to disseminate and integrate normative standard numerical procedures with NIST Reference Materials, with the result of improving measurement traceability and adding value to NIST products and services. The site has been designed especially to benefit the atmospheric, health, and industrial communities that collect and evaluate isotopic data from extensive monitoring networks. Such information is used to formulate effective pollution control strategies for a wide variety of carbonaceous species, including airborne particulate matter, hazardous air pollutants, and “greenhouse” gases.

Procedures (such as algorithmic reduction, standardization, and normalization) needed to process high-precision stable isotope measurement data are non-trivial, and inconsistencies in these procedures across laboratories have contributed to the poor reproducibility of reported ¹³C and ¹⁸O values observed in intercomparisons. This problem had prevented precise value assignments, inflated the uncertainty estimates of isotope Reference Materials (RMs), and limited the full potential and usefulness of field measurements. To improve this situation, we worked with the International Atomic Energy Agency to define standard procedures for stable isotope measurement and data processing. We are now disseminating this information through the Website, where measurement data of samples and RMs may be entered into specified fields. These data are sent from the user to a NIST server, where appropriate calculations are performed and the results instantaneously returned.

Users may validate proprietary algorithms embedded in their laboratory instrumentation, and specify the values of fundamental variables usually fixed in reduction algorithms in order to see the effect upon the calculations. Several international laboratories have independently verified the accuracy of the algorithm for natural abundance measurements, as well as slightly enriched compositions up to about 150 % relative to normal abundances of ¹³C and ¹⁸O. In using this Website, threefold improvements in reproducibility across laboratories have been reported in ¹³C and ¹⁸O values as measured in a recent intercomparison. Future plans for the Website include incorporating uncertainty estimation, and expansion to include other chemical-isotopic species.

CONTACT: Mike Verkouteren, ext. 3933