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History of Value Assignments

As opposed to NIST Standard Reference Materials (SRMs), the value assignments and uncertainties of RMs (at NIST and IAEA) are sometimes updated as new data become available, or when new standard methods for data reduction or scale realization are adopted. Since this site became active, there have been three major intercomparison exercises that affected the value assignments (and estimated uncertainties) of the reference materials used in the interactive algorithm.

• Verkouteren (1999) Preparation, characterization, and value assignment of carbon dioxide isotopic reference materials: RMs 8562, 8563, and 8563. Anal. Chem. 71, 4740-4746. {Notes: 13 laboratory intercomparison, 8 RMs, dual-inlet IRMS, discretionary procedures only, delta-13-C and delta-18-O reported by labs, no normalization}
  

• Verkouteren and Klinedinst (2004) Value assignment and uncertainty estimation of selected light stable isotope reference materials: RMs 8543-8545, RMs 8562-8564, and RM 8566. NIST Special Publication 260-149 (2004 Edition) 56p. {Notes: 7 laboratory intercomparison, 9 RMs, dual-inlet IRMS, mandatory procedures, delta-45-CO2 and delta-46-CO2 reported by labs, delta-18-O values normalized -- original version of SP260-149 reported unnormalized values}

• Coplen et al. (2006) New guidelines for delta-13-C measurements. Anal. Chem. accepted for publication (DOI: 10.1021/ac052027c, web-release Feb 16, 2006 {Notes: 4 laboratory intercomparison, 21 RMs, continuous-flow IRMS, delta-C-13 reported by labs, Bayesian statistical analysis, delta-13-C values normalized}

At NIST, the Reports of Investigation that accompany the distribution of reference materials are updated as new information becomes available.
Users of RMs are encouraged to check these Reports on the NIST website for current information.