HR: 1340h
AN: B13A-0886    [Abstracts]
TI: Uncertainty Estimation for Extreme Isotopic Ratios Measured in Multi-Collector Mass Spectrometers
AU: * Essex, R M
EM: Richard.essex@ch.doe.gov
AF: U.S. Department of Energy New Brunswick Laboratory, 9800 South Cass Avenue Building 350, Argonne, IL 60439, United States
AU: Thomas, R B
EM: Rebecca.thomas@ch.doe.gov
AF: U.S. Department of Energy New Brunswick Laboratory, 9800 South Cass Avenue Building 350, Argonne, IL 60439, United States
AB: Isotopic abundances for elements of interest to the earth science community can vary by many orders of magnitude. Commonly used detector systems (Faraday, Daly, SEM) can not encompass the entire dynamic range of interest and making measurements at the limits of the useful range for a single type of detector can also present special difficulties. Therefore, estimating a measurement uncertainty that accurately represents the level of confidence in the results obtained for an extreme ratio (<1:10,000) requires the careful consideration of several components that are not applicable to more conventional isotopic ratio measurement. To be consistent with widely accepted principles for determination of measurement uncertainty, as expressed in the ISO "Guide to the Expression of Uncertainty in Measurement" (GUM), it is necessary to consistently evaluate the contributions of these additional components and provide an uncertainty budget for measurement results. This is of particular importance for measurements of extreme isotopic ratios because many of the uncertainty components are associated with measurement biases. For mass spectrometric measurements of elements with relatively evenly distributed isotopic abundances (e.g. Sr and Nd), multicollector dynamic analysis techniques can reduce the significant uncertainty contributors to measurement variability and uncertainty of the internal normalization ratio. For more extreme ratios such as ²³⁴U/²³⁸U and ²³⁰Th/²³²Th, not only do uncertainties associated with measurement variability and mass bias corrections have to be considered, but several other components can be significant or even dominant contributors to measurement uncertainty. These components include uncertainties associated with detector inter-calibration, detector linearity, electronic baseline, background corrections, dark noise, dead time, and mass interferences. Although one or several of these parameters may not be significant contributors to the uncertainty for a particular measurement, the relative significance of any individual component can change dramatically depending upon signal intensity. Therefore, a proper uncertainty evaluation will, at least initially, include all of the identifiable components associated with that particular measurement system. The relationship of these various components to the total uncertainty should be presented in an uncertainty budget which provides transparency for the uncertainty estimate and is a useful tool for determining where best to seek improvements in accuracy or precision.
DE: 0452 Instruments and techniques
DE: 1040 Radiogenic isotope geochemistry
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1194 Instruments and techniques
SC: Biogeosciences [B]
MN: 2007 Fall Meeting