Ensuring the reliability of stable isotope ratio data—beyond the principle of identical treatment

Abstract

The need for inter-laboratory comparability is crucial to facilitate the globalisation of scientific networks and the development of international databases to support scientific and criminal investigations. This article considers what lessons can be learned from a series of inter-laboratory comparison exercises organised by the Forensic Isotope Ratio Mass Spectrometry (FIRMS) network in terms of reference materials (RMs), the management of data quality, and technical limitations. The results showed that within-laboratory precision (repeatability) was generally good but between-laboratory accuracy (reproducibility) called for improvements. This review considers how stable isotope laboratories can establish a system of quality control (QC) and quality assurance (QA), emphasising issues of repeatability and reproducibility. For results to be comparable between laboratories, measurements must be traceable to the international δ-scales and, because isotope ratio measurements are reported relative to standards, a key aspect is the correct selection, calibration, and use of international and in-house RMs. The authors identify four principles which promote good laboratory practice. The principle of identical treatment by which samples and RMs are processed in an identical manner and which incorporates three further principles; the principle of identical correction (by which necessary corrections are identified and evenly applied), the principle of identical scaling (by which data are shifted and stretched to the international δ-scales), and the principle of error detection by which QC and QA results are monitored and acted upon. To achieve both good repeatability and good reproducibility it is essential to obtain RMs with internationally agreed δ-values. These RMs will act as the basis for QC and can be used to calibrate further in-house QC RMs tailored to the activities of specific laboratories. In-house QA standards must also be developed to ensure that QC-based calibrations and corrections lead to accurate results for samples. The δ-values assigned to RMs must be recorded and reported with all data. Reference materials must be used to determine what corrections are necessary for measured data. Each analytical sequence of samples must include both QC and QA materials which are subject to identical treatment during measurement and data processing. Results for these materials must be plotted, monitored, and acted upon. Periodically international RMs should be analysed as an in-house proficiency test to demonstrate results are accurate.

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Notes

  1. 1.

    A critical review of the use of these terms is presented by Thompson [1]

  2. 2.

    The form of this equation follows SI guidelines [2]. Historically, the value was multiplied by 1000 to give values in permil or by 106 to give values in per meg (ppm)

  3. 3.

    Also known as high temperature conversion, high temperature pyrolysis, and carbon reduction

  4. 4.

    Analogous problems have been reported when analysing reduced forms of nitrogen by TC/EA [11]

  5. 5.

    The difference between the VSMOW and VPDB scales is not additive. The conversion determined by Coplen [14] is widely accepted but authors should always state when this formula has been applied (δ 18OVSMOW = 1.03091 × δ 18OVPDB + 30.9)

  6. 6.

    At the time of ILC#3, the IAEA assigned the values δ 13C = −10.43 ‰ and δ 15N = +0.4 ‰

  7. 7.

    The δ-value and uncertainty assigned to many of the international RMs has been re-assessed and reassigned over time and it is important to check these periodically. The most reliable source of information is the CIAAW website: http://www.ciaaw.org

  8. 8.

    Because isotopic differences can be measured most precisely when these differences are small, the δ-value of the working gas should be within the range of isotopic compositions to be measured

  9. 9.

    http://isotopes.usgs.gov/lab/referencematerials.html

  10. 10.

    With the exception of hydrogen, RMs spanning a range of approximately 50 ‰ will encompass most of the natural variation observed

  11. 11.

    This is not the same as linearity correction, which compensates for changes in δ-value in response to variations in sample size

  12. 12.

    This may not be true for elements in extreme oxidation states, specifically nitrates

  13. 13.

    LIMS for isotopic measurements can be obtained from http://isotopes.usgs.gov/research/topics/lims.html

  14. 14.

    Peak integration settings will have a profound effect on the data and must be applied uniformly

  15. 15.

    The δ-values measured for very small peaks can vary unpredictably, because of integration issues, but acceptable measurements of peak area are still possible

  16. 16.

    The introduction of a second anchor point for the VPDB scale in 2006 [15] resulted in the reassignment of many of the δ 13C values for international RMs so it is important to report the values assigned together with any analytical data

  17. 17.

    Higher temperatures and/or chemical additives, for example AgCl, have been proposed as remedies

  18. 18.

    Drying over phosphorus pentoxide, at elevated temperature and under vacuum for several days, is usually recommended [13]

  19. 19.

    Although it is possible to perform measurements on SO+ this is generally considered less sensitive and less reliable because of problems with blanks and interference

  20. 20.

    Oxygen isotope corrections for δ 34S are theoretically possible when both SO+ and SO2 + are measured from the same sample [46, 47]. These general formulae apply [36]:

    $$ {\delta^{18}}\mathrm{O}=24.02\times {\delta^{66}}-23.024\times {\delta^{50 }}\,\mathrm{and}\,{\delta^{34}}\mathrm{S}=1.0908\times {\delta^{66}}-0.0908 \times {\delta^{18}}\mathrm{O} $$

    The latter formula also gives the conventional way of calculating δ 34S from δ 66 values under the assumption that samples and standards are prepared with the same oxygen isotope composition, so that δ 18Ο is zero and can be ignored

  21. 21.

    If the copper is omitted higher temperatures can be used to speed equilibration

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Correspondence to J. F. Carter.

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Published in the topical collection Isotope Ratio Measurements: New Developments and Applications with guest editors Klaus G. Heumann and Torsten C. Schmidt.

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Carter, J.F., Fry, B. Ensuring the reliability of stable isotope ratio data—beyond the principle of identical treatment. Anal Bioanal Chem 405, 2799–2814 (2013). https://doi.org/10.1007/s00216-012-6551-0

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Keywords

  • Stable isotope ratio measurements
  • Isotope-ratio mass spectrometry
  • Reference materials
  • Principle of identical treatment
  • Quality assurance
  • Quality control