Cosmogenic isotope measurement inter-comparison

1. Cosmogenic isotope measurement inter-comparison Marian Scott, Tim Jull University of Glasgow, University of Arizona July 2008

2. The CRONUS inter-comparison • To assess comparability of measurements made by the different laboratories • To assist in the definition of overall uncertainty when results from different laboratories used. • Carried out through a programme of comparisons, small number of samples distributed to the laboratories.

3. The quality of the measurement is determined by the laboratory. Sound, reliable, precise and accurate measurement requires trace-ability to community agreed reference materials and standards

4. Reference materials • Basic purpose: improvement of comparability of measurement results • Possible uses • for calibration, to demonstrate trace-ability • for quality control, to verify the performance of a method

5. Reference materials • For calibration, material often artificially produced so its properties are known with low uncertainty • for qualitycontrol, material is often ‘real world’, so that it behaves as similarly as possible to the samples being measured.

6. Objectives of TCN within CRONUS • To explore the comparability of results from the different laboratories • generate consensus values for a range of reference materials • to assist laboratories in independently assessing quality and • to quantify precision and accuracy

7. What information can be quantified from an inter-comparison? • Accuracy (from known activity samples) • Laboratory precision (from duplicate samples) leading ultimately to quantification of • Measurement uncertainty • Quality Assurance (QA)

8. QA is an early warning system - it is retrospective and dynamic, based on judgement of the measurements on backgrounds, standards and reference samples (including internal laboratory materials). It is the laboratory paper trail, and in-house checks What is QA?

9. Quality issues • User concerns • How good are my results?what does the quoted uncertainty represent? • Laboratory concerns • Is my system stable? Are there any sources of contamination in the laboratory? How do the results compare to those expected?,……How good are my measurements?

10. QA involves • Internal checking • Measurements made on a series of replicate samples (Polach (1989) noted ‘internal checking needs suitable quality control and reference materials’.) • Monitoring of background, standards, known-age and reference materials • Independent (external) checking • Laboratory inter-comparisons

11. Time line of activities within CRONUS • Design the inter-comparison • Identify suitable samples (and criteria) done • Agree timescale for results- done- phase one results due in April 2008 • Define format for reporting results done • Inform laboratories and ask for expression of interest to participate (done) • Distribute samples- phase 1: done

12. Timeline • For phase 1 • Distribution July 2007, results returned April 2008 • Archive material for future use • Location of archive- currently in Arizona- sufficient material for at least 10 more intercomparisons of the same size • 23 laboratories sent samples

13. identified inter-calibration standards • Noble gases there are 2 potential “standards” – the pyroxene of Schaefer and an EU sample (from Wieler) . • pyroxene sample distributed • AMS standards prepared and available from Nishizumi- not distributed as part of CRONUS

14. identified inter-calibration samples-distributed July 2007 • (A)ntarctic sample: high in Al-26 and Be-10. Quartz was separated at U of Vermont, etched 3 times in HF and washed. Recommended 5g be used. Approx 37g provided • (N)amibia: a low latitude sample, recommend 20g be used. approx 75g provided • For in-situ C-14, same samples provided in glass vials.

15. typical format for reporting results eg Al-26 Mass of sample (quartz) (g) used in the measurement: AMS Standard used in the measurement Half-life used Background material used measured 26Al/27Al ratio (1 uncertainty) (specify units) mass (number of atoms of 27Al) in sample number of atoms 26Al in lab process blank

16. Participating laboratories Al, Be, C- USA, UK, France, Switzerland, Germany, The Netherlands, Sweden, Australia, Canada (24 in total) Noble gases: USA, UK, Switzerland, France, Germany (14 in total)

17. Results so far in CRONUS • So far, results from 7 laboratories for samples A and N, from 2 laboratories for sample P: laboratories have often reported replicate results • There is general consensus on half-life used for Al, but some variability for Be (1.5,1.51, 1.36, 1.37 x 106) • Standards used • Be:- NIST SRM4325, Nishiizumi SYD Be01-5-4 • Al:- Z92-0222(PRIME, Purdue), Nishiizumi STD Al 0143

18. Results so far in CRONUS • Sample A, • Be analysis • 13 results, coefficient of variation (CV) (stdev/mean*100%) is 4.92 • Al analysis • 6 results, CV 7.92%

19. Results so far in CRONUS • Sample N (lower Be and Al by approx factor of 100 than sample A), • Be analysis • 15 results, coefficient of variation (CV) (stdev/mean*100%) is 9.34% • Al analysis • 6 results, CV 9.2%%

20. Potential analysis • Similar to that used commonly for the C-14 inter-comparisons, defining reproducibility but will use z-scores, defined as standardised deviations from a consensus value • for each sample, we define an agreed value (usually a robust estimate based on all results) • the z-score is defined as the difference between an individual result and the robust value standardised to account for the uncertainty (also based on a robust estimate). • properties of Z-scores well understood- used internationally in proficiency trials.

21. The error in a measurement • A single value, which represents the difference between the measured value and the true value • However, for these samples, we do not have the true or real Al/Be atoms/g, so we use the inter-comparison to define this value (as a consensus from the participating laboratories)

22. Key properties of measurement • Accuracy of the measurement refers to the deviation (difference) from the true value (or sometimes expected or consensus value) • Precision refers to the variation (expected or observed) in a series of replicate measurements (obtained under identical conditions). High precision, low uncertainty

23. Accuracy and precision Accurate and inaccurate and precise Accurate and inaccurate and imprecise

24. Evaluation of accuracy • In FIRI and VIRI, known-age material is used to define the ‘true’ age • The figure over shows a measure of accuracy for individual laboratories

25. Between laboratory variation • reproducibility –identical samples, different laboratories

26. Reliability and reproducibility • Repeatability (r) refers to measurements made under identical conditions in one laboratory, • Reproducibility (R) refers to measurements made in different laboratories, under different conditions. • Reproducibility is the closeness of agreement between test results under conditions where the same method is used in different laboratories.

27. Reliability and reproducibility • The reproducibility value R is the value below which the absolute difference between two single results obtained under reproducibility conditions may be expected to lie with probability 0.95. • A difference larger than R cannot be ascribed to random fluctuations and would warrant investigation of possible sources of systematic differences.

28. Reproducibility (VIRI phase 1)

29. Estimation of r and R • Model: Y = m + B + e where Y is the measurement, m is the average activity, B is the between-laboratory variation and e is the random error. • B is assumed random and var (B) = 2L • e is assumed random and for a single laboratory var(e) = 2W. • 2W assumed constant for all laboratories, with average value 2r.

30. r and R • The repeatability value r is 2.8 r • The reproducibility value R is 2.8 R , where R = (2L + 2W) • 2L , 2W and rmust all be estimated.

31. Conclusions • All measurement is subject to uncertainty, the test of which is to make replicate measurements • Inter-laboratory trials provide generic measures of reproducibility and assessment of laboratory comparability • For cosmogenic isotope work, this is still at an early stage • CRONUS inter-comparison has archived material for future use, but for satisfactory characterisation, we need more results

32. Actions- for 2008 • Finalise acquisition and preparation of the 2nd suite of samples. • Agree a timescale and distribute the samples (ideally distribute shortly after results of phase 1) • await results- for phase 2, assuming distributed September 2008, deadline for results- Feb 2009. • analysis of results from phase 1- reported by August 2008.

33. other potential inter-calibration samples • Antarctic sample: 14C, 10Be, 26Al, 21Ne. • Namibia: 14C, 10Be, 26Al, 21Ne. • Maine: 14C, 10Be, 26Al, 21Ne. • NMT basalt: 36Cl. • Carbonate/sandstone: 36Cl, 10Be, 26Al. Some other potential materials are: • Lake Bonneville basalt sample. • Promontory Point quartzite • Blank quartz

34. acknowledgements • All the participating laboratories, the sample providers (Paul Bierman and Joerg Schaefer), NSF for funding.