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1st Nordplus Seminar

1st Nordplus Seminar. Geochemical mapping and monitoring of trace elements and organic pollution. Vilnius 2007-03-05. The role of metrology in the interpretation of analytical data. Kaj Heydorn Department of Chemistry. Definition of the Measurand. “Quantity intended to be measured”.

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1st Nordplus Seminar

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  1. 1st Nordplus Seminar Geochemical mapping and monitoring of trace elements and organic pollution Vilnius 2007-03-05

  2. The role of metrology in the interpretation of analytical data Kaj Heydorn Department of Chemistry

  3. Definition of the Measurand “Quantity intended to be measured” VIM 3 (2004)

  4. Metrology - the science of measurement • The measurand, e.g. time, length, mass • The object, i.e. the particular specimen being measured

  5. Metrology in Chemistry - MiC • Determination of something, e.g. pH, TOC, alcohol, insulin, gold, C-14 • In something, e.g. an archaelogical specimen, blood, lake water, soil, the universe

  6. Definition of the Measurand I • The determinand, i.e. the chemical species to be determined • The specified amount of material to which the measurement should apply

  7. Specification of measurand • 1) Should the result refer to a particular, specified sample or to a larger system • Shall the sample be subjected to a special pretreatment before analysis • 3) Should the result refer to the time of analysis or be valid for a specified period

  8. Specification of measurand 4) Shall the analysis be carried out as a test according to a precisely specified method 5) Should the result be corrected for bias associated with the analytical method used for its determination 6) Must the result be traceable to the SI- units or other stated references

  9. Validity of a sample Unless the complete history of a sample is known with certainty, the analyst should not spend his time analyzing it. Thiers 1957

  10. SYSTEM Sample analysed

  11. The sampling process • primary samplingrepresentativity versus quantity • secondary sampling uniformity and sample size

  12. SYSTEM Primary sample Secondary sample Test sample

  13. Analytical measurements Until a measurement operation…. has attained a state of statistical control, it cannot be regarded in any logical sense as measuring anything at all. C.E. Eisenhart

  14. Uncertainty of measurement Parameter that characterizes the dispersion of the values that are being attributed to a measurand VIM 3 Draft April 2004

  15. Uncertainty A result without a statement of uncertainty is useless - because No conclusions can be made from it

  16. Definition of the Measurand II • A result without corresponding definition of the measurand is worthless • An uncertainty without corresponding specification of the measurand is misleading

  17. Uncertainty evaluation • Type Astatistical analysis of actual observations • Type B any other method

  18. Uncertainty components 1 Testing = method-based result • Repeatabiliy • Intermediary precision • Reproducibility

  19. Propagation of uncertainty

  20. Reporting analytical results The result of a measurement shall include • Definition of the measurand • Reported value of the measurand • ± its expanded uncertainty • The coverage factor used

  21. Uncertainty A result with an incorrect statement of uncertainty is dangerous - because Wrong conclusions can be made from it

  22. Verification HOW? By comparing predicted and actualvariability of results T-statistic

  23. Expectation Value µj By calculating the weighted mean value

  24. Verification of uncertainty m is the number of duplicate results A chi-square distribution with m degrees of freedom

  25. Example: Fall-out in Denmark

  26. Determination of environmental radioactivity in Denmark • Average amount of 137Cs in the top 5 cm of uncultivated soil • expressed in Bq/m2 at a specified time and for a selected area

  27. Determination of environmental radioactivity in Denmark • Without regard to soil composition soil density temperature and humidity vegetation

  28. Ishikawa diagram for 137Cs measurements

  29. Uncertainty components, uD • Type A Variation with depth • Type B Sampling area Time 3.2 %/cm 2 % negligible

  30. Uncertainty components, uM • Type A Counting statistics + Counting geometry • Type B Calibration 1-2 % 5 % 1.1 %

  31. Uncertainty components, uS • Type A Reproducibility + Location • Type B Representativity 15 % 49 % good

  32. 137Cs in Danish soils autumn 1987 Results for 10 random locations

  33. 137Cs in Danish soils autumn 1989 Results for 10 selected locations

  34. Conclusions • The Null Hypothesis could not be rejected • More measurements must be made, before any conclusions are drawn

  35. Thank you for your attention

  36. Verifikation Type A løbende analytisk kvalitetskontrol Type B repræsentative dobbeltprøver

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