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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 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” VIM 3 (2004)
Metrology - the science of measurement • The measurand, e.g. time, length, mass • The object, i.e. the particular specimen being measured
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
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
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
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
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
SYSTEM Sample analysed
The sampling process • primary samplingrepresentativity versus quantity • secondary sampling uniformity and sample size
SYSTEM Primary sample Secondary sample Test sample
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
Uncertainty of measurement Parameter that characterizes the dispersion of the values that are being attributed to a measurand VIM 3 Draft April 2004
Uncertainty A result without a statement of uncertainty is useless - because No conclusions can be made from it
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
Uncertainty evaluation • Type Astatistical analysis of actual observations • Type B any other method
Uncertainty components 1 Testing = method-based result • Repeatabiliy • Intermediary precision • Reproducibility
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
Uncertainty A result with an incorrect statement of uncertainty is dangerous - because Wrong conclusions can be made from it
Verification HOW? By comparing predicted and actualvariability of results T-statistic
Expectation Value µj By calculating the weighted mean value
Verification of uncertainty m is the number of duplicate results A chi-square distribution with m degrees of freedom
Example: Fall-out in Denmark
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
Determination of environmental radioactivity in Denmark • Without regard to soil composition soil density temperature and humidity vegetation
Uncertainty components, uD • Type A Variation with depth • Type B Sampling area Time 3.2 %/cm 2 % negligible
Uncertainty components, uM • Type A Counting statistics + Counting geometry • Type B Calibration 1-2 % 5 % 1.1 %
Uncertainty components, uS • Type A Reproducibility + Location • Type B Representativity 15 % 49 % good
137Cs in Danish soils autumn 1987 Results for 10 random locations
137Cs in Danish soils autumn 1989 Results for 10 selected locations
Conclusions • The Null Hypothesis could not be rejected • More measurements must be made, before any conclusions are drawn
Thank you for your attention
Verifikation Type A løbende analytisk kvalitetskontrol Type B repræsentative dobbeltprøver