1st nordplus seminar
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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”. - PowerPoint PPT PresentationTRANSCRIPT
1st Nordplus Seminar
Geochemical mapping and monitoringof trace elements and organic pollution
Vilnius 2007-03-05
The role of metrology in the interpretation of analytical data
Kaj HeydornDepartment 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
2) 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
Unless the complete history of a sample is known with certainty, the analyst should not spend his time analyzing it.
Thiers 1957
Validity of a sample
Sample analysed
SYSTEM
The sampling process
primary samplingrepresentativity versus
quantity
secondary sampling uniformity and sample size
Primary sample
Secondary sample
Test sample
SYSTEM
Analytical measurementsUntil 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 3Draft 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 measurandis worthless
An uncertainty without corresponding specification of the measurand is misleading
Uncertainty evaluation
Type A statistical analysis of actual observations
Type B any other method
Uncertainty components 1
Testing = method-based result
• Repeatabiliy
• Intermediary precision
• Reproducibility
Propagation of uncertaintyxixi xjxj xkxk
y
++
u(y)
y = f(xi, xj, xk, .)
pp((y|xy|xii, , xxjj, , xxkk))
Reporting analytical results
• Definition of the measurand• Reported value of the measurand • ± its expanded uncertainty• The coverage factor used
The result of a measurement shall include
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 actual variability of results
T-statistic
kj
j
ni
i ij
jijj
yu
yT
1 12
2
)(
)(
Expectation ValueµjBy calculating the weighted mean value
ni
i i
ni
i i
i
yu
yu
y
12
12
)(
1)(̂
k
j kn1
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 compositionsoil densitytemperature and humidityvegetation
Ishikawa diagram for 137Cs measurements
Measurement Definition
Sampling
137Cs Bq/m2
Time
Area
Topsoil
Counting statistics
Geometry
Calibration
Representativity
Reproducibility
Location
Uncertainty components, uD
Type AVariation with depth
Type B Sampling areaTime
3.2 %/cm
2 %negligible
Uncertainty components, uM
Type ACounting statistics
+ Counting geometry
Type B Calibration
1-2 %5 %
1.1 %
Uncertainty components, uS
Type AReproducibility
+ Location
Type B Representativity
15 %49 %
good
137Cs in Danish soils autumn 1987 Results for 10 random
locationsCs-137 Uncertainty Counting SampleBq/m2 Bq/m2 SD % kg
1441 232 1 1,811844 297 1 1,711242 200 1 2,543320 535 1 2,821269 204 1 2,311942 313 1 2,101177 189 1 2,17959 154 2 2,54817 132 1 1,69960 155 2 2,90
137Cs in Danish soils autumn 1989 Results for 10 selected
locationsData for 1989 Decay cor rected 1987 Value of TBq/m2 Uncertainty Bq/m2 Uncertainty per d.f.1200 188 1377 222 0,3711800 281 1762 284 0,0091130 177 1187 191 0,0472100 328 3172 511 3,1181350 211 1212 195 0,2291720 269 1855 299 0,1141020 159 1125 181 0,188490 77 916 148 6,576560 88 781 126 2,074770 120 917 148 0,597
10 d.f.T = 13,323
p> 0,21
Conclusions
The Null Hypothesis could not be rejected
More measurements must be made, before any conclusions are drawn
Thank youfor your attention
Verifikation
Type A løbende analytisk kvalitetskontrol
Type Brepræsentative dobbeltprøver