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Universität Stuttgart
Using PT systems for TraceabilityFeasibility Study in the Water Sector in Germany
www.iswa.uni-stuttgart.de
Michael Koch1, Ulrich Borchers2, Frank Baumeister1, Olaf Rienitz3, Detlef Schiel31Institute for Sanitary Engineering, Water Quality and Solid Waste Management, Dept. Hydrochemistry, Universität Stuttgart2IWW Water Centre, Mülheim/Ruhr3Physikalisch-Technische Bundesanstalt, Braunschweig
Con
tact
: D
r.-Ing
. M
icha
el K
och,
ISW
A,
Ban
dtäl
e 2,
70569 S
tutt
gart
info
@aq
sbw
.de,
ww
w.a
qsbw
.de
Situation• European harmonization demands comparability• No traceability structure in this sector• Well-organized PT systems→ Links between PT systems and national metrological stan-
dards would improve traceability system
Matrix content from standard addition procedure
• Robust consensus means vs. the spiked amounts (example: Cu)• Matrix content is given by the absolute value of the x-axis
intercept• Uncertainty of the matrix content can be calculated from ge-
neralised least square regression [see ref.]
Possibilities to estimate the true value
1st PT round• Organized by the Institute of Public Health (LÖGD) in North-
rhine-Westfalia in cooperation with IWW Water Centre• Analysis of Cd, Cr, Cu, Ni and Pb in drinking water• Result for one Cu sample as an example:
Conclusions and outlook• Reference measurements using primary methods are limited
to only a few relevant parameters and are very time-consu-ming and expensive
• Reference values can be calculated that are at least trace-able to some extend, as long as the matrix content is low compared to the spike
• This method will be used regularly in the PT rounds of AQS Baden-Württemberg to gather more experience
• More problems are expected for organic analytes
y = 0.9932x + 1.0884R2 = 0.999652
matrix: 1.096 µg/l
0
25
50
75
100
125
150
175
200
-25 0 25 50 75 100 125 150 175 200
Spikes in µg/l
Rob
ust c
onse
nsus
mea
n in
µg/
l
2nd PT round• Organized by “AQS Baden-Württemberg“, PT provider at the
Universität Stuttgart• Analysis of Sb, Cd, Cr, Cu, Ni, Pb and CrVI in drinking water• Result for one Cu sample as an example:
Traceability• Reference measurements are traceable• Consensus mean is not traceable• Formulation values are traceable, provided that the purity of
the raw chemical is traceable• Additional uncertainty from matrix determination does not
„destroy“ this traceability
References• Rienitz, O., Schiel, D., Güttler, B., Koch, M., Borchers, U.:
A convenient and economic way to achieve SI-traceable re-ference values to be used in drinking water interlaboratory comparisons. Accred. Qual Assur., in preparation
• Koch, M., Baumeister, F.: Traceable reference values for routine drinking water PT – first experiences. Accred. Qual Assur., in preparation
method uncertaintyrobust consensus mean
Hampel-estimator (ISO/TS 20645; DIN 38402-45)
ux=1.25·s*/√ps* = robust stdp = number of participants
reference measurements by NMI
e.g. isotope dilution ICP-MS uncertainty budget
from formulation
spik
es f
rom
gr
avim
etry
/vo
lum
etry
+
matrix from PT provi-ders measurement
unce
rtai
nty
budg
et
laboratory uncertainty
matrix from NMI measurements
uncertainty budget
matrix from standard addition
from linear regression
• Consensus mean lower than reference measure-ment
• Matrix determination by PT provider not reliable enough (very low values)
• Sum of matrix values from standard addition procedure and formula-tion in good aggreement with reference measure-ments
• Uncertainties partially not available at this stage of the project
• Again consensus mean lower than reference measurement
• Matrix determination by PT provider was not performed (very low values)
• Sum of matrix values from standard addition procedu-re and formulation in good aggreement with reference measurements
• Uncertainties overlap for re-ference measurement and standard addition/formulati-on values
0,390,4
0,410,420,430,440,450,460,47
Sample EC
once
ntra
tion
in m
g/l
NMI reference measurementrobust consensus meanformulation + matrix (PT provider meas.)formulation + matrix (std.-add. NMI)formulation + matrix (std.-add. mean)
0,08
0,081
0,082
0,083
0,084
0,0850,086
0,087
0,088
0,089
0,09
Sample 5
Con
cent
ratio
n in
mg/
l
NMI reference measurementrobust consensus mean(no direct matrix measurement) formulation + matrix (std.-add. NMI)formulation + matrix (std.-add. mean)