2016-05-18

Setting the requirements for a method (including aspects of MU)

Bertil Magnusson, SP Technical Research Institute of Sweden

For a validation
study we need
to have several
requirements…
Target Measurand Robustness
uncertainty

Focus on these 3 performance characteristics

Sample types Measurement Precision Trueness (bias)
range
2

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6
5
4
3
2
1
0

Measurement cycle Requirements Comparing Process control

With clear and

distinct requirements
on an analytical
method we can
assess fitness for
Translate
Specification Legal limits requirements purpose
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Measurement cycle

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The whole method validation process

“from the customer problem

to the laboratory decision

on whether or not the customer request

can be carried out with an identified method”

Text cited From Eurachem Guide Fitness for purpose of

analytical methods, page 17

Step 1

Write down your method in detail to

Specify your be used in your lab
measurand

Part of the
method
validation
process

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Requirements
Step 1 in the method validation process

Transferring the client issue

into analytical requirement
that can be used
in the validation study

Analytical requirements
Specify the analytical requirement on
performance characteristics such as:

Measurement range

Precision

Trueness

Measurement uncertainty

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Chapter 6.3 Working range - method

Instrument
calibrated
according to
method/
procedure

Chapter 6.3 Working range - instrument

10 working range

8
Signal from instrument

linear range

6 analytical
∆y
LOQ
LOD

sensitivity
4 ∆x

0 2 4 6 8 10 12
A Concentration in standards used for calibration

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Analytical requirements
Specify the analytical requirement on
performance characteristics such as:

Measurement range

Precision

Trueness

Measurement uncertainty

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Precision

12

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Analytical requirements
Specify the analytical requirement on
performance characteristics such as:

Measurement range

Precision

Trueness

Specify max bias

Measurement uncertainty
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Analytical requirements
Specify the analytical requirement on performance
characteristics such as:

Measurement range

Precision

Trueness

Measurement uncertainty

Set target uncertainty

For validation – translate to
precision and trueness

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Requirements when comparing

When comparing 2 results using the same

method precision limits can be used 6

Same lab – same day 5

4

Repeatability limit
3

Same lab – different days 2

Withinlab reproducibility limit 1

0

Different labs

Maybe Reproducibility limit
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Precision limits
Precision limit
In 19 cases out of 20 the difference between 6

2
- results will be less than… 5

4
e.g. the repeatability limit, r
3

2
  
r=k
 
  2 2
 = 2 2sr 1

  /2.8
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Process control

Is the process under control?

Demand on sRw at
the level of interest
NOTE – to process from lab, communicate
95 % confidence interval i.e. 4 sRw

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Process control
Fluid cracking catalyst
poisoned by nickel in crude oil
Nickel mg/kg

Time

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Specification
MSA – Measurement System Analysis –
often physical measurement e.g. length

Bias ≈ 0

Acceptable sRw < 5 % of specification

Good sRw < 1,7 % of specification

SPC – Statistical process control –

Capability inded, Cp

Acceptable sprocess < 10 % of specification

Acceptable sRw < ?
Specification/Tolerance
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Legal limits & EU directives

Directive 98/83/EC of 3 November 1998 on the
quality of water intended for human consumption
At a given concentration level (parametric value)
the performance characteristics for the method
to be used are specified:

Bias < 10 %

Within-lab reproducibility, 2 sRw < 10 %

LOD < 10 %

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Legal limits & EU directives

Directive 2009/90/EC
regarding EU Water Framework Directive 2000/60/EC
At a given concentration level (environmental
quality standard the performance characteristics
for the method to be used are specified:

Measurement Uncertainty, U < 50 %

LOQ < 30 %

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Legal limits – Danish law

Extensive Requirements environmental measurements 2011*
list Performance characteristics sRw CVRw LOD Uabs URel
Water
Ground, Example Ground water
Drinking
Surface,
Marine
Waste
Swimming
Biota…
Soil
Sludge..
Shellfish

*www.retsinformation.dk/pdfPrint.aspx?id=174970 22

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Legal limits - Blood alcohol

Measurand including Concentration of EtOH in a blood
measurement object sample delivered to the laboratory

Upper limit 0,200 mg/g

A decision limit. 0,220 mg/g

Target U calculated to be *0.026 mg/g

*The concentration above which it can be decided with a statistical

certainty of 99.9 % that the permitted limit has been truly exceeded
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Translate requirements

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Translate requirements
From LOQ & U to sRw
The Trollbook – Nordtest Tr 565 proposes

Example 2 - LOQ

Acceptable within-lab reproducibility
sRw < LOQ/10

Example 1 – Measurement uncertainty

Acceptable within-lab reproducibility
sRw < U/4

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Setting requirements
– target uncertainty

Many more examples are given in our
EURACHEM/CITAC guide

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For a validation
study we need
to have several
requirements…
Target Measurand Robustness
uncertainty

With clear and distinct requirements on an

analytical method we can assess fitness for pupose

Sample types Measurement Precision Trueness (bias)

range
27

Thank you for your attention!

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