An Introduction to Quality Assurance in Analytical Science

1
An Introduction to Quality Assurance in
Analytical Science

• Dr Irene Mueller-Harvey
• Mr Richard Baker
• Mr Brian Woodget

2
Part 5 - Expression of Results

• Contents
• The famous trio - validation, traceability and
• measurement uncertainty (slides 3,4)
• Measurement uncertainty (slides 5-9)
• Test report (slide 10)

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3
The Famous Trio
When expressing results of analytical
measurements you will need to bear in mind the 3
inter-linking parameters of
Will the results be traceable to an
accepted reference standard?
Has the method been validated and it is fit
for purpose?
VALIDATION
TRACEABILITY
How sure will you be of the accuracy of the
results obtained?
MEASUREMENT UNCERTAINTY
4
About validation, traceability measurement
uncertainty
As you have seen in Part 1 of this presentation,
all analytical results have an error associated
with them. Measurement uncertainty is an estimate
of the potential size of that error and is
affected by both the time and effort put into
method validation and the traceability of
standard reference materials and substances
Note refer to Part 2 of the presentation for a
description of method validation and to Part 3
for a description of traceability
5
Measurement Uncertainty (1)
Analysis of a popular brand of Cod Liver Oil
capsules has shown them to contain 30 5 ng/kg
of dioxins
Mean result from replicate analyses
Estimate of the measurement uncertainty
Let us consider from where the uncertainty is
likely to arise
Note accredited laboratories (ISO 17025)
carrying out this analysis would need to be aware
of the levels of uncertainty but would not
necessarily have to present this data unless
asked. So the result could well be presented as
30 ng/kg
6
Measurement Uncertainty (2)
Sources of uncertainty arise from ALL aspects of
the analysis - these could include
sampling
Volumetric glassware
Calibration standards
instruments
sample preparation
operator skill
7
MeasurementUncertainty (3)
Analytical balance

• To estimate the measure of overall uncertainty,
  errors likely to occur in all aspects of the
  analysis need to be taken into account in the
  form of individual standard deviations (SD).
  Data may be obtained from
• manufacturers of equipment and reagents (e.g.
  balances,
• volumetric glassware, standard reagents etc)
• data from method validation
• estimates from the literature or from previous
  experience

An overall estimate of the uncertainty can then
be calculated by using accepted procedures.
8
Measurementuncertainty (4)
Standard uncertainties
A measure of the SD of an uncertainty component
u(y) - may be calculated from

• experimental data
• accuracy guaranteed by a piece of equipment
• e.g. balance accurate to 0.1 mg _at_ 95
  confidence
• which may be converted to a SD
• pipette guaranteed to deliver 25 0.2 ml.
  Although no
• confidence level has been stated, SD may again
  be
• calculated.

9
Measurementuncertainty (5)
Combined uncertainties
Dependent upon the type of analysis carried out,
the standard uncertainties may be combined to
produce a combined uncertainty uc(y) One of
three equations may be used, the choice
being dependent upon the complexity of the
analysis and the relationship between the
components parts
A model equation must be devised which describes
in simple algebraic terms the whole analysis
process
10
TEST REPORT HEADER For
Accredited Tests (example for
demonstration only)