Standard Operating Procedure
Measurement Uncertainty in Chemical Analysis
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STANDARD OPERATING PROCEDURE
Document Number: S-536 Version 1.xx
Page 2 of 9
Measurement Uncertainty in Chemical Analysis
Company Name:
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Effective Date
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STANDARD OPERATING PROCEDURE
Document Number: S-536 Version 1.xx
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Measurement Uncertainty in Chemical Analysis
1. PURPOSE
Typically every chemical measurement has an error. Therefore a measurement only
gives an approximation of the true value of the quantity to be measured. The
possible difference to the true value, or the measurement uncertainty, should be
estimated and reported as part of any good analytical practice. This SOP addresses
the process for estimating and documentation of measurement uncertainty.
2. SCOPE
The SOP covers the estimation and reporting of uncertainty of chemical testing.
3. GLOSSARY/DEFINITIONS
Item
Explanation
(Measurement)
Uncertainty
Parameter associated with a result of a measurement that
characterizes the dispersion of the values that could reasonably
be attributed to the measurand (Reference 1).
Standard
Uncertainty
Uncertainty of the result x of a measurement expressed as a
standard deviation (Reference 1).
Combined
Standard
Uncertainty
Standard uncertainty of a result y of a measurement when the
result is obtained from the values of a number of other
quantities, equal to the positive square root of a sum of terms,
the terms being the variances or co-variances of these other
quantities weighted according to how the measurement result
varies with these quantities (Reference 1).
Expanded
Uncertainty
Quantity defining an interval about the result of the
measurement that may be expected to encompass a large
fraction of the distribution of values that could reasonably be
attributed to the measurand (Reference 1).
Coverage
Factor
Numerical factor used as a multiplier of the combined standard
uncertainty in order to obtain an expanded uncertainty
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STANDARD OPERATING PROCEDURE
Document Number: S-536 Version 1.xx
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Measurement Uncertainty in Chemical Analysis
(Reference 1).
Traceability
The property of the result of a measurement of the value of a
standard whereby it can be related to stated references, usually
national or international standards, through an unbroken chain
of comparisons all having stated uncertainties (Reference 1).
True Value
Value consistent with a definition of a given particular quantity
(Reference 1).
Error (of a
measurement)
The result of a measurement minus the true value of the
measurand (Reference 1).
Random Error
The result of a measurement minus the mean that would result
from an infinite number of measurements of the same
measurand carried out under repeatability conditions
(Reference 1).
Systematic
Error
Mean that would result from an infinite number of
measurements of the same measurand carried out under
repeatability conditions minus a true value of the measurand
(Reference 1).
Note: For other definitions, see www.labcompliance.com/glossary.
4. REFERENCE DOCUMENTS
4.1. EURACHEM/CITAC Guide CH 4, Quantifying Uncertainty in Analytical
Measurement, 2nd Edition, 2000.
4.2. Checklist E-186: “Measurement Uncertainty in Chemical Analysis”.
Order from www.labcompliance.com/solutions/examples.
5. RESPONSIBILITIES
5.1. Analysts
5.1.1. Collect measurement data to estimate measurement uncertainty.
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Measurement Uncertainty in Chemical Analysis
5.1.2. Estimate measurement uncertainty.
5.1.3. Report measurement uncertainty for routine analysis.
5.2. Laboratory Supervisor
5.2.1. Prepares a list of tests that require estimation and reporting of
measurement uncertainty.
5.2.2. Reviews uncertainty data experiments, evaluations and reports.
5.3. Quality Assurance Department
5.3.1. Advises on regulations and guidelines related to measurement
uncertainty.
5.3.2. Checks if processes and documentation are in compliance with internal
policies and quality standards/guidelines.
5.3.3. Reviews and approves measurement uncertainty data as part of the
review of test results.
6. FREQUENCY OF USE
6.1. Whenever chemical analyses are performed that may require estimation and
reporting of measurement uncertainty data.
7. PROCEDURE
7.1. Determine Tests that Require Estimation of Measurement Uncertainty
7.1.1. The laboratory supervisor proposes tests that require estimation and
reporting of measurement uncertainty. In general these include:
 Well-recognized quantitative test methods that specify limits to the
values of the major sources of uncertainty of measurement and specify
the form of presentation of calculated results.
 All other quantitative test methods including standard methods and
methods developed in-house.
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Measurement Uncertainty in Chemical Analysis
 All tests where reporting of measurement uncertainty is required by
clients.
7.1.2. Quality Assurance reviews and approves the list.
7.2. Document Methods and Specifications
7.2.1. For methods as defined in 7.1.1 the analyst documents the method and
specifications.
This includes:
 Scope of the method.
 Equipment used to run the method.
 Performance acceptance criteria.
 Laboratories where the method is used.
 Required measurement uncertainty data.
7.3. Identify Uncertainty Sources
7.3.1. The analyst collects and documents information on possible sources of
uncertainty. The following sources should be considered:
 Sampling, sample storage.
 Instruments, equipment (bias, precision).
 Reagent purity, reference standards.
 Environmental and measurement conditions.
 Contamination, carry over.
 Computational effects (calibration model).
 Sample effects (matrix).
7.3.2. The list is reviewed and approved by the laboratory supervisor.
7.4. Define Experiments
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STANDARD OPERATING PROCEDURE
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Measurement Uncertainty in Chemical Analysis
7.4.1. The analyst suggests a list of experiments to be executed to get
uncertainty estimates. Depending on findings from 7.2 and 7.3 the
following should be considered:
 Single laboratory short-term precision.
 Single laboratory day-by-day precision.
 Precision within multiple laboratories.
 Recovery studies through comparison with certified reference
material.
 Recovery studies through spiking blank and sample matrix
reference material.
7.4.2. The list is reviewed and approved by the laboratory supervisor.
7.5. Define other Sources of Uncertainty
7.5.1. The analyst suggests a list of other sources that can contribute to
uncertainty. Depending on findings from 7.2 and 7.3 the following should
be considered:
 Uncertainty of certified reference material as reported in the
certificate as supplied by the supplier.
 Purity of reagents as found in the supplier’s catalogue.
7.5.2. The laboratory supervisor reviews and approves the list.
7.6. Conduct Experiments and Evaluate Results
7.6.1. For each experiment defined and approved in 7.4 the analyst conducts
the experiments.
7.6.2. For each experiment a minimum of ten analyses are performed.
7.6.3. The analyst calculates analyte recovery and the standard deviation of
precision studies for each experiment.
7.7. Calculate Combined Standard and Expanded Measurement Uncertainty
7.7.1. Using data from 7.5 and 7.6.3 the analyst calculates the combined
standard uncertainty data using the formula:
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Measurement Uncertainty in Chemical Analysis
7.7.2. Using results from 7.7.1 the analyst calculates the expanded uncertainty
formula:
Unless otherwise specified a capacity factor of 2 is used that gives a 95%
confidence interval.
7.8. Report Results
7.8.1. The analyst reports the results. The following general recommendations
should be considered:
 Present sufficient information to allow the result to be re-evaluated.
 Document the details on how measurement uncertainty has been
determined.
 Describe the methods used to estimate measurement uncertainty.
 Document all values and the sources of all assumptions and
corrections.
 List all components of uncertainty.
 Limit the numerical values of uncertainty to two digits.
7.8.2. Unless otherwise specified the analyst reports numerical values following
the guidelines for reporting expanded uncertainty:
7.8.3. Reports are signed by the analyst.
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Measurement Uncertainty in Chemical Analysis
7.9. Review of Experiments, Evaluations and Reports
7.9.1. Experiments, evaluations and reports are reviewed and approved by the
laboratory supervisor or delegate.
7.9.2. Experiments, evaluations and reports are reviewed and approved by the
QA manager or delegate.
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