Why use reference materials? The importance of reference materials Different categories of reference materials. Different classes of reference materials. Standards for reference materials. How reference materials are prepared and assessed. How reference materials are used.

1. The Selection and Use of Reference
Materials
Tim Sandle

2. Introduction
1. Why use reference materials?
2. The importance of reference materials
3. Different categories of reference materials.
4. Different classes of reference materials.
5. Standards for reference materials.
6. How reference materials are prepared and
assessed.
7. How reference materials are used.

3. 1. What are reference materials?

4. Why use
reference
materials?
• Analytical measurements depend on many
variables to provide the credible data needed to
make informed decisions.
• The quality of this data is only as good as the
reference material used to calibrate the
measurement device.
• High quality reference materials are available
only from organizations with a robust quality
system.

5. What are
reference
materials?
• Reference materials are used to support measurements.
• Measurements include:
• Chemical composition.
• Biological.
• Clinical.
• Physical.
• Engineering properties e.g. for equipment
calibration.
• Reference materials are classed as:
• Identity’ (e.g. chemical structure, fibre type,
microbiological species etc.)
• Property values (e.g. amount of specified chemical
entity, hardness etc.).

6. 2. The importance of reference materials

7. Importance of reference materials
• Reference materials are an important tool for several aspects of measurement quality.
They are used in:
• Method validation.
• Calibration.
• Estimation of measurement uncertainty.
• Including comparability with measurements made with other laboratories.
• Training.
• Internal QC.
• Proficiency testing.
• Independent evidence of performance.

8. Different reference materials have different
levels of uncertainty
Primary reference
material
Secondary reference
material
In-house or working
reference material
Decreasing
uncertainty

9. 3. Different categories of reference
materials

10. Different categories of reference materials
• Common types of reference materials are
• Pure substances characterised for chemical purity and/or trace impurities.
• Standard solutions and gas mixtures, for calibration purposes.
• Matrix reference materials, characterised for the composition of specified major,
minor or trace chemical constituents.
• Physico-chemical reference materials characterised for properties such as melting
point, viscosity, and optical density.
• Reference objects characterised for functional properties such as taste, odour,
octane number, flash point and hardness. Plus microscopy specimens characterised
for properties ranging from fibre type to microbiological specimens.

11. 4. Classes of reference materials

12. Classes of
reference
materials
• Certified reference material (CRM)
• Limited availability
• Reference materials
• QCM (Quality Control Material)

13. Different classes of reference materials #1
• There are two different classes of reference material :
• A certified reference material (CRM)
• One or more properties will be certified.
• CRMs must be traceable to an original source, where the CRM property values are expressed.
• Each property value must be accompanied by an uncertainty at a stated level of confidence.
• CRMs are desirable for method validation.
• Reference materials (RM)
• RMs are materials whose property values are sufficiently homogeneous and well established.
• Used for the calibration of an apparatus, the assessment of a measurement method, or for assigning
values to materials.
• A working level reference material is adequate for QC e.g. precision control, bias assessment, calibration,
preparation of calibration reference materials and maintaining conventional scales.

14. Different classes of reference materials #3
• QCM (Quality Control Material)
• Very similar to a reference material: A material intended for internal
laboratory quality control. A material or substance one or more of whose
property values are sufficiently homogeneous, stable and well established to
be used for maintaining or monitoring measurement processes.
• But a QCM does not have formally assigned property values or uncertainties.

15. WHO uses different terminology
• WHO guidelines for biological reference materials provide the terms:
• Reference standards: materials that are used as calibrators in assays
• International biological measurement standard: a biological substance
provided to enable the results of biological assay or immunological assay
procedures to be expressed in the same way throughout the world
• Secondary reference standards: Reference standards calibrated against and
traceable to primary WHO materials and intended for use in routine tests
• Reference reagent: a WHO reference standard, the activity of which is defined
by WHO in terms of a unit

16. 5. Standards for reference materials

17. Standards for
reference
materials
• Accreditation to ISO/IEC 17025 and ISO Guide
34 – “Double Accreditation”.
• This represents the pinnacle of quality
system achievement.
• This allows the credible production and
testing of Certified Reference Materials
(CRMs).
• ISO/IEC 17025: 2017 “General requirements for
the competence of testing and calibration
laboratories”.
• For biological standards, there is a separate
WHO guide.

18. ISO/IEC
17025: 2017
• ISO/IEC 17025: 2017 “General requirements for the competence of testing
and calibration laboratories”
• Key principles:
• Capacity
• Sufficient resources.
• Exercise of Responsibility
• Authority to execute responsibilities.
• Scientific Method
• Objectivity of Results
• Qualified personnel in place to independently review results.
• Impartiality of Conduct
• QA in place.
• Traceability of Measurement
• A recognised system of measurement that derives from
accepted, known quantities (SI system) or other intrinsic or well-
characterised devices or quantities
• Repeatability of Test
• Transparency of Process

19. ISO Guide
34:2009
• ISO Guide 34:2009 “General requirements for the
competence of reference material producers”
• Intended for the use by reference material producers in
the development and implementation of their
management system for quality, administrative and
technical operations.
• The general requirements by which a reference material
producer demonstrates that it operates as competent to
carry out the production of reference materials.
• Sets out the management system requirements by which
reference materials shall be produced.
• To be used as part of a reference material producer's
general quality assurance procedures.
• Covers the production of certified and non-certified
reference materials..

20. WHO guide for biological standards
• WHO Expert Committee on Biological Standardization (2004). "Annex 2: Recommendations for the preparation, characterization and
establishment of international and other biological reference standards“
• Provides guidance for certification:
• Name and address of manufacturer;
• Site(s) of manufacturing;
• Trade name and common name of product;
• Marketing authorization number;
• Lot number(s) (including sub-lot numbers and packaging lot numbers if necessary);
• Type of container used;
• umber of doses per container;
• Number of containers or lot size;
• Date of start of period of validity (for example, manufacturing date) and expiry date;
• Storage conditions;
• Signature and function of the person authorized to issue the certificate;
• Date of issue of certificate;
• Certificate number.

21. 6. How reference materials are prepared
and assessed

22. Preparing reference standards
• The preparation of certified reference materials is described in general in ISO Guide 17034 and
ISO Guide 35Preparation of biological reference standards is described in WHO Guidance.
• General steps required in production of a certified reference material typically include:
• Collection or synthesis of material
• Sample preparation (including homogenization, stabilization, bottling etc.)
• Homogeneity testing
• Requires replicated measurements on multiple units or subsamples of the material
• Stability assessment
• Value assignment ("characterization" in ISO terms).

23. Preparing
reference
materials in-
house
• When preparing reference materials, the following must be
taken into account:
• Selection of materials
• Appropriateness, native material versus spikes,
material preparation etc.
• Homogeneity testing.
• Preparation and packaging
• Homogeneity, contamination, stability etc.)
• Stability testing.
• Certification studies.
• Uncertainty estimation.
• Documentation
• QA release
• Certification approval
• Storage, distribution and relifing.

24. Assessing
reference
materials #1
• The user must assess the appropriateness and
fitness for purpose of any RM based on the
customer and analytical requirements.
• The suitability of a reference material will depend
on the details of the analytical specification, e.g.:
• Measurand including analyte,
• Measurement range (concentration),
• Matrix match and potential interferences,
• Sample size,
• Homogeneity and stability,
• Measurement uncertainty ,
• Value assignment procedures (measurement
and statistical).

25. Assessing
reference
materials #2
• In addition the user must:
• Assess the validity of the ‘certification’ and
uncertainty data.
• Have a record of both the producer and the material.
• This includes when and where a reference
material has been used for interlaboratory
comparison.
• Ensure the availability of the certificate and report.
• The report should cover: characterisation,
certification and statistical analysis procedures.
• Demonstrated conformance of the production of the
reference materials with appropriate quality
standards.

26. 7. How reference materials are used

27. How reference materials are used #1
• Method Validation and Measurement Uncertainty
• Estimation of bias (the difference between the measured value and the true
value) is key to method validation.
• Appropriate reference materials can provide valuable information, within the
limits of the uncertainty of the certified value(s) and the uncertainty of the
method being validated.
• Shows the estimation of bias differences between two or more methods.
• It is important that the reference materials are within the scope of the
method in terms of matrix type, analyte concentration etc.
• It is also important that the uncertainty associated with an refence material
should be no greater than one third of that of the sample measurement.

28. How reference materials are used #2
• Verification of the Correct Use of a Method
• The successful application of a valid method depends on its correct use, both
with regard to operator skill and suitability of equipment, reagents and
standards.
• Reference materials can be used for training, for checking infrequently used
methods and for trouble shooting when unexpected results are obtained.

29. How reference
materials are
used #3
• Calibration
• A pure substance reference material can be
used for calibration of the measurement
stage of a method.
• However, the uncertainty associated with
reference material purity will contribute to
the total uncertainty of the measurement.
• For example, a reference material
certified as 99.9% pure, with an
expanded uncertainty U (k=2) of 0.1%
will contribute an uncertainty
component of 0.1% to the overall
measurement uncertainty.

30. How reference
materials are
used #4
• Quality Control and Quality Assurance
• Reference materials should be characterised
with respect to homogeneity, stability, and
the certified property value(s).
• For in-house QC adequate homogeneity and
stability are essential.
• For QA proficiency testing, homogeneity is
essential and sample stability must be
assessed and controlled.

31. How reference
materials are
used #5
• Further information in ISO Guide 33:2015
“Reference materials -- Good practice in using
reference materials”.
• Describes good practice in using reference
materials and certified reference materials,
in measurement processes.
• Includes the assessment of precision and
trueness of measurement methods, quality
control, assigning values to materials,
calibration, and the establishment of
conventional scales.
• The guide also relates key characteristics of
various types of reference material to the
different applications.

32. Summary
1. What are reference materials?
2. Why use reference materials?
3. What are reference materials?
4. The importance of reference materials
5. Different categories of reference materials.
6. Different classes of reference materials.
7. Standards for reference materials.
8. How reference materials are prepared and
assessed.
9. How reference materials are used.

33. Thank you
Any questions?