Reference materials play an important role within a broader quality assurance programme; they serve as an important tool for implementation of numerous aspects of quality measurements, mainly for the validation of methods, calibration, estimation of measurement uncertainty, training and internal quality control, as well as external proficiency testing or inter-laboratory testing.
RMs are used to support measurements concerned with chemical composition, biological, clinical, physical, engineering properties and with various fields such as taste and odour. They may be characterised for their "identity" (e.g. chemical structure, fibre type, microbiological species), or "property values" (e.g. the quantity of specified chemical entity, hardness etc.)
Some commonly encountered types of reference materials are as follows:
- Pure substances characterised for chemical purity and/or trace impurities.
- Standard solutions and gas mixtures, often prepared gravimetrically from pure substances and used for calibration purposes.
- Matrix reference materials, characterised for the composition of specified major, minor or trace constituent. Such materials may be prepared from matrices, which contain the suitable components, or by preparing synthetic mixtures.
- Physico-chemical reference materials characterised for properties such as melting point, viscosity and optical density.
- Reference objects or artefacts characterised for functional properties such as taste, odour, octane number, flash point and hardness. This type also includes microscopy specimens characterised for properties ranging from fibre type to microbiological specimens.
ISO recognises two classes of materials: certified reference materials (CRMs) and reference materials (RMs). CRM must by definition be traceable to an accurate realisation of the unit in which the property values are expressed. Each property value must be accompanied by a measurement uncertainty at a stated level of confidence. RMs are materials whose property values are sufficiently homogenous and well defined to be used for the calibration of an apparatus, the assessment of a measurement method, or for assessing values to materials. The following classes of reference materials are also encountered:
- Primary reference material ↑
- Secondary reference materials ↑ decreasing uncertainty
- In-house or working reference materials ↑
Various reference materials are needed for various functions; e.g. a certified reference material for the validation of methods, whereas for the quality control a working reference material is sufficient.