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Different types of reference material

In some cases the range and variation in types of reference material can be quite bewildering. In other cases the choice may be far more limited. In general, reference materials can be broken into three main categories:

1. Certified Reference Materials (CRMs)

2. Reference Materials (RMs)

3. Quality Control Materials (QCMs)

CRMs

A certified reference material (CRM) is defined by ISO REMCO (ISO Committee on Reference Materials) as:

“a reference material characterized by a metrologically valid procedure for one or more specified properties, accompanied by a certificate that states the value of the specified property, its associated uncertainty, and a statement of metrological traceability.”

Notes added to the definition are:

1. The concept of value includes qualitative attributes such as identity or sequence. Uncertainties for such attributes may be expressed as probabilities.

2. Metrologically valid procedures for the production and certification of reference materials are given in, among others, ISO Guides 34 and 35.

3. ISO Guide 31 gives guidance on the contents of certificates.

RMs

ISO REMCO (ISO Committee on Reference Materials) defines a reference material (RM) as:

“A material, sufficiently homogeneous and stable with respect to one or more specified properties, which has been established to be fit for its intended use in a measurement process.”

Notes added to the definition are:

1. RM is a generic term.

2. Properties can be quantitative or qualitative, e.g. identity of substances or species.

3. Uses may include the calibration of a measurement system, assessment of a measurement procedure, assigning values to other materials, and quality control.

4. An RM can only be used for a single purpose in a given measurement.

Alternative definitions

The following definitions from the International Vocabulary of Metrology (VIM) are analogous to the ISO REMCO definitions given above.

certified reference material

“reference material, accompanied by documentation issued by an authoritative body and providing one or more specified property values with associated uncertainties and traceabilities, using valid procedures.”

reference material

“material, sufficiently homogeneous and stable with reference to specified properties, which has been established to be fit for its intended use in measurement or in examination of nominal properties.”

See the International Vocabulary of Metrology – Basic and General Concepts and Associated Terms (VIM) (ISO Guide 99) for further information and notes.

Key differences between RMs and CRMs

• CRMs have a certificate
• A value given on the certificate of a CRM is accompanied by an estimate of the uncertainty
• RM is a generic term – A CRM is an RM, but an RM is not necessarily a CRM
• CRMs represent a higher level of metrological rigour
• RMs may be suitable for some roles, but CRMs should be preferred in most cases

QCMs

Quality Control Materials are less clearly defined than the other categories of reference material. ISO REMCO is currently producing a guidance document for the production of reference materials used for quality control purposes. This guide seeks to define QCMs and the processes which should be applied in their manufacture. In general QCMs should not be considered suitable for use in validation, calibration or in establishing traceability. Rather, their purpose is to provide relatively cheap and readily available samples for use in ongoing AQC (Analytical Quality Control), where the analyst’s historical data is used to construct control charts and set warning and action limits. In these cases the absolute values associated with the material are not important, as it is the variation over time which is being measured. However, QCMs must be sufficiently stable and homogeneous that the variation measured can be ascribed primarily to the analytical method rather than the material. The values associated with QCMs are generally referred to as indicative, and they should be treated as such.

Note on characterised values

Many reference materials carry values which represent a mixture of metrological levels. For instance, a soil matrix material certified for extractable metal content may also have indicative values for nutrient content (nitrate, phosphate, sulphate, chloride etc.). In these cases, it is the level assigned to the value (i.e. “certified” or “indicative”) which defines the metrological level of the data, and not the overall designation of the material (indicative values given on a CRM certificate are the same metrological level as indicative values quoted for a QCM). Common terminology which is used includes “Certified Value”, ”Reference Value”, “Assessed Value”, “Indicative Value” and “Information Value”. The precise meanings of these terms depend upon the manufacturer of the reference material. It is important for the analyst to be clear about what the different values present in CRM/RM documentation represent.

Other categorisation

Reference materials may also be classified into groups depending on whether they are pure materials or matrix reference materials. A method may require the use of one or both of these types to establish traceability (e.g. a pure pesticide standard used to make calibration solutions, and a soil matrix material certified for pesticides used for method validation). Reference materials may also be classified by the sector which they serve, for instance; environmental reference materials or clinical reference materials.