What is a reference material?

A reference material provides a kind of benchmark for a measurement. It is therefore used in, for example, method development and validation, calibration and quality assurance.

Following the definition given in the amendment to ISO Guide 30 [1], a reference material is 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".

The term "reference material" is a generic term, i.e. it comprises materials which are investigated and documented at different levels:

• Certified reference materials (defined as "reference material, characterized by a metrologically valid procedure for one or more specified properties, accompanied by a certificate that provides the value of the specified property, its associated uncertainty, and a statement of metrological traceability").

• Non-certified reference materials (not accompanied by a certificate, sometimes called "reference materials" in the sense of materials qualified only to a limited extent compared to "certified reference materials").

   

Frequently, reference materials are also classified according to their use, for instance by calling them "calibrants/calibrators", "quality control (QC) materials", "proficiency testing materials", etc.

"Standard Reference Material SRM®" (certified reference material provided by NIST) and "European Reference Material ERM®" (certified reference material provided by the ERM co-operation) are examples of brands for certified reference materials provided by specific RM-producers.

From the metrological point of view, a kind of hierarchy can be established based on the uncertainty of the certified values and in relation to their position in the traceability chain:

• Primary reference materials (a material having the highest metrological qualities and whose value is determined by means of a primary method).

• Secondary reference materials (reference material whose chemical composition is assigned by comparison with a primary reference material of the same chemical composition, or with several such primary reference materials).

• "In-house" reference materials, QC materials, etc.

   

The uncertainty associated with the property values usually increases from reference material types 1 through 3 as does the length of the traceability chain. In several cases, neighbouring material types may be situated on the same level of hierarchy.

For definitions and additional information see:

[1] ISO GUIDE 30:1992(E)/Amd.1:2008

[2] ISO Guide 30 (1992) Terms and definitions used in connection with reference materials

[3] International vocabulary of metrology — Basic and general concepts and associated terms (VIM), JCGM 2008 (http://www.bipm.org/utils/common/documents/jcgm/JCGM_200_2008.pdf)

[4] H. Emons, A. Fajgelj AMH. van der Veen, R. Watters; New definitions on reference materials; Accred Qual Assur (2006) 10: 576-578

[5] H. Emons, TPJ. Linsinger, BM. Gawlik; Reference Materials: terminology and use. Can't one see the forest for the trees? Trends in Analytical Chemistry (2004) 23: 442-448

[6] W. Hässelbarth (1996); Primary reference materials for chemical composition analysis - Proposal for an operational definition; Fresenius J Anal Chem (1996) 354: 263-265