Key Topics

Reference materials

Valid and reliable measurements depend on the regular use of reference materials. Their use reduces the costs and risks associated with incorrect measurements.

What is a Reference Material?

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.

A certified reference material (CRM) is defined 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.

A CRM differs from an RM in that the value certified is traceable to fundamental units and it includes a stated uncertainty.

Selection Of Reference Materials

In selecting a reference material for a particular application, it is the analyst's responsibility to assess its suitability. As such, it is necessary to consider all or some of the following factors before selecting a material:

• matrix match and potential interferences
• measurands (analytes)
• measurement range (concentration)
• measurement uncertainty
• certification procedures used by the producer
• documentation supplied with the material (e.g. certificate, report, etc.)

Use of Reference Materials

Reference materials are one of the cornerstones in producing reliable chemical measurements and, as such, have a wide range of applications in the laboratory. These include:

• method validation
• instrument calibration
• verification of instrument, laboratory & analyst performance
• uncertainty estimation
• internal quality control

Of these, the most common uses are for the calibration of instruments, usually using standard solutions made up from known purity reference materials, and for the validation of test methods using matrix reference materials that, ideally, will closely resemble the samples being analysed.

Both types of reference material are essential to the analytical process. Whereas a standard solution is necessary for calibrating the response of a measuring instrument, a matrix reference material is equally necessary to assess suitability of a complete analytical process.

In general, reference materials should be used on a regular basis within the framework of a sound quality assurance system. When used in this manner, the analyst can feel confident that the analytical system is under statistical control and is producing results that are fit for purpose.

Useful information on the use of reference materials can be found at the following links:

• European Reference Materials Website
• Eurachem guide on the selection and use of reference materials

LGC operates the Reference Materials Advisory Service (REMAS) which provides advice and information on reference materials such as:

• Availability of reference materials in a particular field of application
• How to select and use an appropriate reference material
• Detailed information on the appropriate reference materials such as certified/reference values, name of producer, form of the material, etc
• Information available from the international reference material database, COMAR, that contains data on over 10,000 reference materials

For access to this service, please contact the NMS helpdesk at nmshelp@lgc.co.uk.