Standard ≠ reference material

The term ‘standard’ is one of the most misused terms in the analytical sciences. Indeed, the sometimes indiscriminate use of the term can make its meaning completely ambiguous. Misunderstandings often stem from a lack of appreciation of the differences between a reference materialcertified reference materialstandard reference materialcalibrator and the extent to which these represent measurement standards.

Examples of good and bad practice

Bad practice

The Pt SRM 3140 standard was ...
The concentration of sample and bracketing standard was ...
Reproducibility of the bracketing Pt standard SRM 3140 is ...
In-house standard …
Individual measurements of 35 pg Os standards ...
Fifteen standard analyses ...
External standards ...
External standardisation was based on soda-lime silicate glasses (SRM 610, 612, 614) using Jochum et al. (2011) …

Good practice

The Pt certified reference material NIST SRM 3140 was ...
Mass fractions of sample and bracketing calibrators were ...
The measurement repeatability of the bracketing calibrator NIST SRM 3140 was ...
In-house reference material ..
Individual measurements of 35-pg test portions of Os reference materials ...
Fifteen analyses of reference material BHVO-2 ...
Reference materials …
Calibration was achieved using soda-lime silicate glasses (NIST SRM 610, 612, 614) with the reference values of Jochum et al. (2011) …  or  Soda-lime silicate glasses (NIST SRM 610, 612, 614) with the reference values of Jochum et al. (2011) were used as calibrators … 

 

Commentary

In the geochemical context, a reference material is a rock or mineral that has been sufficiently well characterised to be fit-for-purpose in characterising analytical measurements. It could be argued that the metrological qualities of RMs are not very demanding, except with respect to being sufficiently homogeneous and stable. Reference values do not have to be accompanied by uncertainties (but they can be) nor provided with evidence of traceability (but they could be). 

However, the metrological properties associated with a certified reference material are much more demanding. In particular, as well as having the characteristics of a reference material, ‘property values’ (that is, certified values) must be accompanied by an uncertainty and be traceable using valid procedures described on an accompanying certificate of analysis. The characterisation of certified reference materials is an area of active development, with international coordination being lead by REMCO, the reference material committee of the International Organisation for Standardisation (ISO), which is active in the production and revision of a number of influential guides.

Confusion arises in the status of geochemical reference materials because many of the best known ones were characterised before modern procedures for certification were developed. Arguably, they are therefore not compatible with current definitions of certification. In addition, further confusion arises because some reference materials, which have not been characterised following modern requirements for certification,  are issued with a certificate of analysis rather than a ‘reference material data sheet’, as should be the case; certificates of analysis should be associated exclusively with certified reference materials.

A further common misuse of terminology is the use of ‘SRM’ as being synonymous with ‘CRM’. Standard Reference Material (SRM) in a registered trade mark of the US National Institute of Science and Technology (NIST) and is used by that organisation to refer to certified reference materials produced by NIST that meet additional NIST-specific certification criteria. It is a reflection of the acclaimed quality of NIST CRMs that the term SRM is misused in this way, but this is incorrect.

One way to understand the correct use of terminology in this area is to distinguish what the material is, what it is used for, and what that use represents.

Thus, RMs and CRMs are clearly materials with specified metrological properties, as summarised above.

Both these materials can be used as calibrators (to quantify the result of a measurement from the signal observed from an analytical instrument) and to validate a procedure (that is to provide evidence that the measurement results are reliable). In both these cases, RMs and CRMs are regarded as measurement standards.

The formal (VIM) definition of a ‘measurement standard’ is fairly opaque (see below). However, the examples given in VIM 3 show that a measurement standard can be a reference material, or a certified reference material, that provides quantity values with measurement uncertainties for the mass concentrations in a particular material. Put another way, the use of a measurement standard as a calibrator can be used to demonstrate metrological traceability of results from a measuring instrument.

Current recommendations define nine different types of measurement standard (international, national, primary, secondary, reference, working, travelling, transfer and intrinsic), but this level of detail is unlikely to trouble the average geoanalyst, being more relevant to the activities of national agencies.

DEFINITIONS 

Reference material (RM)

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. (VIM 3: 5.13) 

Certified reference material (CRM)

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. (VIM 3: 5.14)

NIST Standard Reference Material® (SRM) 

A CRM issued by NIST that also meets additional NIST-specific certification criteria and is issued with a certificate or certificate of analysis that reports the results of its characterizations and provides information regarding the appropriate use(s) of the material (NIST SP 260-136). Note: An SRM is prepared and used for three main purposes: (1) to help develop accurate methods of analysis; (2) to calibrate measurement systems used to facilitate exchange of goods, institute quality control, determine performance characteristics, or measure a property at the state-of-the-art limit; and (3) to ensure the long-term adequacy and integrity of measurement quality assurance programs. The terms "Standard Reference Material" and the diamond-shaped logo which contains the term "SRM," are registered with the United States Patent and Trademark Office

Measurement standard (etalon)

Realization of the definition of a given quantity, with stated quantity value and associated measurement uncertainty, used as a reference. (VIM 3: 5.1)

Calibrator 

Measurement standard used in calibration (VIM 3: 5.12)

Calibration

Operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication(VIM 3: 2.39) 

Validation 

Verification, where the specified requirements are adequate for an intended use. (VIM 3: 2.45)

Verification 

Provision of objective evidence that a given item fulfils specified requirements. (VIM 3: 2.44)


 

Index to terms

Concept

Metrological terms considered

Metrological terms covered

Say no to ppm

The ambiguities associated with the use of ‘ppm’

 

Sigma is out, standard deviation is the way to go!

What symbols are used to represent the properties of population- and sample-distributions

 

Standard ≠ reference material

Avoiding the use of the term standards when referring to (certified) reference materials or calibrators

Reference material, certified reference material, standard reference material, calibrator, calibration, validation, measurement standard (étalon), verification

Please, no more errors from your laboratory

Explaining the difference between uncertainty and error

Measurement uncertainty, measurement error, systematic measurement error, measurement bias, random measurement error, confidence level

Precision

Distinguishing between repeatability, intermediate precision and reproducibility and discouraging the use of ‘internal precision’

Measurement precision, repeatability condition of measurement, intermediate precision of measurement, reproducibility condition of measurement, repeatability, intermediate precision, reproducibility

Small bias yes, high accuracy no

Explaining the difference between accuracy, bias and trueness

Measurement accuracy, measurement trueness, measurement bias

 

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