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NIST Traceability

By Paul Gaines, Ph.D.

Inorganic Ventures is frequently asked the question, "Are your standards NIST-traceable?" The term traceability is used to describe the reliability of measurements, but it is not always clear exactly what that means. The purpose of this guide is to provide the user with facts about traceability, as well as a more specific question to ask reference material (RM) manufacturers.

Background

The concept of traceability dates back to the Convention du Metre, signed by seventeen countries in 1875. All length measurements are ultimately made in comparison to the international prototype meter located in Paris. Formally a diplomatic organization, the General Conference of Weights and Measures (CGPM) was created by the Metre Convention. The name International System of Units (SI) was given to the system by the eleventh CGPM in 1960. At the fourteenth CGPM in 1971, the current version of the SI was completed by adding the mole as base unit for amount of substance, bringing the total number of base units to seven (see Table 1 below).

Table 1: SI Base Units
 SI base unit
Base quantityNameSymbol
lengthmeterm
masskilogramkg
timeseconds
electric currentampereA
thermodynamic temperaturekelvinK
amount of substancemolemol
luminous intensitycandelacd

Terms and Explanations

  • Traceability has been defined as "the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties1." This definition has achieved global acceptance in the metrology community.
  • ISO (International Organization for Standardization) is a federation of national standards bodies from more than one hundred countries whose mission is to promote activities related to standardization in order to facilitate international exchange of goods and services and to develop co-operation among its members in the areas of intellectual, scientific, technological, and economic activity (REMCO 1995). The ISO functions through its technical committees. Subcommittees and working groups to produce international agreements that are published as international 'technical' standards.
  • Standards is a term surrounded by ambiguity. The word 'standard' can either be defined as a written specification (i.e. - 'technical standard') or a chemical reference material intended to define the concentrations of specified components (i.e. - 'measurement standard'). This guide uses the latter definition.
  • REMCO is ISO's Committee on Reference Materials. It was established in 1975 to carry out and encourage a broad international effort for harmonization and promotion of certified reference materials (CRMs) and their applications. REMCO task groups have produced a number of ISO Guides establishing definitions of reference materials and setting forth internationally agreed 'technical' standards for the production, certification, and use of reference materials. The primary ISO accreditations dealing with certified reference material manufacturers are clarified in our guide, ISO Guide 34, 17025, and 9001 Explained.
  • NIST (National Institute of Standards and Technology) is "responsible for developing, maintaining, and disseminating national standards - realizations of the SI - for the basic measurement quantities, and for many derived measurement quantities. NIST is also responsible for assessing the measurement uncertainties associated with the values assigned to these measurement standards. As such, the concept of measurement traceability is central to NIST's mission." — source
  • SRM (Standard Reference Material) is a federally registered trademark of NIST and the US Federal Government. This term describes the certified reference materials distributed specifically by NIST.

Discussion

Traceability to the SI can be achieved through NIST's Standard Reference Material (SRM) program. NIST has developed a very comprehensive line of SRMs in a wide variety of matrices. Their organization functions as the path to achieving traceability to the SI. All commercial standard manufacturers have the responsibility to establish traceability to a specified NIST SRM through an unbroken chain of comparisons, each with a stated uncertainty. The certificate of analysis should make it clear exactly how the manufacturer accomplished this traceability.

Therefore, the question of whether our standards are NIST-traceable should be replaced with a more specific question like, "Do your certificates of analysis make reference to the NIST SRM(s) with stated uncertainty through which you claim to establish traceability?"

Admittedly, this question sounds a bit longwinded, but the point is that a 'measurement' standard produced by a standard manufacturer other than NIST is not traceable to NIST. Rather, it may be traceable to a specific and defined NIST SRM. This is an important distinction and one that NIST fully supports.

The NIST SRM number should always be provided on the certificate of analysis according to the traceability definition given above.

The Big Question

Ask yourself this: As an analyst, what would I need to see on the Certificate of Analysis to support a claim of traceability?

NIST has answered this question with the following statement:

"To support a claim, the provider of a measurement result or value of a standard must document the measurement process or system used to establish the claim and provide a description of the chain of comparisons that were used to establish a connection to a particular stated reference. There are several common elements to all valid statements or claims of traceability:
  • A clearly defined particular quantity that has been measured.
  • A complete description of the measurement system or working standard used to perform the measurement.
  • A stated measurement result or value, with a documented uncertainty.
  • A complete specification of the stated reference at the time that it was compared to the measurement system or working standard.
  • An 'internal measurement assurance' program for establishing the status of the measurement system or working standard at all times pertinent to the claim of traceability."

An internal measurement assurance program can be simple or complex, depending on the level of uncertainty at issue and what is necessary to demonstrate its credibility. The user of a measurement result is responsible for determining what is adequate to meet his or her own needs.

Bottom Line

It is your responsibility as the end user of a "measurement" standard to assess the validity of a claim of traceability. Likewise, it is the responsibility of the standard manufacturer to provide the necessary information on the Certificate of Analysis that the user assesses. This mutual interest shared by both parties establishes a greater sense of trust in the quality of the standard.

1. International Standard Organization VIM, 2nd ed., definition 6.10, 1993.