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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.
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
Terms and Explanations
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:
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.
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.
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