Traceability

Discussion

Traceability to the SI can be achieved through NIST's 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. Laboratories can purchase SRMs from NIST in the process of method validation and calibration. In addition, chemical standards for use in calibration and method validation are produced commercially. Most, if not all, of the commercial manufacturers claim traceability.

The definition of traceability requires that a statement of uncertainty be made with each comparison in the chain. If a laboratory chooses to purchase standards from a commercial supplier rather than from NIST directly, it should be with the understanding that the stated uncertainty cannot be smaller than the uncertainty of the SRM used by the commercial manufacturer for comparison. This is due to the fact that the comparison process has a standard uncertainty that must be added onto the standard uncertainty of the NIST SRM, which is used in making the comparison.

Example: A commercial supplier certifies a 10,000 µg/mL (nominal value) solution of Cu and determines that the standard deviation of all systematic and random errors in their certification process is 25 µg/mL. In addition the NIST SRM used for comparison has a certified value of 10,000 ± 30 µg/mL Cu. NIST uses a coverage factor of 2 in reporting the uncertainty. Therefore the standard deviation of the SRM is 15 µg/mL. The reported uncertainty of the CRM produced by the commercial supplier would then be calculated taking into account the standard deviation of their production process and the standard deviation of the NIST SRM. The reported uncertainty, using a coverage factor of 2, is calculated taking the square root of the sum of the squares, i.e., ((25)² + (15)²)1/2 x 2 = 58 µg/mL.

A chemical standard obtained from a commercial supplier that claims traceability to a specified NIST SRM should have the following information on the certificate of analysis to support a claim of traceability (the following is cited from the NIST website):

"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.

It is the responsibility of 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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