6.1 NTRM Batch Traceability

The traceability of a gas NTRM batch relies on the unbroken chain of comparisons to NIST’s primary gas standards. Through gravimetric preparation of the primary gas standards, NIST assures traceability to the mole, a basic unit of the International System of Units, known as SI. Since the beginning of the NIST gas standards program, the NIST primary gas standards have been the foundation of the gas SRM program. They are prepared and maintained by NIST, and regularly undergo stringent internal evaluation and intercomparisons with other international gas standards. In the national hierarchy of gas standards, they are at the pinnacle. The NIST certified standards, such as gas SRMs, are compared to these primary gas standards before certification. These NIST certified standards enable producers to prepare and maintain the NTRM batches that are provided to the public or used to produce other NIST traceable gas standards.

There is an unbroken series of analyses comparing NTRM gas mixtures to NIST certified standards, and a direct NIST involvement in the NTRM certification. NIST maintains records of certified NTRMs and certification periods. If an NTRM batch is still in use after the certified period expires, it must be reanalyzed and the data submitted to NIST for concurrence concerning stability. NIST reserves the right to request any NTRM cylinder be sent to NIST for analysis if there are any questions concerning stability or concentration. The direct involvement of NIST in the quality assurance of each NTRM batch should provide continued acceptability of the certified concentrations with time.

6.2 Mixtures Produced Referencing NTRMs and SRMs

Not all of the analytical activities of gas analysis laboratories require the stringent traceability of an SRM or NTRM. In these instances a gas mixture with a somewhat lower level of traceability may be all that is required. Gas mixtures prepared using a protocol referencing gas SRMs or NTRMs may be declared as NIST traceable if the general guidelines given below are followed. It is important to realize that these guidelines represent the minimum required in order for NIST to recognize a gas mixture as traceable. The minimum requirements for a NIST traceable gas mixture are;

1. The cylinders will be clean, of standard size and construction, and of known compatibility and history with the composition of the proposed standard. The cylinders will be equipped with valves of the appropriate material that conform to Compressed Gas Association (CGA) recommendations for the particular gas mixture. The reagent gases must be of high purity and quality to ensure that impurity levels and mixture stability meets NIST specifications.

2. The concentration of the mixture is within 5 % relative of the SRM or NTRM concentration to which it is related by analysis. The composition of the mixture should be identical to that of the SRM or NTRM. Multicomponent mixtures may be traceable to binary mixtures as long as there are no component interactions, and the balance gas is identical to the referenced mixtures (or chemically inert to the components). Stability must be assured through procedures similar to section 3.5 or demonstrated by data available to customers.

3. All sources of uncertainty in the concentration assignment are included in the final uncertainty statement. The model given in section 4.4 should be followed. The combined uncertainty of the referenced SRM or NTRM must be included in the mixtures expanded uncertainty.

4. An expiration date for the mixture must be given to the customer. The period of time the standard is valid will not exceed the original certification period of the corresponding SRM or NTRM. Example: If the original NTRM was certified for 2 years, the mixture can be given a maximum validation period of 2 years.

5. Documentation provided the customer will include concentration, expanded uncertainty, expiration date, significant impurities, and the specific SRM or NTRM referenced.

An excellent document describing a method to prepare a gas mixture referencing an SRM or NTRM is given in reference 3.