The NIST Traceable Reference Material (NTRM) Program for Gas Standards

An NTRM ^(CM) (NIST Traceable Reference Material) is a reference material produced by a commercial supplier with a well-defined traceability linkage to NIST. This linkage is established via criteria and protocols defined by NIST that are tailored to meet the needs of the metrological community to be served. Reference materials producers adhering to these requirements will be allowed to use the NIST "NTRM" certification mark. The concept was established to allow NIST to respond the increasing needs for high quality reference materials with constant human and financial resources.

The gas NTRM program was established in 1992 in partnership with EPA and Specialty Gas Companies as a means for providing end-users with the wide variety of certified gas standards needed to implement the "Emissions Trading" provision of the 1990 Clean Air Act. Gas NTRMs are produced and distributed by Specialty Gas Companies with NIST oversight of the production and involvement in the analysis, and can be developed for any pollutant, concentration and balance gas combination for which an NIST primary gas standard mixture exists. Certified concentration values are assigned by NIST according to a published protocol.

Since 1995, twelve commercial vendors have produced over 18,000 NTRMs (>680 batches). It is estimated that these NTRMs have resulted in the production of over 1,000,000 EPA Protocol gas standards that are traceable to NIST. Production of an NTRM batch required 5 months on average, whereas, SRM production requires over 1 year.

Plans are being put in place to extend the Gas NTRM model for the commercial production of reference materials with a well-defined traceability to NIST, to other sectors of the commercial reference materials community.

For more information of the gas NTRM program please contact Joe Hodges (joseph.hodges [at] nist.gov (joseph[dot]hodges[at]nist[dot]gov)) or Mike Kelley (mekelley [at] nist.gov (mekelley[at]nist[dot]gov)).

Gas NTRM Program Executive Summary

The NIST Traceable Reference Material (NTRM) program is intended to provide a mechanism to increase the availability of accurate gas standards by the creation of a series of secondary standards.

A gas NTRM is a gas mixture which has the same components as a NIST primary gas standard; however the concentrations need not be identical. The gas mixture is produced by a specialty gas company (producer) and is contained in a compressed gas cylinder. Any producer wishing to create an NTRM or a series of NTRMs must contact NIST for discussion before initiating production. These discussions will establish the scope of the program to aid in coordination, and also to establish specific analytical and statistical guidelines to be followed during production. After there is agreement, work by the producer can commence. A producer will create a multi-cylinder batch of a gas mixture containing the same analytes as an existing NIST primary gas standard. In the Environmental Protection Agency (EPA) Certified Reference Material (CRM) program, the concentration of the analytes in the mixture must be within 1% relative of an SRM. In the NTRM program the concentration can be anywhere within the bounds of a suite of NIST primary gas standards. For instance carbon monoxide in nitrogen primary standards range in concentration from 10 ppm to 13%, an NTRM concentration can lie anywhere within these bounds.

After a candidate NTRM batch is filled, each cylinder is analyzed by the producer according to established protocols using NIST certified standards as calibration gases. When a candidate NTRM is close in concentration to a NIST certified standard, it is analyzed by means of "direct comparison" and when it is between NIST certified standard concentrations it is analyzed by "interpolation". This analysis is used to establish homogeneity across the batch. The data is then sent to NIST for review. If it meets all requirements established in the preproduction negotiations, then NIST will select a subset of the batch (minimum of two cylinders) to be sent to NIST.

Sufficient analyses will be performed at NIST for concentration value assignment and to determine levels of any impurities that would compromise the use of the mixture. Whereas in the EPA CRM program the producer was responsible for concentration value assignment, in the NTRM program it is solely the responsibility of NIST. The results of the homogeneity analyses will be used to set batch uncertainty. This data combined with the NIST data will be used to assign a single concentration and uncertainty to the batch. It is expected that for most NTRM batches the spread in concentrations as determined by the producer will not exceed 0.5% relative. As this uncertainty will impact directly the overall uncertainty, the more precise and homogeneous the producer can make the batch, the better the resulting certification.

NIST will print certificates and provide them to the NTRM producer. An NTRM is recognized by EPA as being equivalent to a CRM and can be used to produce EPA protocol gas standards.

The producer is responsible for warehousing and distributing the NTRMs. The producer is also responsible for assuring concentration stability with time; different gas species will have differing certification periods after which a reanalysis will have to be performed and the data submitted to NIST. Non-reactive species such as CO, C₃H₈, and CO₂ will have longer certification periods than NO, SO₂, and H₂S.

The cost of the NIST involvement in this program will be covered by the organization producing the NTRMs. There will be a higher fee for analysis by interpolation than there will be for direct comparison. Again, the NIST involvement includes evaluation of producer's data, sample analyses, concentration value assignment, certificate generation and stability data review.

One of the purposes of this program is to provide NIST traceable gas standards in a timely fashion. Sufficient communication and coordination between the specialty gas companies and NIST should allow for a smooth process and result in a NIST turn around time not exceeding three months.

For more information of the NTRM program please contact Joe Hodges (joseph.hodges [at] nist.gov (joseph[dot]hodges[at]nist[dot]gov)) or Mike Kelley (mekelley [at] nist.gov (mekelley[at]nist[dot]gov)).

RESOURCES

• PittCon 2000 Presentation by Frank Guenther
• PittCon 2000 Presentation by Bill Dorko