Traceable Calibration Gases: SRMs, NTRMs, and Protocol Gases

The Gas Metrology Group in the Chemical Sciences Division is tasked with providing traceability in gas analysis for the U.S. This group works with regulators, gas suppliers, and international partners to provide the highest level gas standards to function as the top of the traceability chain. Traceability is then realized through NIST gas standards sold as SRMs, or through gas industry standards known as NTRMs, and ultimately to end users through EPA protocol gas standards.

The Gas Metrology Group prepares the highest level gas standards, known as Primary Standard Gas Mixtures based on precise gravimetric methods and accurate purity analysis. These Primary Standards are verified at NIST, stability confirmed through reanalysis over many years, and compared internationally through the CCQM. Gas Standard Reference Materials (SRMs) are then certified against these primary standards and sold to users to provide a source of traceability for gas analysis.

NIST Traceable Reference Materials (NTRMs) are prepared by a specialty gas company (vendor) at their facility using NIST specifications. The batch of NTRMs is then jointly certified by the vendor and NIST. Each of the NTRM gas standards are analyzed against a NIST standard at the vendor’s site, and the data provided to NIST. NIST analyzes the data and if the data passes NIST selection criteria, a subsample of the batch is sent to NIST. NIST certifies the selected gas standards using internal NIST gas standards which are traceable to our Primary Standards. If NIST analyses and the vendor’s data agree, the batch is certified with a single concentration and uncertainty. The vendor is then free to use or sell these NTRM standards. Most of the NTRM standards are ultimately used to certify EPA protocol gases, which are then sold to the public.

• New low concentration nitric oxide SRMs, SRM 2737 (500 nmol/mol) and SRM 2738 (1000 nmol/mol) have been produced.
• New SRM 2765, 100 nmol/mol propane and new SRM 2619a, 500 µmol/mol Carbon Dioxide to support new automobile research.
• New low concentration sulfur dioxide SRM 1689 (5 µmol/mol) has been produced to support acid rain regulations.
• Certified over 1900 gas mixtures NTRM standards supporting EPA regulations
• In 2007, 447 NTRMs were produced; 1385 were produced in 2008.
• As of October 1, 2009 there were 349 gas mixture NTRM batches in certification.