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Traceable Calibration Gases: SRMs, NTRMs, and Protocol Gases


The Gas Sensing 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.


Photography of gas cylinder storage facility with cylinders stored horizontally in movable cabinets.

Primary gas standard storage facility at the NIST Gaithersburg campus.

Credit: Lane Sander

The Gas Sensing 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.

Major Accomplishments

Associated Publications

1. Srivastava, A., Long, S. E., Norris, J. E., Bryan, C. E., Carney, J., and Hodges, J. T., "Comparison of Primary Laser Spectroscopy and Mass Spectrometry Methods for Measuring Mass Concentration of Gaseous Elemental Mercury," Analytical Chemistry, 93, 1050-1058 (2021). 

2. Long, S. E., Norris, J. E., Carney, J., and Ryan, J. V., "Provision of primary NIST traceability to support vapor phase mercury emissions monitoring of combustion sources using isotope dilution inductively coupled plasma mass spectrometry," Atmospheric Pollution Research, 11, 909-919 (2020). 

3. Norris, J. E., Choquette, S. J., Viallon, J., Moussay, P., Wielgosz, R., and Guenther, F. R., "Temperature measurement and optical path-length bias improvement modifications to National Institute of Standards and Technology ozone reference standards," Journal of the Air & Waste Management Association, 63, 565-574 (2013). 

4. Leiva, M. A., Araya, C., Mancilla, C., Seguel, R., and Norris, J. E., "Uncertainty of ozone measurements with the primary standard reference photometer (SRP45)," Talanta, 86, 71-81 (2011). 

5. Viallon, J., Moussay, P., Norris, J. E., Guenther, F. R., and Wielgosz, R. I., "A study of systematic biases and measurement uncertainties in ozone mole fraction measurements with the NIST Standard Reference Photometer," Metrologia, 43, 441-450 (2006). 

Created January 27, 2009, Updated November 7, 2023