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.