Development and Verification of Air Balance Gas Primary Standards for the Measurement of Nitrous Oxide at Atmospheric Levels

Abstract

The Gas Metrology Group at the National Institute of Standards and Technology (NIST) became active in developing primary standards at ambient levels of N₂O in the 1980s, and this has continued through to the present. In recent years, interest in NIST-traceable standards has increasednot only at the ambient level of approximately 325 nmol mol^–1 (ppb) but at micromole per mole (ppm) levels as well. In order to support two in-process dry whole air standard reference materials (SRMs 1720 and 1721) and the NIST Traceable Reference Materials (NTRM) program, a project was implemented in the Gas Metrology Group to produce a complete suite of new primary standard materials (PSMs) of N₂O with synthetic air (O₂/N₂) as the balance gas. Six levels of dilution, approximately 1 order of magnitude apart, were gravimetrically prepared and verified. Each level serves as the “parent mix” for the next level. This discussion describes the process of producing each level and then verifying its amount-of-substance fraction. Expanded uncertainties, <i>k</i> = 2, of 0.025% relative to the gravimetric amount-of-substance fraction were obtained at the ambient level. One standard from the final group of standards at the ambient amount-of-substance fraction level was compared with standards from the National Oceanographic and Atmospheric Administration and the Scripps Institution of Oceanography, two organizations experienced in gas standards preparation and ambient whole air measurements, and shows agreement to 0.07 nmol mol^–1 (0.02% relative) and 0.20 nmol mol^–1 (0.06% relative), respectively