NIST Gravimetrically Prepared Atmospheric Level Methane in Dry Air Standards Suite

The Gas Metrology Group at the National Institute of Standards and Technology was tasked, by a congressional climate change act, to support the atmospheric measurement community through standards development of key greenhouse gases. This paper discusses the development of a methane (CH₄) primary standard gas mixture (PSM) suite to support CH₄ measurement needs over a large amount-of-substance fraction range 0.3–20 000 μmol mol^–1, but with emphasis at the atmospheric level 300–4000 nmol mol^–1. Thirty-six CH₄ in dry air PSMs were prepared in 5.9 L high-pressure aluminum cylinders with use of a time-tested gravimetric technique. Ultimately 14 of these 36 PSMs define a CH₄ standard suite covering the nominal ambient atmospheric range of 300–4000 nmol mol^–1. Starting materials of pure CH₄ and cylinders of dry air were exhaustively analyzed to determine the purity and air composition. Gas chromatography with flame-ionization detection (GC-FID) was used to determine a CH₄ response for each of the 14 PSMs where the reproducibility of average measurement ratios as a standard error was typically (0.04–0.26) %. An ISO 6134-compliant generalized least-squares regression (GenLine) program was used to analyze the consistency of the CH₄ suite. All 14 PSMs passed the <i>u</i>-test with residuals between the gravimetric and the GenLine solution values being between −0.74 and 1.31 nmol mol^–1; (0.00–0.16)% relative absolute. One of the 14 PSMs, FF4288 at 1836.16 ± 0.75 nmol mol^–1 (<i>k</i> = 1) amount-of-substance fraction, was sent to the Korea Research Institute of Standards and Science (KRISS), the Republic of Korea’s National Metrology Institute, for comparison. The same PSM was subsequently sent to the National Oceanic and Atmospheric Administration (NOAA) for analysis to their standards. Results show agreement between KRISS-NIST of +0.13% relative (+2.3 nmol mol^–1) and NOAA-NIST of −0.14% relative (−2.54 nmol mol^–1)

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

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