The National Institute of Standards and Technology ambient level methane in air Standard Reference Material historical record

Abstract

The National Institute of Standards and Technology (NIST) has been certifying lots, or series, of Standard Reference Materials (SRMs) containing ambient level methane in air for over 40 years. The historical record contains six traditional series of SRM 1658 (1 μmol mol⁻¹), five of SRM 1660 (4 μmol mol⁻¹), and seven of SRM 1659 (10 μmol mol⁻¹) methane in air. All series of any one particular SRM can be linked to each other through the historical suites of gravimetric primary standard mixtures (PSMs) developed at NIST. One gas mixture cylinder from a series is chosen as the lot standard (LS), retained and held at NIST, and periodically compared to the PSMs to assure its stability. Recently, 6 of the original 18 LS still in service in the Gas Metrology Group inventory, and cylinder samples held at NIST from 6 other SRM lots, were analyzed against a newly prepared suite of PSMs using cavity ring-down spectroscopy. Data were analyzed using a generalized least squares linear regression. The results indicate that, within the original 95 % confidence intervals, the methane concentration has remained the same for all the SRM LS and lot samples. The current predicted concentrations of the LS and samples for SRMs 1659 and 1660 are within 0.002 to 0.051 μmol mol⁻¹, or ≤0.5 %, relative of the original certificate value. SRM 1658 LS and samples are within 0.0001 to 0.0023 μmol mol⁻¹, or ≤0.2 % relative. These results illustrate the consistency, repeatability, and stability of these methane in air SRMs over the historical 35+-year record. It also demonstrates that the historical gravimetric primary methane in air suites have remained accurate and consistent over time.