Protocol gas standards are employed in measurements made for compliance purposes; one of the major uses is for measurements of regulated gaseous species from stationary source emitters, i.e., stack gases. Many of these stationary sources are regulated through the Cap and Trade program; therefore, accurate measurements are critical due to the economic and health consequences. EPA protocol gas mixture production is governed by Document EPA-600/R-97/121. Section 2.1.10, which states that, on a periodic basis, EPA will conduct an audit of these gas mixtures. The audit is to be blind with a selection of at least two samples from every U.S. manufacturing site. The samples are to consist of NO (50 µmol/mol to 1000 µmol/mol), SO2 (50 µmol/mol to 100 µmol/mol), and CO2 (5% to 20% mol/mol) in balance N2.
The objective of the audit is to determine whether the concentrations of the components of the gas mixtures are within the limits stated on the certificate provided by the vendor. These data can be used to facilitate the accurate reporting of continuous emissions monitoring (CEM) and stack measurements, to provide an independent quality check of the vendors, and to connect end users with good-quality vendors. NIST was the independent analytical laboratory for the 2006 and 2008 audits. NIST has a stake in this process, as the gases and the measurements made using them are claimed to be NIST traceable.
In 2006 only 11 samples were received, and only 3 out of 17 vendors (6 of 37 sites) were represented. In the 2008 audit 87 samples were received and all 11 vendors (18 sites) were represented. (The number of sites decreased due to industry consolidation.) The audit gas mixture has three components: NO, SO2 and CO2. The presence of % mol/mol levels of CO2 changes the viscosity and molar mass of the audit gas, compared to single-component standards. It is important to ensure that such a change does not cause an unexpected bias in the analytical results. This bias is dependent on the analytical technique chosen and could also be subject to interference from one component to another, e.g., CO2 reduces NO chemiluminescence.
The figure illustrates the relationship between NIST and the gas vendors with the uncertainty expressed as k = 1. If the analytical uncertainty claims of NIST (≤0.5%) and the gas vendors (≤1.0%) are valid, and there is no bias (unexpected or expected) or there is a bias and it is accurately corrected, then the difference between the NIST analysis and the vendor certified concentrations of the audit mix should ideally be ≤1.0% relative and as a worst case, no more than 2 % relative. The audit therefore validates the traceability claims of the three-component mixtures to NIST.
NIST developed a variety of methods that achieved an analytical uncertainty of ≤0.5% for NO, SO2, and CO2. The expected CO2 interference in the chemiluminescense analysis was accurately modeled. There was an unexpected interference of pressure broadening (due to CO2) that was modeled for non-dispersive infrared (NDIR) and fourier transform infrared (FTIR) measurements.