(144c) Experimental Measurement and Thermodynamic Modeling of Adsorption-Induced Composition Changes in CO2/Air Reference Gas Cylinders

Measurement of CO2 emissions and atmospheric CO2 composition relies on a suite of reference gas mixtures with assigned CO2 compositions that are developed and disseminated with the cooperation of metrologists in various international institutions. The stability of these reference materials is central to anchoring precise measurements to primary standards and the International System of Units. For atmospheric CO2 concentrations, long term stability of assigned values is important for their use [1] and continues to be the subject of research by NMIs and CCLs [2]. However, it is known for gas mixtures with low assigned concentrations of certain analytes that the mole fraction of those analytes in the effluent gas may change as the gas cylinder is consumed1. This change in mole fraction may be attributed to desorption of the analyte species from the interior surface of the storage cylinder and may be sufficiently large that the actual composition of the effluent gas stream falls outside the uncertainty limits of the assigned composition value. At NIST, a multidisciplinary effort is underway examining the stability of CO2-containing natural air in various compressed gas metal cylinders typically used in the dissemination of these gas standards [3]. Here, we focus on the pressure dependence of desorption of CO2 from cylinder walls. Using new technologies that enable sensitive, real-time measurements of effluent gas composition, we have developed an experimental method and thermodynamic model for quantifying gas composition as cylinder contents are discharged.

To illustrate, we present experiments in which cylinders are discharged at rates much faster than during normal use but under near-isothermal conditions. These data reveal a relatively large desorption of CO2 below a threshold partial pressure which depends on temperature. While the underlying cause of the rise in CO2 mole fraction has been explained [4], the exact mechanism is difficult to elucidate. To better understand this effect, we introduce a thermodynamic model that includes a simple description of the adsorption equilibrium of CO2 on the inner cylinder surface. Despite its simplicity, the model can quantify the rise in CO2 mole fraction as the cylinder discharges. Furthermore, by incorporating competitive adsorption in this model, we can also predict and describe non-monotonic changes in CO2 composition when two or more gases in the mixture adsorb to the cylinder surface (e.g., CO2 and trace species such as water, Ar, CH4, or other contaminants). We have used this approach to quantify the capacity and relative affinity of CO2 to adsorb on metal surfaces as a function of temperature and pressure. In general, this experimental and theoretical approach enables rapid quantification of the minimum useful pressure of reference standards for CO2 and other relevant gases. We envision this approach to support a variety of new gas reference materials. Its targeted application by standards organizations and user communities will help ensure adherence to increasingly stringent amount-of-substance specifications.

1. W. R. Miller Jr., G. C. Rhoderick, and F. R. Guenther; Anal. Chem., 2015, 87, pp 1957–1962; DOI: 10.1021/ac504351b.
2. B. D. Hall, A. M. Crotwell, B. R. Miller, M. Schibig and J. W, Elkins: Atmos. Meas. Tech., DOI:10.5194/amt-12-517-2019;
3. Schmidt, Wong, Siderius, Harris, & Hodges, in press.
4. M. C. Leuenberger, M. F. Schibig and P. Nyfeler: Atmos. Meas. Tech., 2015, 8, pp. 5289 – 5299; DOI: 10.5194/amt-8-5289-2015.