Influence of CO₂ adsorption on cylinders and fractionation of CO₂ and air during the preparation of a standard mixture

Abstract

We evaluated carbon dioxide (CO2) adsorption on the internal surface of the cylinder and the fractionation of CO2 and air during the preparation of standard mixtures with atmospheric CO2 level through multistep dilution. The CO2 molar fractions in the standard mixtures deviated from the gravimetric values by -0.207±0.060 µmol mol−1 on average, which is larger than the compatibility goal (0.1 µmol mol−1) recommended by the World Meteorological Organization. The deviation was consistent with those calculated using two fractionation factors: one was estimated by the mother–daughter transfer experiment in which CO2–air mixtures were transferred from a mother cylinder to an evacuated daughter cylinder, and another was computed by applying the Rayleigh model to the change in CO2 molar fractions in a source gas as its pressure was depleted from 11.5 to 1.1 MPa. The mother–daughter transfer experiments showed that the deviation was caused by the fractionation of CO2 and air during the transfer of the source gas (CO2–air mixture with a higher CO2 molar fraction than that in the prepared gas mixture). The CO2 fractionation was less significant when the transfer speed decreased to less than 3 L min−1, indicating that thermal diffusion mainly caused the fractionation. The CO2 adsorption on the internal cylinder surface was experimentally evaluated by emitting a CO2–air mixture from a cylinder. When the cylinder pressure was reduced from 11.0 to 0.1 MPa, the CO2 molar fractions in the mixture exiting the cylinder increased by 0.16±0.04 µmol mol−1. By applying the Langmuir adsorption–desorption model to the measured data, the amount of CO2 adsorbed on the internal surfaces of a 10 L aluminum cylinder when preparing a standard mixture with atmospheric CO2 level was estimated to be 0.027±0.004 µmol mol−1 at 11.0 MPa.