Gas adsorption and desorption effects on cylinders and their importance for long-term gas records

Abstract

Abstract.It is well known that gases adsorb on many surfaces, in particular metal surfaces. There are two main forms responsible for these effects (i) physisorption and (ii) chemisorption. Physisorption is associated with lower binding energies in the order of 1-10 kJ mol −1 , compared to chemisorption which ranges from 100 to 1000 kJ mol −1 . Furthermore, chemisorption only forms monolayers, contrasting physisorption that can form multilayer adsorption. The reverse process is called desorption and follows similar mathematical laws; however, it can be influenced by hysteresis effects. In the present experiment, we investigated the adsorption/desorption phenomena on three steel and three aluminium cylinders containing compressed air in our laboratory and under controlled conditions in a climate chamber, respectively. Our observations from completely decanting one steel and two aluminium cylinders are in agreement with the pressure dependence of physisorption for CO 2 , CH 4 , and H 2 O. The CO 2 results for both cylinder types are in excellent agreement with the pressure dependence of a monolayer adsorption model. However, mole fraction changes due to adsorption on aluminium (< 0.05 and 0 ppm for CO 2 and H 2 O) were significantly lower than on steel (< 0.41 ppm and about < 2.5 ppm, respectively). The CO 2 amount adsorbed (5.8 × 10 19 CO 2 molecules) corresponds to about the fivefold monolayer adsorption, indicating that the effective surface exposed for adsorption is significantly larger than the geometric surface area. Adsorption/desorption effects were minimal for CH 4 and for CO but require further attention since they were only studied on one aluminium cylinder with a very low mole fraction. In the climate chamber, the cylinders were exposed to temperatures between −10 and +50 • C to determine the corresponding temperature coefficients of adsorption. Again, we found distinctly different values for CO 2 , ranging from 0.0014 to 0.0184 ppm • C −1 for steel cylinders and −0.0002 to −0.0003 ppm • C −1 for aluminium cylinders. The reversed temperature dependence for aluminium cylinders points to significantly lower desorption energies than for steel cylinders and due to the small values, they might at least partly be influenced by temperature, permeation from/to sealing materials, and gas-consumptioninduced pressure changes. Temperature coefficients for CH 4 , CO, and H 2 O adsorption were, within their error bands, insignificant. These results do indicate the need for careful selection and usage of gas cylinders for high-precision calibration purposes such as requested in trace gas applications.

Investigation of adsorption/desorption behavior of small volume cylinders and its relevance for atmospheric trace gas analysis

“…A significant difference between this study and the previous studies (Leuenberger et al, 2015;Brewer et al, 2018;Schibig et al, 2018) is the onset of adsorption effects. All previous studies have shown that for high pressure cylinders, usage below 20 bar is problematic.…”

Stability of gaseous volatile organic compounds contained in gas cylinders with different internal wall treatments

“…There has been much interest of late in the initial adsorption of carbon dioxide (CO 2 ) in air balance mixtures from the initial filling of a cylinder followed by desorption as the pressure in the cylinder is reduced, resulting in an increase in the mole fraction of CO 2 (Leuenberger et al, 2015;Miller et al, 2015). This adsorption/desorption can be modeled for different cylinder volumes using the Langmuir isotherm (Langmuir, 1916(Langmuir, , 1918.…”

Preparation of primary standard mixtures for atmospheric oxygen measurements with less than 1 µmol mol−1 uncertainty for oxygen molar fractions

“…as isotopic abundances in the troposphere are considered to be constant (Junk and Svec, 1958;Baertschi, 1976;Li et al, 1988;Barkan and Luz, 2005). Because differences between isotopic ratios of N 2 , O 2 , and Ar in the AIST reference air and air samples at Hateruma were small enough to be negligible, their fractionations due to preparation of the AIST reference air are ignored.…”

“…However, there are many deterioration risks of the O 2 /N 2 ratio in aluminium cylinders used for reference air and the primary standard mixtures. These include fractionations of O 2 and N 2 induced by pressure, temperature, and water vapour gradients (Keeling et al, 1998b); adsorption-desorption of the constituents on the inner surface (Leuenberger et al, 2015); and permeation or leakage of the constituents from or through the valve (Sturm et al, 2004;Keeling et al, 2007). In order to avoid these risks, the cylinders are handled in accordance with certain best practices, including orienting cylinders horizontally to minimize thermal and gravitational fractionation (Keeling et al, 2007;Leuenberger et al, 2015).…”