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

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 ranging from 100 to 1000 kJ mol⁻¹. Furthermore, chemisorption forms only 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. We proved the pressure effect on physisorption for CO₂, CH₄ and H₂O by decanting a steel and an aluminium cylinder completely. The results are in excellent agreement with a monolayer adsorption model for both cylinders. However, adsorption on aluminium (0.3 ppm and 0 ppm for CO₂ and H₂O) was about 20 times less than on steel (6 ppm and 30 ppm, respectively). In the climate chamber the cylinders were exposed to temperatures between −10 to +50 °C to determine the corresponding temperature coefficients of adsorption. Again, we found distinctly different values for CO₂ ranging from 0.0011 to 0.0133 ppm °C⁻¹ for steel cylinders and −0.0003 to −0.0005 ppm °C⁻¹ for aluminium cylinders. The reversed temperature dependence for aluminium cylinders is most probably due to temperature and gas consumption induced pressure changes. After correction, aluminium cylinders showed no temperature independence. Temperature coefficients for CH₄, CO and H₂O 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.