A new look at the sorption kinetics in reference gas standards

The preparation of calibration gas mixtures in cylinders using the gravimetric method (ISO 6142-1) has enabled the production of a wide range of mixtures down to the pmol/mol level with low uncertainties reaching 0.01% relative and beyond. The gravimetric method has limited use however for reactive components that adsorb on the cylinder wall or valve. For such components the adage ‘what comes in = what comes out’ no longer holds. To quantify gas losses in cylinders due to adsorption on the inner surfaces, two methods are typically used: by comparison against other gas mixture preparation methods (e.g. dynamic methods) or by decanting part of a mixture in an identical cylinder followed by cross-comparison. Here we present a new method to elucidate the sorption dynamics based on the use of isotopes (here 12 C-methanol and 13 C-methanol isotopes). The amount fraction evolution of both isotopes in gas phase is followed in time using laser spectroscopic methods. This way, the kinetics of desorption (mainly 13 C-methanol) and adsorption ( 12 C-methanol) can be followed in time. In this paper we will present the results from a pilot study on methanol mixtures at trace amount fractions prepared in different cylinder materials and treatments.