Water vapor stable isotope memory effects of common tubing materials

Water molecules in vapor can exchange with gaseous water molecules sticking to surfaces of sampling tubing, and exchange rates are unique for each water isotopologue and tubing material. Therefore, water molecules on tubing walls take some time to reach isotopic equilibrium with a new vapor isotopic signal. This creates a memory effect that is observed as attenuation time for signal propagation in continuous stable water vapor isotope measurement systems. Tubing memory effects in δ D and δ 18 O measurements can limit the ability to observe fast changes, and because δ D and δ 18 O memory are not identical, this introduces transient deuterium excess (D-excess, defined as δ D - 8 × δ 18 O ) artifacts in time-varying observations. To our knowledge, a comprehensive performance comparison of commonly used tubing material water exchange properties in laser-based measurement systems has not been published. We compared how a large isotopic step change propagated through five commonly used tubing materials for water isotopic studies – perfluoroalkoxy (PFA), fluorinated ethylene propylene (FEP), polytetrafluoroethylene (PTFE), high-density polyethylene (HDPE), and copper – at two different temperatures and an airflow rate of 0.635 L min −1 through approximately 100 ft (30.5 m) of 1 / 4 in. (6.4 mm) outer diameter (o.d.) tubing. All commonly used ...