Water vapor stable isotope memory effects of common tubing materials

<p>Water molecules in vapor can exchange with water molecules sticking to surfaces of sampling tubing, and exchange rates are unique for each 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 observed as attenuation time for signal propagation in continuous laser-based stable water vapor isotope measurement systems. Tubing memory effects in δD and δ<sup>18</sup>O measurements can limit the ability to observe fast changes, and because δD and δ<sup>18</sup>O memory are not identical, this introduces transient deuterium excess (D-excess, defined as δD – 8* δ<sup>18</sup>O) artifacts in time-varying observations. A comprehensive performance comparison of commonly used tubing material water exchange properties has not been published to our knowledge. We compared how a large isotopic step change propagated through five commonly used tubing materials, PFA, FEP, PTFE, HDPE, and copper, at two different temperatures and an air flow rate of 0.635 L min<sup>-1</sup> through approximately 100 feet (~30.5 m) of ¼ in. (6.35 mm) outer diameter (OD) tubing. All tubing materials performed similarly to each other in terms of attenuation times, reaching 95 % completion in less than 45 seconds in all but 2 experiments with slight variations based on temperature. Bev-A-Line XX was also tested, unheated, but it did not reach isotopic equilibrium after an hour, and we cannot recommend its use in water vapor applications. While shorter tubing length and smaller inner diameters shortens the delay of signal propagation through the tubing, they don’t greatly change the shape of the attenuation curve or the delay-adjusted attenuation times under these conditions. Our results show that these commonly-used plastic tubing materials are not inferior to copper in terms of isotopic memory under these conditions, and they are easier to work with and are less expensive than copper. Our experience and results from other published studies indicate that maximizing air flow rates through the analyzer is the most effective way to minimize memory effects when accurate high-frequency deuterium-excess measurements are desired.</p>