Comment on amt-2023-56

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Water molecules in vapor exchange with water molecules sticking to surfaces of sampling tubing, and exchange rates are unique for each isotopologue and tubing material. Therefore, tubing walls take some time to reach isotopic equilibrium with a new vapor isotopic signal, creating a memory effect observed as attenuation time for signal propagation in continuous laser-based stable water vapor isotope measurement systems. Memory effects in &delta;D and &delta;¹⁸O measurements can limit the ability to observe fast changes, and because &delta;D and &delta;¹⁸O memory are not identical, this introduces transient deuterium excess (D-excess, defined as &delta;D &ndash; 8* &delta;¹⁸O) 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 tubing materials, PFA, FEP, PTFE, HDPE, and copper, at two different temperatures and an air flow rate of 1.1 L min^-1 through approximately 100 feet (~30.5 m) of &frac14; inch (6.35 mm) outer diameter (OD) tubing. All tubing materials performed similarly to each other in terms of attenuation times regardless of temperature. While inner diameter and length of tubing affect lag times of signal propagation, they don&rsquo;t change the shape of the attenuation curve or the attenuation times. This indicates that the speed of isotopic equilibrium of the tubing walls can be described as a first order chemical reaction controlled by the concentration of reactive surface sites rather than the total number of sites. Likewise, use of a high-surface area particle filter at this air flow rate did not affect the speed of the isotopic signal attenuation. However, the addition of a mass flow meter did affect the speed of the attenuation, and we recommend investigating the influence of similar devices during measurement inlet and system design. Our results show that 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. Users are still advised to maximize air flow rates through both analyzer and tubing to minimize memory effects especially when accurate time-varying deuterium-excess measurements are required.