Testing common tubing materials for memory effects during water vapor stable isotope measurements

<p>In this study, we compared how a large isotopic step change propagated through five tubing materials, PFA, FEP, PTFE, HDPE, and copper, at two different temperatures and a flow rate of 1.1 SLPM through approximately 100 feet of 1/4 inch OD tubing. All tubing materials performed similarly to each other in terms of attenuation times regardless of temperature. While surface area and length of tubing affect lag times of signal propagation, they don’t change the shape of the attenuation curve or the e-folding time of the exponential decay. This is due to the speed of isotopic equilibrium with tubing walls, which can be described as a first order chemical reaction with the concentration of reactive surface sites. Likewise, use of a high-surface area air filter at this flow rate only affected lag times, but not the overall shape of the attenuation curve. Our results show that plastic tubing materials are not inferior to copper in terms of isotopic memory, and they are easier to work with and lower cost. Users are still advised to maximize air flow rates to minimize memory effects especially when accurate time-varying deuterium-excess measurements are required. Our results indicate that flow rates greater than 1.1 SLPM should yield D-excess e-folding times below 6 minutes for 1/4-inch OD tubing.<br />
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