Water vapor stable isotope memory effects of common tubing materials

<p>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 δD and δ18O measurements can limit the ability to observe fast changes, and because δD and δ18O memory are not identical, this introduces transient deuterium excess (D-excess, defined as δD – 8* δ18O) 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 1/4 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’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.</p>

气相水分子可与采样管内壁附着的水分子发生交换，且交换速率因同位素异构体（isotopologue）与管材材质的不同而存在差异。因此，管壁需要一定时间才能与新的气相同位素信号达到同位素平衡，进而产生记忆效应——在基于连续激光的稳定水汽同位素测量系统中，该效应表现为信号传播的衰减时间。δD（氘同位素比值）与δ¹⁸O（氧-18同位素比值）测量中的记忆效应会限制对快速变化过程的观测能力，且二者的记忆效应并不一致，这会在时变观测中引入瞬态氘盈余（deuterium excess，定义为D-excess，即δD – 8×δ¹⁸O）伪影。据我们所知，目前尚未有针对常用管材水交换特性的全面性能对比研究。本研究对比了在两种温度、1.1 L·min⁻¹气流速率下，总长约100英尺（约30.5米）、外径1/4英寸（6.35毫米）的5种管材——全氟烷氧基烷烃（PFA）、氟化乙烯丙烯共聚物（FEP）、聚四氟乙烯（PTFE）、高密度聚乙烯（HDPE）与铜——对大型同位素阶跃变化的传播情况。所有管材在衰减时间方面表现相近，且不受温度影响。尽管管材内径与长度会影响信号传播的滞后时间，但不会改变衰减曲线的形状或衰减时间。这表明管壁的同位素平衡速率可被描述为一级化学反应，其受控于反应性表面位点的浓度，而非位点总数。同样，在该气流速率下使用高表面积颗粒过滤器，不会对同位素信号衰减的速率产生影响。然而，加装质量流量计确实会改变衰减速率，因此我们建议在测量进气口与系统设计阶段，调研类似设备的影响。本研究结果显示，在上述实验条件下，塑料管材在同位素记忆效应方面并不劣于铜材，且相较于铜材更易于加工、成本更低。不过仍建议用户最大化分析仪与管材的气流速率，以最小化记忆效应，尤其是在需要高精度时变氘盈余测量的场景中。