08 May 2023
 | 08 May 2023
Status: this preprint is currently under review for the journal AMT.

Water vapor stable isotope memory effects of common tubing materials

Alexandra L. Meyer and Lisa R. Welp

Abstract. 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 ¼ 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.

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Alexandra L. Meyer and Lisa R. Welp

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2023-56', Anonymous Referee #1, 14 May 2023
    • AC3: 'Reply on RC1', Alexandra Meyer, 17 Oct 2023
  • RC2: 'Comment on amt-2023-56', Anonymous Referee #2, 23 May 2023
    • AC4: 'Reply on RC2', Alexandra Meyer, 17 Oct 2023
  • RC3: 'Comment on amt-2023-56', Anonymous Referee #3, 31 May 2023
    • AC2: 'Reply on RC3', Alexandra Meyer, 17 Oct 2023
  • EC1: 'Comment on amt-2023-56', Thomas Röckmann, 31 May 2023
    • AC5: 'Reply on EC1', Alexandra Meyer, 17 Oct 2023
  • CC1: 'Comment on amt-2023-56', Jonathan Keinan, 09 Jun 2023
    • AC1: 'Reply on CC1', Alexandra Meyer, 17 Oct 2023
Alexandra L. Meyer and Lisa R. Welp

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Water vapor stable isotope memory effects of common tubing materials Alexandra Meyer and Lisa Welp

Alexandra L. Meyer and Lisa R. Welp


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Short summary
Water molecules stick to air intake tubing wall surfaces and exchange with ambient vapor. This can slow signal change measurements. We tested whether material type affects this stickiness. Less stickiness would lead to an instrument seeing signal changes faster. We unexpectedly saw no difference in signal speed between material types. Water vapor stable isotope users can more confidently use plastic tubing and compare measurements across observation systems that used different tubing materials.