Technical note: an alternative water vapor sampling technique for stable isotope analysis

Abstract. Recent developments in laser spectroscopy enabled to carry out direct measurements of δ²H and δ¹⁸O of air water vapor in the field. However, certain experimental sites or project budgets do not ease the deployment of this technology to obtain the needed measurements. We carried out three consecutive experiments aiming to provide an alternative method to sample air vapour in the field, and preventing fractionation during the process. The first experiment determined the minimum air sample volume required to obtain measurements of δ²H and δ¹⁸O with a laser spectrometer. The second one test evaluated the capacity to retrieve continuously similar isotopic signatures of the collected samples from one location. The third experiment assessed the applicability of this methodology under an experimental set up in a coniferous forest in The Netherlands. Stable isotope measurements of water vapor by laser spectroscopy can be obtained with a sample volume of 450 mL of air. This allows to measure each sample during a period of 300 s, obtaining isotope signatures with standard deviations lower than 0.1 ‰, and 0.5 ‰, for δ¹⁸O and δ²H, respectively. Air samples collected with bags were homogeneously mixed, allowing to retrieve a better temporal variation in the field than the cold traps employed.