Water vapor exposure chamber for constant humidity and hydrogen and oxygen stable isotope composition

Water vapor exposure experiments have applications for studying water physisorption and chemisorption hydration and hydroxylation reactions on a wide variety of material surfaces. The stable isotopes of hydrogen and oxygen in the water molecule are useful tracers of water exchange mechanisms and/or rates in such vapor exposure experiments.

We designed and built a humidity chamber system that uses membrane-mediated liquid-vapor exchange of water followed by mixing with dry air to control the relative humidity of air and its δ² H and δ¹⁸ O isotopic composition. We tested the stability and precision of the humidity and its isotopic composition on hourly to 90-day timescales.

The humidity chamber design reported here is capable of providing relative humidity control to within ±1%, and consistent δ² H and δ¹⁸ O values of the water vapor that are similar to our cavity ringdown spectroscopy (CRDS) measurement precision (δ² H_vap  ± 0.7‰ and δ¹⁸ O_vap  ± 0.24‰). We quantify the isotopic enrichment effects of Rayleigh distillation in the system and provide information on water reservoir sizes large enough to buffer isotopic enrichment effects to within measurement precision.

The humidity chamber design reported here provides a means to create constant δ² H and δ¹⁸ O values over the course of an exposure experiment. The design has applications to a wide range of studies of water sorption on material surfaces from foods and pharmaceuticals to geological materials.