An Improved Vacuum Distillation Method for Extracting Soil Water for Stable Isotope Analyses

A simple, fast and portable vacuum distillation setup and methodology to extract water from soil and plant samples for water stable isotope analyses (oxygen-18 [¹⁸O] and hydrogen-2 [²H]) was developed. The methodology was tested on two soil types (a sandy loam and a silty clay loam) over different extraction times and soil moisture (near field capacity and permanent wilting point). Using an immersion cooler instead of liquid nitrogen or solid carbon dioxide (dry ice) as the cooling agent, the results showed that full recovery (extracting more than 98 percent of the total water) and reproducible isotopic ratios (± 0.08‰ for δ¹⁸O and ± 0.83‰ for δ²H) can be reached after 30 min with a dry sandy soil and 120 min with a dry clay soil at 100°C distillation temperature. The methodology allows large number of samples to be prepared for analysis quickly and reliably. (author)