2.3. Isotopic analysis
Precipitation, groundwater, plant water, and soil water samples were all extracted by the cryogenic vacuum distillation technique (LI-2100pro) in order to avoid the influence of high salinity moisture on the accuracy of the instrument. After all samples were equilibrated to room temperature, extraction was started, setting 3 h for plant water and soil water, 2h for precipitation and groundwater samples to achieve full recovery of sample water according to West et al. (2006). All of the extracted water from samples were transferred into 2 ml vials for analysis, and then analyzed stable isotopes (δ2H and δ18O) using an isotope ratio spectrometer (LGR-TLWIA-912). The instrument was equipped with an autosampler (PAL-LSI) for sample injection, and post-processing software (LWIA Post Analysis Full Installer v4.4.1) for test diagnosis, checking, and quantifying problems in the analysis (e.g., interference from organic pollutants, injection volume error) through detailed analysis of high-resolution absorption spectra. The organic contamination on plant water need correction procedures to eliminate the influence according to (Schultz et al., 2011). The measurement precision of the liquid water isotope analyzer was 0.3‰ for δ2H and 0.1‰ for δ18O. The isotopic compositions of δ18O and δ2H were expressed as an isotope ratio:
\begin{equation} \delta sam(\%0)=\left(\frac{R\text{sam}}{R\text{std}}-1\right)\times 1000\%0\nonumber \\ \end{equation}
where δsam was the isotopic difference for the samples relative to the Vienna Standard Mean Ocean Water (VSMOW) standard, and Rsam and Rstd were the molar abundance ratios (18O/16O and2H/1H) in the sample and standard, respectively.