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.