Optimization Of Determination Of Water Stable Isotope By Direct Vapor Equilibration

The stable isotope tracing technique of water(²H,¹⁸O)has emerged as a vital technological tool for studying ecological hydrological processes.Prior to determining stable water isotope information,it is customary to extract water from soil and plant samples.The conventional method,Cryogenic Vacuum Distillation（CVD）,is laborious and time-consuming,and its accuracy and compatibility in water sample collection have recently been questioned.In recent years,the Direct Vapor Equilibration（DVE）method has gained popularity as a novel approach due to its convenience and efficiency.It is believed to provide a better representation of water involved in hydrological cycles and enables high-frequency,in-situ measurement of plant or soil water isotopes.However,certain limitations have hindered its development.Therefore,this study aims to enhance the water vapor equilibrium method by focusing on liquid water and plant water.Through a series of controlled indoor mechanistic experiments and plant rehydration experiments,various containers of different materials and sealing methods were used to investigate the impact of container airtightness,background water vapor concentration,and sample volume on the outcomes of the water vapor equilibrium method.Furthermore,the influence of water vapor concentration variations on the measurement results was explored by creating gradients of different water vapor concentrations.Additionally,the effects of organic compounds produced by alcohol solutions and plant samples on the water vapor equilibrium method were examined by analyzing the water vapor isotopic information of ethanol and methanol solutions with different concentrations and rehydrated plant samples.The following conclusions were primarily drawn:（1）Two types of glass headspace bottles exhibited no significant water loss or deviation inδ2H andδ18O measurement values compared to the true values（p>0.05）within a 30-day period,thereby recommending them as preferred containers for this method.The measurement values did not significantly differ under two distinct background water vapor concentrations（p>0.05）,indicating that the influence of background water vapor on water isotope measurement can be disregarded.However,substantial measurement deviations were observed when the sample/container volume ratio was below 1:300,emphasizing the need for sample volumes not less than 1/300 of the container volume.（2）Changes in water vapor concentration exhibited significant biases in the measurement results（p<0.05）.The relationship between the measured values and water vapor concentration followed a logarithmic curve across the entire concentration range（3000～30000ppmv）,which was influenced by the isotopic abundance of the sample.At high water vapor concentrations（>10000 ppmv）,a linear relationship was observed between the measured values and water vapor concentration,with no significant difference found among samples with different isotopic abundances（p>0.05）.（3）The introduction of organic compounds,such as methanol and ethanol,significantly affected the measurement results of the water vapor equilibrium method,with the magnitude of the measurement bias increasing with higher concentrations of organic compounds.Methanol exhibited a more pronounced effect compared to ethanol.Except for maize straw,the branches of three woody plant species—apple,willow,and black locust—generated substantial amounts of organic compounds during the equilibrium process,influencing the measurement of the water vapor equilibrium method,irrespective of plant species or water content.The ¹⁷O-excess demonstrated a linear relationship with the measurement bias and can serve as an indicator of spectral contamination.In summary,the measurement of the water vapor equilibrium method can be influenced by factors such as container leakage,insufficient sample volume,fluctuations in water vapor concentration,and the presence of volatile organic compounds.Optimization of the method can be achieved by adopting glass bottles as preferred containers,ensuring the sample volume exceeds 1/300 of the container volume,establishing concentration-dependent curves and calibration equations,and identifying and excluding samples contaminated with organic compounds.This research provides valuable insights for researchers utilizing this method.