A soil water distillation technique using He-purging for stable isotope analysis

Abstract

Summary A new method of soil water extraction for oxygen and hydrogen isotopic analysis has been developed; this method uses a helium flow system as an alternative to the conventional vacuum extraction method. The method significantly increases the efficiency of sample preparation and simplifies the extraction. During the water distillation, a helium carrier gas transfers water vaporized at 95 °C from the soil sample to a cold trap at liquid nitrogen temperature. An extraction time of 180 min is used to distill the water from the fine-grained soil completely. The proposed He-purging distillation technique makes it possible to distill approximately a dozen samples simultaneously. The method was tested using liquid water samples and clayey soil samples hydrated with water of known isotopic composition. The standard deviations for these tests were 0.08‰ for δ18O and 0.7‰ for δD. An intercomparison test was conducted for the helium and vacuum extraction methods using natural soil samples. The correlation coefficients between the methods were 0.9926 and 0.9939 for δD and δ18O, respectively. The proposed He-purging distillation method can achieve high precision for clayey soil samples with low water content and has the potential to provide adequate isotopic data in hydrological and ecological studies. The method is relatively fast, efficient, and inexpensive. We also recommend using the method to distill salt solutions (sea water, mineralized water) before determining the δD and δ18O values using a chromium or carbon reduction method to avoid “salt effects”.