Isotopic (δ18O and δ2H) Integrity of Water Samples Collected and Stored by Automatic Samplers

Abstract

Core Ideas Isotopic integrity (δ18O/δ2H) of water samples stored in autosamplers was assessed. Uncertainty in isotope values increased with increasing storage time and temperature. Addition of mineral oil to samples decreased evaporation and isotope fractionation. Stable water isotopes are increasingly becoming part of routine monitoring programs that use automatic samplers. The objectives of this study were to quantify the uncertainty in isotope signatures due to the length of sample storage (1–24 d) inside autosamplers over a range of air temperatures (5–35°C) and to evaluate the effectiveness of two evaporation reduction measures (mineral oil and high density polyethylene balls). Results of the laboratory study showed that up to 11.8% of the sample volume evaporated when samples were stored in an autosampler at 35°C for 24 d. To prevent significant water isotope fractionation, samples should be retrieved from autosamplers <7 d following sample collection when air temperatures are <22°C and <3 d following sample collection when the air temperature is 35°C. If samples need to be stored in an autosampler for longer periods of time, we found that mineral oil added to sample bottles effectively decreases evaporation and the potential for isotope fractionation.