Abstract: Because of very low air temperatures, precipitation and water vapor over continental Antarctica and water vapor near the tropopause are depleted in ¹⁸O isotopes. For instance, the ratio of the ¹⁸O isotope (δ¹⁸O) in water vapor near the tropopause can reach −150‰, which is considerably lower than that in Standard Light Antarctic Precipitation (SLAP; −55.50‰). In mass spectrometry, the calibration of isotopic measurements based on linear interpolation is only valid if the isotopic ratios of the samples fall within the ratio range of reference samples. To meet the needs of high-precision measurements of isotopically depleted water samples, it is necessary to prepare laboratory reference standards that are isotopically depleted. Laser spectroscopy has also been widely used for isotopic measurements of water but the validity of linear interpolation for calibration still needs to be validated by experimental results. We used the principle of mass conservation to determine the δ¹⁸O value of ¹⁶O-enriched water produced by Sigma-Aldrich ( −940.40‰). We used different mixing ratios of laboratory water standard and ¹⁶O-enriched water to prepare four solutions with δ¹⁸O values of −76.53‰, −97.08‰, −117.93‰, and −137.50‰, respectively. We measured the isotopic ratios of the solutions using the L2140-i isotope and gas concentration analyzer from Picarro. Results support the validity of calibration based on linear interpolation; mean deviation is only 0.14‰. In this paper, we provided a method for high-precision measurements of stable isotopes in precipitation and water vapor over continental Antarctica and water vapor near the tropopause.

Key words: water stable isotopes, isotopic depletion, polar precipitation, standard sample preparation, linearity of isotopic measurements