Journal Highlight: Cryogenic separation of an oxygen–argon mixture in natural air samples for the determination of isotope and molecular ratios

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  • Published: Apr 13, 2015
  • Author: separationsNOW
  • Channels: Sample Preparation
thumbnail image: Journal Highlight: Cryogenic separation of an oxygen–argon mixture in natural air samples for the determination of isotope and molecular ratios
The precise cryogenic separation of oxygen–argon mixtures from natural air samples has been accomplished using a single 5A (30/60 mesh) molecular sieve column at liquid nitrogen temperature, which is associated with isotopic fractionation.

Cryogenic separation of an oxygen–argon mixture in natural air samples for the determination of isotope and molecular ratios

Rapid Communications in Mass Spectrometry, 2015, 29, 775-781
Habeeb Rahman Keedakkadan and Osamu Abe

Abstract: The precise and simplified cryogenic separation of oxygen–argon mixtures from natural air samples presented here was made possible using a single 5Å (30/60 mesh) molecular sieve column. The method involves the trapping of eluted gases using molecular sieves at liquid nitrogen temperature, which is associated with isotopic fractionation. We tested the proposed method for the determination of isotopic fractionations during the gas exchange between water and atmospheric air at equilibrium. The dependency of fractionation was studied at different water temperatures and for different methods of equilibration (bubbling and stirring). Isotopic and molecular fractionations during gas desorption from molecular sieves were studied for different amounts and types of molecular sieves. Repeated measurements of atmospheric air yielded a reproducibility (±SD) of 0.021 ‰, 0.044 ‰, 15 per meg and 1.9 ‰ for δ17O, δ18O, Δ17O and δ(O2/Ar) values, respectively. We applied the method to determine equilibrium isotope fractionation during gas exchange between air and water. Consistent δ18O and Δ17O results were obtained with the latest two studies, whereas there was a significant difference in δ18O values between seawater and deionized water. We have revised a helium-free, cryogenic separation of oxygen–argon mixtures in natural air samples for isotopic and molecular ratio analysis. The use of a single 13X (1/8" pellet) molecular sieve yielded the smallest isotopic and molecular fractionations, and this fractionation by molecular sieves can be corrected by the amount of molecular sieve used in the experiment. The reproducibility of the method was tested by the measurement of the oxygen isotope ratios of dissolved oxygen at equilibrium with atmospheric air. We confirmed that the choice of methods for making air-equilibrated water was not related to the magnitude of isotope fractionation, whereas there was a difference between seawater and deionized water.

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