Oxygen isotopic fractionation of O2 during adsorption and desorption processes using molecular sieve at low temperatures

RATIONALE Cryogenic trapping using molecular sieves is commonly used to collect O2 extracted from silicates for 17O/16O and 18O/16O analyses. However, gases which interfere with 17O/16O analysis, notably NF3, are also trapped and their removal is essential for accurate direct measurement of the 17O/16O ratio. It is also necessary to identify and quantify any isotopic fractionation associated with the use of cryogenic trapping using molecular sieves. METHODS The oxygen isotopic compositions of O2 before and after desorption from, and adsorption onto, 13X and 5A molecular sieves (MS13X and MS5A) at 0 degrees C, -78 degrees C, -114 degrees C, and -130 degrees C were measured in order to determine the oxygen isotopic fractionation at these temperatures. We also investigated whether isotopic fractionation occurred when O2 gas was transferred sequentially into a second cold finger, also containing molecular sieve. RESULTS It was confirmed that significant oxygen isotopic fractionation occurs between the gaseous O2 and that adsorbed onto molecular sieve, if desorption and adsorption are incomplete. As the fraction of released or untrapped O2 becomes smaller with decreasing trapping temperature (from 0 to -130 degrees C), the isotopic fractionation becomes larger. Approximately half of the total adsorbed O2 is released from the molecular sieve during desorption at -114 degrees C, which is the temperature recommended for separation from NF3 (retained on the molecular sieve), and this will interfere with 17O/16O measurements. CONCLUSIONS The use of a single cold finger should be avoided, because partial desorption is accompanied by oxygen isotopic fractionation, thereby resulting in inaccurate isotopic data. The use of a dual cold finger arrangement is recommended because, as we have confirmed, the transfer of O2 from the first trap to the second is almost 100%. However, even under these conditions, a small isotopic fractionation (0.18 +/- 0.05 parts per thousand in 17O values and 0.26 +/- 0.06 parts per thousand in 18O values) occurred, with O2 in the second trap being isotopically enriched in the heavier isotopes. Copyright (c) 2014 John Wiley & Sons, Ltd.