An On-Line Technique for the Determination of the δ18O and δ17O of Gaseous and Dissolved Oxygen

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Abstract
Few studies have used the stable isotopic composition of O2 as a tracer of gas transport or biogeochemical processes in environmental research. Here we demonstrate field sampling techniques for gaseous and dissolved O2 and describe an analytical method for measuring δ18O and δ17O values of O2 in air, soil gas, and water samples using continuous-flow isotope-ratio mass spectrometry (CF-IRMS). A Micromass CF−IRMS was altered to accommodate a sample gas injection port prior to a CO2 and H2O trap and GC column. The GC column was a 1-m, 5-Å molecular sieve column held at 35 °C. The resolved sample O2 was introduced to the IRMS via an open split. δ18O and δ17O values were determined by measurement of O2 isotopes at m/z 34/32 and 33/32 and comparison to a reference pulse of O2. Repeated injections of atmospheric oxygen yielded a repeatability (±SD) of ±0.17‰ for δ18O and ±0.5‰ for δ17O. IRMS source linearity was excellent for O2 over a sample size range of 60−400 μL. The smallest sample for routine δ18O and δ17O determinations was ∼80 μL of O2, with a sample analysis time of 180 s. Preliminary results from a riverine and soil gas study illustrate natural oxygen isotope fractionation processes.