Rapid and Precise Determination of the δ18O of Dissolved and Gaseous Dioxygen via Gas Chromatography−Isotope Ratio Mass Spectrometry

Despite the importance of O 2 in biogeochemical processes, relatively few environmental studies have incorporated stable isotope information to assess the origins and cycling of this gas. A major limitation to the routine use of δ 18 O has been the cost and complexity associated with traditional off-line preparation, dual-inlet techniques. A gas chromatograph−isotope ratio mass spectrometry (GC−IRMS) technique providing rapid and precise δ 18 O−O 2 values is presented. The procedure utilizes a 5-Å molecular sieve column held at a constant temperature of 50 °C to separate O 2 and N 2 in time. A precision (± SD) of ±0.3‰ or better for δ 18 O−O 2 is demonstrated on gaseous and dissolved samples spanning an environmentally relevant range in size of 20−700 μM. The potential for utilizing the technique for δ 17 O−O 2 analysis (precision of ±0.5‰) and δ 15 N−N 2 analysis (precision of ±0.2‰) on air samples is also demonstrated. Preliminary results from two unique environments, the subtropical North Pacific and Cornell University experimental ponds, are presented to demonstrate potential applications of the technique.