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

Abstract

Despite the importance of O2 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 δ18O 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 δ18O−O2 values is presented. The procedure utilizes a 5-Å molecular sieve column held at a constant temperature of 50 °C to separate O2 and N2 in time. A precision (± SD) of ±0.3‰ or better for δ18O−O2 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 δ17O−O2 analysis (precision of ±0.5‰) and δ15N−N2 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.