Automated analyses of ¹⁸O/¹⁶O ratios in dissolved oxygen from 12-ml water samples

First published in Limnology and Oceanography: Methods 2:35-41

We introduce a new technique to routinely determine the ¹⁸O/¹⁶O ratio of O₂(aq) from 12-mL Exetainer vials. Results were expressed in a d-notation versus air and the Vienna Standard Mean Ocean Water (VSMOW). Samples were prepared by creating a He-headspace and stripping O₂(aq) from solution by shaking for 30 min on a wrist shaker. Subsequent isotope analysis of the extracted O₂(g) was achieved by converting the entire headspace into a large sampling loop using a double-hole needle. This enabled admission of sufficient O₂(g) into a packed A5-Å-molecular sieve column, where it was separated from N₂ before admission to the isotope ratio mass spectrometer. The latter was tuned to an m/z ratio of 32, thus enabling direct determination of molecular O₂(g) without conversion to CO₂. External standards consisted of dry air samples in helium-flushed vials and had between 1.5 and 16.8 parts per thousand O₂(g) in a He matrix and a known isotopic composition of 0‰ air (+23.8‰ VSMOW). The method allows automated analyses of up to ~180 samples in one single batch and will provide new quantitative information about oxygen turnover in aqueous systems, including rates of gas transfer, redox processes, respiration, and photosynthesis. Repeat d¹⁸O_O2(aq) measurements on samples with concentrations between 15.6 µmol L^–1 and saturation revealed standard deviations of 0.3‰. This is a typical precision encountered in continuous flow applications, and the method is available for studies using either ¹⁸O-labeled water to evaluate O₂ gross production by incubation experiments or for natural abundance studies when isotope shifts are larger than 0.8‰. It may also become useful in microbiological and medical applications and can serve to quantify plant-gas exchange and soil gas processes.