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

Abstract

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 d18OO2(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