RAPID COMMUNICATIONS IN MASS SPECTROMETRY Rapid Commun. Mass Spectrom. 2003; 17: 2809–2814 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/rcm.1267 High-precision measurements of 17O/16O and 18O/16O

A method for high-precision and high-accuracy mass spectometric measurements of the ratios among the three oxygen isotopes, and of the O2/Ar ratio, is presented. It involves separation of the O2-Ar mixture from air and includes a fully automated system that ensures highly reliable sample processing. Repeated measurements of atmospheric oxygen yield the repeatability (SE t, standard error of the mean (n 12) multiplied by Student’s t-factor for a 95 % confidence limit) of 0.004, 0.003 and 0.2 % for d18O, d17O and dO2/Ar, respectively. Copyright # 2003 John Wiley & Sons, Ltd. The natural distribution of 17O/16O and 18O/16O ratios of atmospheric and dissolved O2 have recently been applied to estimating rates of global and oceanic productivity.1–5 Such applications require very high precision in measure-ments of 17O/16O and 18O/16O (of the order of0.01 % or bet-ter). A main obstacle for such performance is the interference of nitrogen gas in the ion source of the mass spectrometer, which causes a dependence of d18O and d17O on the N2/O2 ratio of the sample gas relative to the working referenc