Absolute ratios 17O/16O and 18O/16O in water standards and atmospheric O2

Full text: We have successfully upscaled an existing analytical method and used it to obtain high precision measurements of three oxygen isotopes in water. The method is based on water fluorination using CoF3 at 370 deg. C under a flowing stream of helium carrier gas. The original method developed by Baker et al. (Anal. Chem., 74, 1665, 2002,) is useful for analyzing nanoliter size samples with a precision of ∼0.3 per mille. In the upscaled system the sample size is about 2-3 μL and it allows obtaining much better precision (∼0.01 per mille). The main advantages of this method are that it is quick, easy and the reagent is relatively safe to handle. Water is injected via high temperature septum in the reactor (nickel tube with preconditioned reagent) and reacted with CoF3. The produced O2 and HF are passed with He carrier (25 ml/min) through traps, where HF is removed, to a second trap, containing coarse 5A molecular sieve. This trap is held at -196 deg. C and adsorbs the produced O2 gas. When all oxygen is trapped, the stream of the carrier is diverted and the O2 sample is transferred to a collection SS tube immersed in a liquid He container. The total processing time of each sample is about 30 min. Because one major problem with isotopic analysis of water is a memory effect, special care was taken to minimize and eliminate this. Operation after injection is fully automated. All isotopic measurements were done on a multi-collector mass spectrometer (Finnigan Delta- Plus and Delta PlusXL) which allows simultaneous monitoring of m/e 32, 33 and 34. δ18O and δ17O were measured versus our working O2 standard calibrated against atmospheric oxygen as a primary standard. In order to obtain the necessary statistics and to decrease the analytical error, each mass spectrometric measurement consists of three separate runs during which the ratio of sample to reference is determined 30 times. The pressures of the sample and reference gas are balanced before each of the three runs. The reported δ-values are averages of three runs, and the analytical error in δ18O and δ17O is about 0.01 per mille. Because natural water samples vary in wide isotopic range (up to 70 per mille) it was necessary to check the overall linearity of our mass spectrometers. With this purpose we prepared two water samples with a difference in δ18O of about 100 per mille and mixed them in different proportions. Regression of calculated vs. measured δ18O yields a nearly perfect one to one relationship (Y=1.00003X; R2 = 0.999997). This shows that our spectrometers are linear and suitable for high precision comparison of water samples and standards over a wide range of isotopic differences. We calibrated the international water standards SLAP and GISP vs. VSMOW for the first time for both δ17O and δ18O . The obtained δ17O values of SLAP can be used for δ17O -scale normalization: SMOW-SLAP. We also, for the first time, made direct measurements of δ17O and δ18O of O2 obtained from VSMOW vs. atmospheric oxygen. The obtained values of δ17O and δ18O were used for determination the absolute isotope ratios in GISP, SLAP and air O2. (author)