High precision measurements of 17 O/ 16 O and 18 O/ 16 O ratios in H 2 O

We have optimized the method of water fluorination using the solid reagent CoF 3 to produce O 2 . This allows isotope ratio measurements by dual‐inlet mass spectrometry with very high precision of 0.01 to 0.03‰ for both δ 17 O and δ 18 O. Using this method, δ 17 O and δ 18 O of atmospheric O 2 were determined as 12.08 and 23.88‰ vs. VSMOW, respectively. Likewise, δ 17 O and δ 18 O of GISP were −13.12 and −24.73‰, and for SLAP they were −29.48 and −55.11‰ vs. VSMOW, respectively. Analysis of these data in a ln( δ 17 O + 1) vs. ln( δ 18 O + 1) plot yields a line with a regression coefficient ( λ ) of 0.5279 ± 0.0001 (R 2  = 0.). We also determined the fractionation factors 17 α and 18 α in liquid‐vapor equilibrium, and found that the ratio ln 17 α /ln 18 α is constant (0.529 ± 0.001) over the temperature range 11.4 to 41.5°C. Copyright © 2005 John Wiley & Sons, Ltd.