Water - δ17O, δ18O, Δ17O


We employ the water fluorination method using Cobalt (III) fluoride to convert H2O into O2 for subsequent triple oxygen isotope analysis on a Thermo MAT253. Water is fluorinated to produce O2 as previously described (Barkan and Luz 2005, Baker et al. 2002, Barkan and Luz 2007) and the O2 is collected as in Abe (2008). Briefly, 2 mL of water was injected into a 370 °C 15 cm long nickel column containing 7 g CoF3 converting H2O into O2 and creating HF and CoF2 as byproducts. Helium carried O2 at 30 mL min-1 through a trap submerged in liquid nitrogen to collect HF, which is later vented to a fume hood; the NaF trap originally recommended by Baker et al. (2002) is not used. The O2 sample is collected in a trap for 20 min, and then transferred to a 14 cm stainless steel cold finger. Both the trap and cold finger contained approximately 40 mg of 5A (4.2 to 4.4 Å) molecular sieve and were submerged in liquid nitrogen. To minimize memory effects, following Barkan and Luz (2005) we injected and discarded a minimum of four injections when switching between waters with a difference in δ18O value of greater than 5 permil. The cold fingers are sealed using a bellows valve.

The O2 samples are warmed to 60 °C and expanded for 10 min into the sample bellows through a custom multiport on a dual-inlet ThermoFinnigan MAT 253 isotope ratio mass spectrometer (Thermo Electron, Bremen, Germany) with Faraday cup amplifiers for m/z 32, 33, and 34 of 1*109 Ω, 1*1012 Ω, and 1*1011 Ω, respectively. The custom designed multiport used air-actuated bellows valves, air pressure control originally intended for a stock microvolume, and modified ISL scripts. The O2 sample was analyzed for m/z 32, 33, and 34 abundance ratios to determine d18O and d17O values with reference to O2 gas (δ17O measured s/VSMOW=-4.319 permil, δ18O measured s/VSMOW=-8.255 permil). Each mass spectrometric measurement comprised 90 sample-to-reference comparisons. Each of these comparisons consisted of 26 s of integration and 15 s of idle time. After every 30 comparisons, the m/z 32 signals of the sample and reference gases were balanced to 10 V and the mass spectrometer was peak-centered on m/z 33. The reference bellows was automatically refilled before each sample to a pressure that was equal to that of the sample. Over the course of these measurements the mass spectrometer exhibited an internal reproducibility of 0.002 permil, 0.004 permil, and 0.0037 permil (3.7 per meg) for δ17O, δ18O, and 17Oexcess values, respectively. We use VSMOW, SLAP, and GISP to calibrate our water fluorination system. We have also measured select in-house reference waters for 17Oexcess17O).

Standard Operating Procedures

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