Oxygen isotope exchange with quartz during pyrolysis of silver sulfate and silver nitrate

RATIONALE Triple oxygen isotopes of sulfate and nitrate are useful metrics for the chemistry of their formation. Existing measurement methods, however, do not account for oxygen atom exchange with quartz during the thermal decomposition of sulfate. We present evidence for oxygen atom exchange, a simple modification to prevent exchange, and a correction for previous measurements. METHODS Silver sulfates and silver nitrates with excess 17O were thermally decomposed in quartz and gold (for sulfate) and quartz and silver (for nitrate) sample containers to O2 and byproducts in a modified Temperature Conversion/Elemental Analyzer (TC/EA). Helium carries O2 through purification for isotope-ratio analysis of the three isotopes of oxygen in a Finnigan MAT253 isotope ratio mass spectrometer. RESULTS The Δ17O results show clear oxygen atom exchange from non-zero 17O-excess reference materials to zero 17O-excess quartz cup sample containers. Quartz sample containers lower the Δ17O values of designer sulfate reference materials and USGS35 nitrate by 15% relative to gold or silver sample containers for quantities of 2–10 µmol O2. CONCLUSIONS Previous Δ17O measurements of sulfate that rely on pyrolysis in a quartz cup have been affected by oxygen exchange. These previous results can be corrected using a simple linear equation (Δ17Ogold = Δ17Oquartz * 1.14 + 0.06). Future pyrolysis of silver sulfate should be conducted in gold capsules or corrected to data obtained from gold capsules to avoid obtaining oxygen isotope exchange-affected data. Copyright © 2012 John Wiley & Sons, Ltd.