Sulfur and oxygen isotope analysis of sulfate at micromole levels using a pyrolysis technique in a continuous flow system

The discovery of a mass-independent isotopic composition (Delta O-17 = (delta O-17 - 0.512 * delta O-18) not equal 0) in aerosol sulfate and the identification of its origin (aqueous-phase oxidation by O-3 and H2O2) have renewed interest in measuring the oxygen isotopic content of sulfate. In this paper, we present a new method to measure both delta O-17 and delta O-18 in SO4, with the possibility of sulfur isotope analysis on the same sample. The technique takes advantage of the easy pyrolysis of Ag2SO4 to SO2, O-2, and Ag metal in a continuous flow system. Because the technique is not quantitative in oxygen (yield similar to 45% for O-2), a calibration is needed. Correction factors of +0.87 and + 0.44 parts per thousand were obtained for delta O-18 and delta O-17, respectively. A technique to convert micromole levels of sulfate in any form to silver sulfate is described. To reach this goal, a solid electrolyte (Nafion membrane) is used in an electrolysis apparatus. Reproducibilities for micromole sample sizes are (1 sigma) 0.5, 0.3, and 0.1 parts per thousand for delta O-18, delta O-17, and Delta O-17, respectively. No memory effects or isotopic exchange during the treatment of the sample is observed. The main advantages of this new method over the existing ones are no fluorinating agent is needed, both oxygen and sulfur isotopes can be measured on the same sample, only very small amounts of sulfate are needed (down to 100 mug (1 mu mol)), it is relatively fast and inexpensive, and the possibility exists to couple this technique to an on-line analysis.