Analysis of oxygen-17 Excess of Nitrate and Sulfate at Sub-Micromole Levels Using the Pyrolysis Method

Rapid Commun Mass Spectrom. 2013 Nov 15;27(21):2411-9. doi: 10.1002/rcm.6703.

Abstract

Rationale: The oxygen-17 excess (Δ(17)O) of nitrate and sulfate contains valuable information regarding their atmospheric formation pathways. However, the current pyrolysis method to measure Δ(17)O requires large sample amounts (>4 µmol for nitrate and >1 µmol for sulfate). We present a new approach employing a Gas Bench interface which cryofocuses O2 produced from sample pyrolysis, enabling the analysis of sub-micromole size samples.

Methods: Silver nitrate or sulfate at sub-micromole levels in a sample container was thermally decomposed to O2 and byproducts in a modified Temperature Conversion/Elemental Analyzer (TC/EA). Byproducts (mainly NO2 for silver nitrate and SO2 for silver sulfate) were removed in a liquid nitrogen trap and the sample O2 was carried by ultra-pure helium (He) gas to a Gas Bench II interface where it was cryofocused prior to entering an isotope ratio mass spectrometer.

Results: Analysis of the international nitrate reference material USGS35 (Δ(17)O = 21.6‰) within the size range of 300-1000 nmol O2 gave a mean Δ(17)O value of (21.6 ± 0.69) ‰ (mean ±1σ). Three inter-laboratory calibrated sulfate reference materials, Sulf-α, Sulf-β and Sulf-ε, each within the size range of 180-1000 nmol O2, were analyzed and shown to possess mean Δ(17)O values of (0.9 ± 0.10)‰, (2.1 ± 0.25)‰ and (7.0 ± 0.63)‰, respectively.

Conclusions: The analyses of nitrate and sulfate reference materials at sub-micromole levels gave Δ(17)O values consistent with their accepted values. This new approach of employing the Gas Bench to cryofocus O2 after the pyrolysis of AgNO3 and Ag2SO4 particularly benefits the effort of measuring Δ(17)O in sample types with a low abundance of nitrate and sulfate such as ice cores.