Analysis of oxygen-17 excess of nitrate and sulfate at sub-micromole levels using the pyrolysis method

RATIONALEThe oxygen-17 excess (O-17) of nitrate and sulfate contains valuable information regarding their atmospheric formation pathways. However, the current pyrolysis method to measure O-17 requires large sample amounts (>4 mu mol for nitrate and >1 mu mol for sulfate). We present a new approach employing a Gas Bench interface which cryofocuses O-2 produced from sample pyrolysis, enabling the analysis of sub-micromole size samples. METHODSSilver nitrate or sulfate at sub-micromole levels in a sample container was thermally decomposed to O-2 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 O-2 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. RESULTSAnalysis of the international nitrate reference material USGS35 (O-17=21.6) within the size range of 300-1000nmol O-2 gave a mean O-17 value of (21.6 +/- 0.69) parts per thousand (mean +/- 1 sigma). Three inter-laboratory calibrated sulfate reference materials, Sulf-, Sulf- and Sulf-epsilon, each within the size range of 180-1000nmol O-2, were analyzed and shown to possess mean O-17 values of (0.9 +/- 0.10) parts per thousand, (2.1 +/- 0.25) parts per thousand and (7.0 +/- 0.63) parts per thousand, respectively. CONCLUSIONSThe analyses of nitrate and sulfate reference materials at sub-micromole levels gave O-17 values consistent with their accepted values. This new approach of employing the Gas Bench to cryofocus O-2 after the pyrolysis of AgNO3 and Ag2SO4 particularly benefits the effort of measuring O-17 in sample types with a low abundance of nitrate and sulfate such as ice cores. Copyright (c) 2013 John Wiley & Sons, Ltd.