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RATIONALE

The oxygen isotope anomaly, Δ17O, of N2O and nitrate is useful to elucidate nitrogen oxide dynamics. The previously developed method for Δ17O measurement presents difficulty in maintaining optimal conditions of the gold tube for thermal decomposition of N2O to O2 and the Δ17O value is also sample size dependent.

METHODS

Trace amounts (5–40 nmol) of N2O were decomposed quantitatively to O2 in a quartz tube by microwave discharge. The O2 was purified using gas chromatography. Triple oxygen isotopes were measured using isotope ratio mass spectrometry. Each step was connected online and was applied to the analysis of nitrate in precipitation samples collected in Yokohama, Japan.

RESULTS

Precision (1σ) of Δ17O analysis was better than 0.26‰ when more than 20 nmol of N2O with a small Δ17O value (approx. 1‰) was measured. It was better than 0.76‰ when more than 60 nmol of nitrate was converted into N2O using the denitrifier method and then measured on the developed system. The obtained Δ17O values in precipitation samples (14.5–26.4‰) agreed with findings from previous studies.

CONCLUSIONS

A novel on-line analytical method was developed to measure the triple oxygen isotopes of N2O using microwave discharge to decompose N2O. This easy-to-use method is free from conditioning of reaction devices, and is applicable to molecules other than N2O such as NO and NO2. Copyright © 2013 John Wiley & Sons, Ltd.