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An automated, laser-based measurement system for nitrous oxide isotope and isotopomer ratios at nanomolar levels.

RAPID COMMUNICATIONS IN MASS SPECTROMETRY(2019)

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Abstract
Rationale Nitrous oxide (N2O) is an atmospheric trace gas regulating Earth's climate, and is a key intermediate of many nitrogen cycling processes in aquatic ecosystems. Laser-based technology for N2O concentration and isotopic/isotopomeric analyses has potential advantages, which include high analytical specificity, low sample size requirement and reduced cost. Methods An autosampler with a purge-and-trap module is coupled to a cavity ring-down spectrometer to achieve automated and high-throughput measurements of N2O concentrations, N2O isotope ratios (delta N-15(bulk) and delta O-18 values) and position-specific isotopomer ratios (delta N-15(alpha) and delta N-15(beta) values). The system provides accuracy and precision similar to those for measurements made by traditional isotope ratio mass spectrometry (IRMS) techniques. Results The sample sizes required were 0.01-1.1 nmol-N2O. Measurements of four N2O isotopic/isotopomeric references were cross-calibrated with those obtained by IRMS. With a sample size of 0.50 nmol-N2O, the measurement precision (1 sigma) for delta N-15(alpha), delta N-15(beta), delta N-15(bulk) and delta O-18 values was 0.61, 0.33, 0.41 and 0.43 parts per thousand, respectively. Correction schemes were developed for sample size-dependent isotopic/isotopomeric deviations. The instrumental system demonstrated consistent measurements of dissolved N2O concentrations, isotope/isotopomer ratios and production rates in seawater. Conclusions The coupling of an autosampler with a purge-and-trap module to a cavity ring-down spectrometer not only significantly reduces sample size requirements, but also offers comprehensive investigation of N2O production pathways by the measurement of natural abundance and tracer level isotopes and isotopomers. Furthermore, the system can perform isotopic analyses of dissolved and solid nitrogen-containing samples using N2O as the analytical proxy.
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Key words
nitrous oxide isotope,isotopomer ratios,nitrous oxide,nanomolar levels
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