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

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 (δ¹⁵N_bulk and δ¹⁸O values) and position-specific isotopomer ratios (δ¹⁵N_α and δ¹⁵N_β 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 (1σ) for δ¹⁵N_α, δ¹⁵N_β, δ¹⁵N_bulk and δ¹⁸O values was 0.61, 0.33, 0.41 and 0.43‰, 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.