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

Abstract

Nitrous oxide (N2 O) 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 N2 O concentration and isotopic/isotopomeric analyses has potential advantages, which include high analytical specificity, low sample size requirement and reduced cost. An autosampler with a purge-and-trap module is coupled to a cavity ring-down spectrometer to achieve automated and high-throughput measurements of N2 O concentrations, N2 O isotope ratios (δ15 Nbulk and δ18 O values) and position-specific isotopomer ratios (δ15 Nα and δ15 Nβ values). The system provides accuracy and precision similar to those for measurements made by traditional isotope ratio mass spectrometry (IRMS) techniques. The sample sizes required were 0.01-1.1 nmol-N2 O. Measurements of four N2 O isotopic/isotopomeric references were cross-calibrated with those obtained by IRMS. With a sample size of 0.50 nmol-N2 O, the measurement precision (1σ) for δ15 Nα , δ15 Nβ , δ15 Nbulk and δ18 O 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 N2 O concentrations, isotope/isotopomer ratios and production rates in seawater. 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 N2 O 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 N2 O as the analytical proxy.