Development and verification of a novel isotopic N ₂ O measurement technique for discrete static chamber samples using cavity ring‐down spectroscopy

Rationale N 2 O isotopomers are a useful tool to study soil N cycling processes. The reliability of such measurements requires a consistent set of international N 2 O isotope reference materials to improve inter‐laboratory and inter‐instrument comparability and avoid reporting inaccurate results. All these are the more important given the role of N 2 O in anthropogenic climate change and the pressing need to develop our understanding of soil N cycling and N 2 O emission to mitigate such emissions. Cavity ring‐down spectroscopy (CRDS) could potentially overcome resource requirements and technical challenges, making N 2 O isotopomer measurements more feasible and less expensive than previous approaches (e.g., gas chromatography [GC] and isotope ratio mass spectrometry [IRMS]). Methods A combined laser spectrometer and small sample isotope module (CRDS & SSIM) method enabled N 2 O concentration, δ 15 N bulk , δ 15 N α , δ 15 N β and site preference (SP) measurements of sample volumes <20 mL, such as static chamber samples. Sample dilution and isotopic mixing as well as N 2 O concentration dependence were corrected numerically. A two‐point calibration procedure normalised δ values to the international isotope‐ratio scales. The CRDS & SSIM repeatability was determined using a reference gas (Ref Gas). CRDS & SSIM concentration measurements were compared with those obtained by GC, and the isotope ratio measurements from two different mass spectrometers were compared. Results The repeatability (mean ± 1σ; n = 10) of the CRDS & SSIM measurements of the Ref Gas was 710.64 ppb (± 8.64), 2.82‰ (± 0.91), 5.41‰ (± 2.00), 0.23‰ (± 0.22) and 5.18‰ (± 2.18) for N 2 O concentration, δ 15 N bulk , δ 15 N α , δ 15 N β and SP, respectively. The CRDS & SSIM concentration measurements were strongly correlated with GC ( r = 0.99), and they were more precise than those obtained using GC except when the N 2 O concentrations exceeded the specified operating range. Normalising CRDS & SSIM δ values to the international isotope‐ratio scales using isotopic N 2 O standards (AK1 and Mix1) produced accurate results when the samples were bracketed within the range of the δ values of the standards. The CRDS & SSIM δ 15 N bulk and SP precision was approximately one order of magnitude less than the typical IRMS precision. Conclusions CRDS & SSIM is a promising approach that enables N 2 O concentrations and isotope ratios to be measured by CRDS for samples <20 mL. The CRDS & SSIM repeatability makes this approach suitable for N 2 O “isotopomer mapping” to distinguish dominant source pathways, such as nitrification and denitrification, and requires less extensive lab resources than the traditionally used GC/IRMS. Current study limitations highlighted potential improvements for future users of this approach to consider, such as automation and physical removal of interfering trace gases before sample analysis.