Development and verification of a novel isotopic N₂ O measurement technique for discrete static chamber samples using...

N₂ O isotopomers are a useful tool to study soil N cycling processes. The reliability of such measurements requires a consistent set of international N₂ 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 N₂ O in anthropogenic climate change and the pressing need to develop our understanding of soil N cycling and N₂ O emission to mitigate such emissions. Cavity ring-down spectroscopy (CRDS) could potentially overcome resource requirements and technical challenges, making N₂ O isotopomer measurements more feasible and less expensive than previous approaches (e.g., gas chromatography [GC] and isotope ratio mass spectrometry [IRMS]).

A combined laser spectrometer and small sample isotope module (CRDS & SSIM) method enabled N₂ O concentration, δ¹⁵ N^bulk , δ¹⁵ N^α , δ¹⁵ 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 N₂ 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.

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 N₂ O concentration, δ¹⁵ N^bulk , δ¹⁵ N^α , δ¹⁵ 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 N₂ O concentrations exceeded the specified operating range. Normalising CRDS & SSIM δ values to the international isotope-ratio scales using isotopic N₂ 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 δ¹⁵ N^bulk and SP precision was approximately one order of magnitude less than the typical IRMS precision.

CRDS & SSIM is a promising approach that enables N₂ O concentrations and isotope ratios to be measured by CRDS for samples <20 mL. The CRDS & SSIM repeatability makes this approach suitable for N₂ 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.