An approach for calibrating laser‐based N₂O isotopic analyzers for soil biogeochemistry research

Abstract

RationaleTechnological advances have motivated researchers to transition from traditional gas chromatography/isotope ratio mass spectrometry to rapid, high‐throughput, laser‐based instrumentation for N2O isotopic research. However, calibrating laser‐based instruments to yield accurate and precise isotope ratios has been an ongoing challenge. To streamline the N2O isotope research pipeline, we developed the calibration protocol for laser‐based analyzers described here. While our approach is targeted at laboratory soil incubations, we anticipate that it will be broadly applicable for diverse types of stable isotope research.MethodsWe prepared standards diluted from USGS52 and from a commercial cylinder to develop a calibration curve spanning from 0.3 to 300 ppm N2O. To calibrate over this broad range, we binned each isotopocule (N2O, N15NO, 15NNO, and NN18O) into low, medium, and high concentration ranges and then used mathematically similar polynomial functions to calibrate the isotopocules within each concentration range. We also assessed the temporal stability of the instrument and the capacity for our calibration approach to work with isotopically enriched gas samples.ResultsOur calibration approach yielded generally accurate and precise data when isotopocules were calibrated in concentration ranges, and the measurements appeared to be temporally stable. For all isotopocules at natural abundance, the residual percentage error was smallest in the medium N2O range. There was more noise in the corrected isotopomers and isotopologue at natural abundance in samples with the lowest and highest N2O concentrations. Corrected isotopomer results from isotopically enriched samples were very precise.ConclusionsDeveloping our calibration strategy involved learning several key lessons: (1) calibrate isotopocules in distinct concentration ranges, (2) use mathematically similar models to calibrate the isotopocules in each range, (3) calibrated N2O concentrations and δ values tend to be most accurate and precise in the medium N2O range, and (4) we encourage users to take advantage of isotopic enrichment to capitalize on laser‐based instrument strengths.

Using isotope pool dilution to understand how organic carbon additions affect N2O consumption in diverse soils

“…We removed CO 2 , water vapor and volatile organics from the sample air using a Nafion–carbosorb–silica gel scrubbing system. Removing these gases minimizes the optical peak‐broadening effects inherent to laser‐based analyzers (Stuchiner et al, 2020).…”

“…These values were used to calculate average N 2 O and isotopomer concentrations. All reported concentrations were then corrected against calibrations using the model and approach described in Stuchiner et al (2020).…”

Characterizing the Importance of Denitrification for N2O Production in Soils Using Natural Abundance and Isotopic Labeling Techniques

“…Across all soil saturations, HNHW and the subalpine soil tended to have the most enriched SP values, and HNLW, LNHW, and LNLW soils reported lower SP values (Figure 3). In some cases n = 1 or 2 replicates per treatment as a consequence of instrument errors leading to anamolous SP values (Stuchiner et al., 2020). All SP n ‐values used for analysis are summarized in Table S2 in Supporting Information .…”

“…We previously described the instrumental determination of N 2 O concentration and isotopic composition (Stuchiner et al., 2020). Briefly, our analyzer measures concentrations of N 2 O ( 14 N 14 N 16 O) and all its isotopomers ( 14 N 15 N 16 O, 15 N 14 N 16 O, and 14 N 14 N 18 O) using cavity enhanced laser absorption spectroscopy (Los Gatos Research N 2 O Isotopic Analyzer model 914–0027; ABB‐Los Gatos Research, Mountain View, CA, USA) in a continuous flow‐through system without pre‐concentration of the incoming gas.…”

“…This measure is taken to minimize the optical peak-broadening effects inherent to laser-based analyzers, as these effects can decrease the accuracy of reported N 2 O concentrations. For further details, see Stuchiner et al (2020).…”