Analysis of clumped isotopes in nitrous oxide by laser spectroscopy: method development and first applications
Nitrous oxide (N2O) is one of the most important greenhouse and ozone-depleting gases. Mitigation of N2O emissions is, however, challenging since its source and sink processes have not been well-understood yet. This work presents a new analytical technique for doubly isotopically substituted molecules (isotopocules) of N2O based on quantum cascade laser absorption spectroscopy (QCLAS). The so called “clumped isotopes” are expected to be new tracers for the characterization of the global N2O budget and its biogeochemical cycle.
The analytical setup is a combination of a QCLAS instrument and an automated gas inlet system. The first focus was to set up and optimize the technique for simultaneous analysis of both clumped and singly substituted species (14N15N18O,15N14N18O, 15N15N16O, and 14N15N16O, 15N14N16O, 14N14N17O, 14N14N18O including the unsubstituted species 14N14N16O). Based on simulated absorption spectra, spectral regions for two laser sources were carefully selected. Three pure N2O gases were synthesized to test the simulated spectral positions of the clumped species, each gas consisting mainly of one of the three clumped species.
In the second part, a new calibration scheme for quantification of the clumped species was established, using a combination of the thermal equilibration of a working standard N2O gas and its high-accuracy gravimetric mixtures. The developed technique was successfully tested for the singly substituted isotopocules against another QCLAS method that uses an established calibration approach. For the clumped species, our QCLAS technique was validated against recently developed high-resolution isotope ratio mass spectrometry (IRMS). This validation revealed clear advantages of the spectroscopic method, especially in terms of sample amount, analysis time, and the measurement precision, repeatability, and accuracy.
The first application of the method was a study on isotopic signatures of N2O produced by denitrifying bacteria Pseudomonas aureofaciens, as denitrification is one of the biggest natural sources of N2O. In the second application, N2O was gradually photolyzed by UV light in a custom-made photoreactor at two wavelengths to simulate the stratospheric N2O photolysis, the most important natural N2O sink.
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