232 CHIMIA 2019, 73, No. 4 Laureates: Junior Prizes of the sCs faLL Meeting 2018 doi:10.2533/chimia.2019.232 Chimia 73 (2019) 232–238 © Swiss Chemical Society Quantifying Isotopic Signatures of N O Using Quantum Cascade Laser 2 Absorption Spectroscopy Kristýna Kantnerováa§, Béla Tuzsona, Lukas Emmeneggera, Stefano M. Bernasconib and Joachim Mohna* §SCS-Metrohm Award for best oral presentation in Analytical Sciences Abstract: Nitrous oxide, N O, is the environmentally most relevant constituent of the biogeochemical nitrogen 2 cycle. Human activities, e.g. the agricultural use of mineral fertilizers, accelerate nitrogen transformations, leading to higher emissions of this strong greenhouse gas. Investigating the stable isotopic composition of N O provides 2 a better understanding of formation mechanisms to disentangle its variable source and sink processes. Mid- infrared (mid-IR) laser spectroscopy is a highly attractive technique to analyze N O isotopocules based on their 2 specific ro-vibrational absorption characteristics. Specifically, quantum cascade laser absorption spectroscopy (QCLAS) in combination with preconcentration has shown to be powerful for simultaneous and high-precision analysis of the main N O isotopocules. Recently, in the scope of my PhD project, we have been advancing this 2 analytical technique for the analysis of the very rare doubly substituted N O isotopic species 15N14N18O, 14N15N18O, 2 and 15N15N16O, also known as clumped isotopes. Currently, we are investigating the potential of these novel iso- topic tracers to track the complex N O production and consumption pathways. Improved understanding of the 2 nitrogen cycle will be a major step towards N O emission reduction. 2 Keywords: Biogeochemical nitrogen cycle · Clumped isotopes · Greenhouse gas · Laser spectroscopy · Nitrous oxide Kristýna Kantnerová was born in the Czech billion, 10-9 moles of trace gas per mole of dry air)in the preindus- Republic in 1991. In 2016, she obtained her trial era (before 1750)[4] to 329.9 ± 0.1 ppb in 2017,[5] and the in- Master’s degree in Analytical Chemistry crease steadily continues. Developing mitigation options for this and Quality Engineering from University trace gas requires a profound understanding of its biogeochemical of Chemistry and Technology, Prague with cycle and control parameters. honors. She conducted her Master’s thesis Although the global N O budget and the main source areas – 2 in the Molecular Electrochemistry group soils under natural vegetation, agricultural land use, and oceans – at the J. Heyrovsky Institute of Physical are well known, fluxes exhibit very high spatial and temporal vari- Chemistry. Since 2016, she is in the group ability, which makes predictions and mitigation very challenging. of Dr. Joachim Mohn at Empa Dübendorf In addition, the specific biogeochemical production pathways of and in the group of Prof. Stefano Bernasconi at ETH Zürich. N O display a high complexity, with many potential microbial and 2 Her PhD work is focused on developing a quantum cascade laser abiotic transformations. A detailed description of processes known based analytical technique for the analysis of the doubly substi- to lead to N O formation or consumption and of challenges in their 2 tuted (‘clumped’) isotopic species of nitrous oxide and exploring characterization can be found in a number of review articles.[6,7] their usage. Currently, the most important pathways in soils are considered to be: bacterial heterotrophic denitrification and nitrifier-denitrifi- 1. Introduction cation, nitrification, and chemodenitrification. However, as some Mitigating anthropogenic emissions of greenhouse gases of these processes start from the same substrates (e.g. nitrates or (GHG) into the Earth’s atmosphere is one of the biggest chal- nitrites), and even exhibit the same reaction intermediates, quanti- lenges that the scientific community and the society in general fication of their individual contributions is still not possible. is currently facing. Emissions of nitrous oxide, N O, contribute In this context, natural abundance stable isotope signatures 2 to global warming by 6%[1] due to its strong global warming po- can be used as fingerprints to trace transformations of sub- tential, which is 300 times higher than that of CO .[2] In addition, stances present in the environment. Likewise, the stable iso- 2 N O is the most important anthropogenically emitted substance topic composition of GHG, such as N O, provides important 2 2 responsible for the depletion of stratospheric ozone.[3] Its atmo- information about source and sink processes.[8,9] The natural spheric abundance has increased from around 270 ppb (parts-per- variations in stable isotope abundances are due to the fact that *Correspondence: Dr. J. Mohna E-mail: [email protected] aLaboratory for Air Pollution / Environmental Technology, Empa – Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf; bGeological Institute, Department of Earth Sciences, ETH Zürich, Sonneggstrasse 5, CH-8092 Zürich Laureates: Junior Prizes of the sCs faLL Meeting 2018 CHIMIA 2019, 73, No. 4 233 in many biotic and abiotic processes, the reaction rates dif- fer between isotopic species, e.g. reduction of 15NO – versus 2 14NO –, leading to a so-called isotopic fractionation.[10] As the 2 isotopic fractionation is distinct for certain reaction pathways, this information can be used to differentiate source processes from each other. The N O molecule (N=N=O) involves in total 2 12 distinct ‘isotopically substituted molecules’ or ‘isotopoc- ules’, representing all possible combinations of the nitrogen isotopes 14N and 15N and the oxygen isotopes 16O, 17O, and 18O. The term ‘isotopocule’ is an umbrella term of ‘isopologue’ and ‘isotopomer’ (isotopic isomer).A particular feature for the asymmetric N O molecule is that the central (α, N*NO) and the 2 end (β, *NNO) N atom are distinguishable, which has particular potential for source attribution as discussed below. A list of all N O isotopocules together with their average abundance and 2 molecular mass can be found in Table 1. The abundance of iso- topocules is usually reported in the δ notation and the deviation from a reference standard expressed in ‰ values: = −1 (1) = − (2) Fig. 1. Dual isotopocule mapping approach with isotopic signatures of several biotic and abiotic sources of N O (adapted from refs [13,15]). 2 where X denotes e.g. 15Nα, 15Nβ, 15Nbulk (for average 15N content), R is the ratio between the amount fraction of the rare isotopic spe- not only by the reaction through which it is produced but also cies and the amount fraction of most abundant species 14N14N16O, by the composition of the substrate. In contrast, the difference e.g. R(15N14N16O/14N14N16O) = x(15N14N16O)/x(14N14N16O), for the between 15N substitution in α and β position, SP, is independent sample and the reference gas.[11] The difference between 15N sub- from the isotopic composition of the precursor and therefore of stitution in α and β position, defined according to Eqn. (2), is particular interest to differentiate biogeochemical reaction path- denoted as site preference (SP). Reference standards provide the ways in the nitrogen cycle.[13] Generally, there is a stronger affin- link to the international scale for isotope ratios, which is atmo- ity of 15N towards the central than towards the end N position in spheric N (Air-N ) for nitrogen isotopes and Vienna Standard the N O molecule, due to a lower zero-point energy of 14N15N1xO 2 2 2 Mean Ocean Water (VSMOW) for oxygen isotopes.[12] as compared to 15N14N1xO.[14] While isotopocule mapping has In the dual N O isotopocule mapping approach, isotope sig- shown potential to study individual reactions, in particular if 2 natures are combined to allocate source processes (Fig. 1). The additional supporting parameters or modelling approaches are bulk isotopic composition of N O (δ15Nbulk, δ18O) is influenced available, overlapping isotopocule signatures often prevent a 2 quantitative assignment. Theoretical predictions indicate that multiply substituted, so- Table 1. The N O isotopocules, their natural average abundance, and 2 called ‘clumped’ isotopocules, provide additional constraints on molecular masses. For the multiply substituted isotopic species, random reaction pathways. Under equilibrium conditions, clumping of (stochastic) distribution of isotopes among all isotopocules is assumed. heavier isotopes in a chemical bond is thermodynamically favor- able because ‘heavier’ chemical bonds are more stable.[16] The isotopocule relative mass m(X)/u extent of this phenomenon is temperature-dependent, meaning abundance that the abundance of ‘clumped isotopes’ in a sample bears in- 14N14N16O9.903·10–1 44.0011 formation about the equilibrium formation temperature and the reversibility of a process.[17–19] In carbonate paleothermometry, 14N15N16O3.641·10–3 44.9981 this formalism is used to study past climate on Earth.[20–23] Under non-equilibrium kinetic conditions, the clumped signature pro- 15 14 16 –3 N N O3.641·10 44.9981 vides information on the reversibility of a reaction and thus ad- 14N14N18O1.986·10–3 46.0053 ditional dimensionality for interpretation.[24] Apart from their use in source studies, clumped N O isotopes have been suggested as 2 14N14N17O3.762·10–4 45.0053 powerful tracers for the main sink process, stratospheric photoly- sis.[25] Concentrations of clumped isotopic species are given in 15 15 16 –5 N N O1.339·10 45.9951 the ∆ notation: 14N15N18O7.300·10–6 47.0023 15 14 18 –6 N N O7.300·10 47.0023 ∆556 = −1 − −1 − −1 (3) 14N15N17O1.383·10–6 46.0023 15N14N17O1.383·10–6 46.0023 where R556, R456, R546 are the abundance ratios of the isoto- pocules 15N15N16O, 14N15N16O, and 15N14N16O relative to 14N14N16O, 15N15N18O2.684·10–8 47.9994 in the sample and in a reference gas with stochastic (random) isotopic composition. SP18 is defined as the difference between 15N15N17O5.085·10–9 46.9993 ∆458 and ∆548. 234 CHIMIA 2019, 73, No. 4 Laureates: Junior Prizes of the sCs faLL Meeting 2018 Traditionally, isotopocules of N O are analyzed by iso- ity for the low abundant N O isotopocules (e.g.