2.8 Enhanced Raman Spectroscopy of Greenhouse Gases
2.8.3 Enhanced Raman Spectroscopic Analysis of Greenhouse Gases
Fig. 2.19 Combined Raman spectra containing the rovibrational bands of14N2,14N15N,16O2,
18O16O,18O2,16O2,12C16O2,13C16O2, and12C18O16O. Adapted with permission from Knebl et al. (2019). Copyright 2019 American Chemical Society
• The Raman scattering intensity offers perfect linearity with the analyte concentra- tion (Eq.2.10) and allows for robust instrument calibration over a broad concen- tration range from ppm to pure compounds. Trace gases can be quantified on the background of a higher concentrated gas matrix.
• The fast Raman spectroscopic measurement enables online monitoring of temporal changes in gas concentrations during process.
• Stable gas isotopes can be distinguished (due to changes in the reduced mass and thus spectral position, see Fig.2.19) and be used as tracers to follow specific pathways.
• Raman devices can be highly miniaturised for field deployment.
2.8.3 Enhanced Raman Spectroscopic Analysis
2 Methodology for Measuring Greenhouse Gas Emissions … 75
study pathways of the nitrogen cycle (e.g. denitrification, N2fixation) (Keiner et al.
2015a; Kumar et al.2018; Jochum et al.2017).
The RQ value was analysed as an indicator of changes in plant metabolism under drought stress (Fig. 2.20) (Hanf et al. 2015b). It was discovered that pine (Pinus sylvestris) can switch from carbohydrate-dominated respiration to a mixture of substrates during several days of drought stress, but spruce (Picea abies) cannot (Hanf et al.2015b). The onsite analysis of depth profiles of soil gases in the Hainich critical zone exploratory showed that the concentrations of O2and CO2were largely decoupled, and complex processes in previously uncharacterised environments can be studied (Sieburg et al.2017). The ability to monitor the inert tracer sulphur hexaflu- oride (SF6) alongside biogenic gases under consideration allows for thorough online gas leakage correction to avoid under- or overestimation of biological activity such as respiration or photosynthesis (Fig.2.21) (Jochum et al.2015b).
The discriminatory power of Raman spectroscopy was used to monitor several stable gas isotopes simultaneously to investigate the labelling of young poplar trees Fig. 2.20 Example for
monitoring dark respiration rates of a branch of untreated pine. The individual respiration rates (both O2
and CO2) are used to calculate subsequent RQ values (Hanf et al.2015b).
Adapted with permission from (Hanf et al.2015b).
Copyright 2015 Royal Society of Chemistry
Fig. 2.21 Example of an experimentally acquired multigas Raman spectrum, consisting of the biogenic gases O2, CO2, N2, H2, CH4, and the tracer gas SF6. Adapted with permission from Jochum et al. (2015b).
Copyright 2015 American Chemical Society
Fig. 2.22 Raman spectroscopic multigas monitoring of the
denitrification of15N-nitrate byPseudomonas
stutzeri(Keiner et al.2015a).
The concentration courses of
15N2, CO2,15N2O, and the calculated pH value are shown as well as the sum of the nitrogen gases15N2+
15N2O. Adapted with permission from Keiner et al.
(2015a). Copyright 2015 Elsevier
under aphid infestation with13CO2to analyse the possible incorporation of13C in defense compounds (Keiner et al.2014,2015b). A combination of13C-labelling and RQ analysis was applied to investigate the microbial degradation of13C-labelled benzene in soil against the background of the heterotrophic soil respiration (Jochum et al.2015a). By combining13CO2and12CO2as well as18O2and16O2measurements in one setup, it was proposed to use carbon dioxide and oxygen isotopologues to track and disentangle different overlaying processes and to help elucidating the contribu- tions of photosynthesis, photorespiration, and respiration to the net gas exchange of plants (Knebl et al.2019).
The nitrogen evolution was continuously monitored over the stepwise enzymatic denitrification of labelled and unlabeled nitrate byPseudomonas stutzeri(Fig.2.22) (Keiner et al.2015a). The simultaneous quantification of the whole gas phase also enabled the contactless and sterile online acquisition of the pH changes in the P.
stutzericulture by the stoichiometry of the redox reactions during denitrification and the CO2-bicarbonate equilibrium. Continuous pH-monitoring–without the need to insert an electrode into a sterile solution–elucidated an increase in the slope of the pH value coinciding with an accumulation of nitrite, which in turn led to a temporary accumulation of N2O, due to an inhibition of N2O reductase (Keiner et al.2015a). The gas quantification was complemented with the analysis of nitrate and nitrite concen- trations for the online monitoring of the total nitrogen element budget (Fig. 2.23) (Keiner et al. 2015a). In an investigation of the thiosulfate- and hydrogen-driven autotrophic denitrification by a microbial consortium enriched from groundwater of an oligotrophic limestone aquifer, the turnover reactions of electron donors (thiosul- fate and H2) were traced, as well as electron acceptor (nitrate), gaseous intermedi- ates, and end products (15N2,15N2O, CO2, H2,14N2, and O2) in the headspace, using Raman gas spectroscopy (Kumar et al.2018). N2 production and H2 consumption rates were calculated under denitrifying conditions and followed the electron donor usage of the bacterial consortium. Recently, the biological nitrogen fixation of a Medicago sativa–Rhizobiumconsortium was, for the first time, directly investigated at natural background and without a proxy or isotopic labelling, by continuously
2 Methodology for Measuring Greenhouse Gas Emissions … 77
Fig. 2.23 Concentration courses of nitrate, nitrite, nitrous oxide, and dinitrogen during the succes- sive reduction NO3−→NO2−→NO→N2O→N2(Keiner et al.2015a). Continuous Raman gas concentrations of15N2,15N2O, and CO2are represented by solid lines. The total nitrogen balance from all nitrogen components was calculated and is given as grey line (Ntot) (Keiner et al.
2015a). Adapted with permission from Keiner et al. (2015a). Copyright 2015 Elsevier
analysing the amount of atmospheric N2in static environmental chambers (Jochum et al.2017).
Enhanced Raman gas spectroscopy combines the unmatched analytical prowess of Raman spectroscopy with the enhancement of small signals through the sophisticated use of optical cavities (CERS) and hollow-core optical fibres (FERS). Enhanced Raman spectroscopy is a powerful technique for simultaneous multigas analysis, including N2, N2O, O2, H2, CH4, CO2, and stable isotopes (13C,15N, and18O). This high selectivity enables the study of complex gas exchange processes, including pathways of the nitrogen cycle.