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In a series of anaerobic batch experiments, the stable carbon isotopes, δ13CCH?4? and δ13CCO?2?, were measured in biogas produced from various sources (maize, cellulose, inoculum) to identify the degradation kinetics and specific methanogenic pathways. Isotopic analysis was performed using a new absorption laser spectrometer in addition to conventional MS. A comparison of the isotopic evolution shows large isotope dynamics for maize and cellulose, indicating a temporal change in degradation pathways and/or a change in the relative contribution from different carbon fractions within the substrate. Further batch experiments with isotopically labelled acetate (either 13CH3CO2Na or CH 313CO2Na) were carried out to study the degradation of acetate in inoculum systematically. The results suggest that the acetate is completely oxidized into CO2 which in turn is partly reduced to CH4. Furthermore, the distinct isotopic signature CH4 and CO2 (for acetate‐methyl labelling as well as for acetate‐carboxy labelling) indicate that only a minor part of the produced methane derives from acetate. A substantial fraction of methane may have been produced at an earlier stage of the reaction chain or by other potential methane precursors such as formate or methanol.

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Abstract. Biologically produced molecular hydrogen (H 2 )is characterised by a very strong depletion in deuterium. Although the biological source to the atmosphere is small compared to photochemical or combustion sources, it makes an important contribution to the global isotope budget of H 2 . Large uncertainties exist in the quantification of the individual production and degradation processes that contribute to the atmospheric budget, and isotope measurements are a tool to distinguish the contributions from the different sources. Measurements of δD from the various H 2 sources are scarce and for biologically produced H 2 only very few measurements exist.Here the first systematic study of the isotopic composition of biologically produced H 2 is presented. In a first set of experiments, we investigated δD of H 2 produced in a biogas plant, covering different treatments of biogas production. In a second set of experiments, we investigated pure cultures of several H 2 producing microorganisms such as bacteria or green algae. A Keeling plot analysis provides a robust overall source signature of δD = −712 ‰ (±13 ‰) for the samples from the biogas reactor (at 38 • C, δD H 2 O = +73.4 ‰), with a fractionation constant ε H 2 -H 2 O of −689 ‰ (±20 ‰) between H 2 and the water. The five experiments using pure culture samples from different microorganisms give a mean source signature of δD = −728 ‰ (±28 ‰), and a fractionation constant ε H 2 -H 2 O of −711 ‰ (±34 ‰) between H 2 and the water. The results confirm the massive deuterium depletion of biologically produced H 2 as was predicted by the calculation of the thermodynamic fractionation factors for hydrogen exchange between H 2 and water vapour. Systematic errors in the isotope scale are difficult to assess in the absence of international standards for δD of H 2 .As expected for a thermodynamic equilibrium, the fractionation factor is temperature dependent, but largely independent of the substrates used and the H 2 production conditions. The equilibrium fractionation coefficient is positively correlated with temperature and we measured a rate of change of 2.3 ‰ / • C between 45 • C and 60 • C, which is in general agreement with the theoretical prediction of 1.4 ‰ / • C.Our best experimental estimate for ε H 2 -H 2 O at a temperature of 20 • C is −731 ‰ (±20 ‰) for biologically produced H 2 . This value is close to the predicted value of −722 ‰, and we suggest using these values in future global H 2 isotope budget calculations and models with adjusting to regional temperatures for calculating δD values.

Methane production by anaerobic digestion of biomass has recently become more attractive because of its potential for renewable energy production. Analytical tools are needed to study and optimize the ongoing processes in biogas reactors. It is considered that optical methods providing continuous measurements at high temporal resolution of carbon isotope ratios of methane (delta(13)C(CH4)) might be of great help for this purpose. In this study we have tested near-infrared laser optical spectrometry and compared it with conventional continuous-flow isotope ratio mass spectrometry (CF-IRMS) using several methane carbon isotope standards and a large number of biogas samples from batch anaerobic reactors. Results from measurements on these samples were used to determine and compare the precision of the two techniques and to quantify for systematic offsets. With pure standards analytical precision of measurements for delta(13)C(CH4) was found to be in the range of 0.33 and 0.48 per thousand, and 0.09 and 0.27 per thousand for the optical method and CF-IRMS, respectively. Biogas samples showed an average mean deviation of delta(13)C(CH4) of 0.38 per thousand and 0.08 per thousand for the optical method and CF-IRMS, respectively. Although the tested laser optical spectrometer showed a dependence of delta(13)C(CH4) on CH(4) mixing ratio in the range of 500 to 8000 ppm this could be easily corrected. After correction, the delta(13)C(CH4) values usually varied within 0.7 per thousand from those measured by conventional CF-IRMS and thus results from both methods agreed within the given analytical uncertainties. Although the precision of the conventional CF-IRMS is higher than the tested optical system, both instruments were well within the acceptable delta(13)C(CH4) precision required for biogas methane measurements. The advantages of the optical system are its simplicity of operation, speed of analysis, good precision, reduced costs in comparison to IRMS, and the potential for field applications.