Coupling of thermal analysis with quadrupole mass spectrometry and isotope ratio mass spectrometry for simultaneous determination of evolved gases and their carbon isotopic composition

By coupling an isotope ratio mass spectrometer (IRMS) and a quadrupole mass spectrometer (QMS) to a thermal analysis system, we have been able to continuously measure δ13C and identify the evolved gases during the thermal decomposition of a range of lignocellulosic materials derived from soils and/or plant tissue. Here we describe the application of this approach to characterise wheat straw during fungal degradation by the oyster mushroom Pleurotus ostreatus. For samples of straw collected over 63 days, TG–DSC showed progressively decreasing contributions of cellulose (300–350 °C) and lignin (400–450 °C) with concomitant increases in the extents of aromatisation and polycondensation (450–500 °C). TG–DSC–QMS analysis showed changes with time in the evolution of different C and N species. H2O and CO2 were the dominant evolved gases observed during the combustion of undegraded and fungally decomposed wheat straw. The relative ion intensities of the gas species NO (m/z 30) and CO2 (m/z 44) observed at 350 °C increased at 530 °C with increasing wheat straw decomposition. This suggests that fungal degradation results in increasing proportions of C and N incorporated within recalcitrant structures.IRMS analysis showed that fungal decomposition of wheat straw involves homogenization of an initially heterogeneous δ13C signal with increasing extent of fungal decay. Undegraded wheat straw has two components: cellulosic material with δ13C of −23.8‰ and lignin with δ13C of −26.1‰. After 9 weeks fungal degradation, δ13C values converged to give −21.3 ± 0.8‰. This is consistent with preferential loss during degradation of lignin that is depleted in 13C compared to cellulose, and accumulation of 13C-rich components within the degraded straw.