Appraisal of analytical pyrolysis for direct compound specific isotope analysis (δ2H, δ13C Py-CSIA) of wood.

Abstract

Changes in climatic and environmental conditions can affect both, plant chemical and isotope composition. Most studies use bulk isotope values, which represent a weighed mean average of the different plant compounds. An isotopic characterization of individual biogeochemical compounds is needed in order to differentiate the isotopic composition of the main plant components i.e. polysaccharides, lignin, polypeptides, lipids and waxes, etc. Here, we investigate the link between molecular and isotope composition of bark from three Eucalyptus species (E. grandis, E. dunni and a hybrid E. grandis x E. dunni) using Py-CSIA. Bulk samples and lignin extracted were analysed by Py-CSIA; this technique, although not widely used, has been previously developed and applied, with different variants, to characterize plant products. Isotopic measurements were normalized to the accepted scale using a n-alkanes mixture, type B, containing 15 dissolved n-alkanes (C-16 to C-30) with increasing concentrations along three pentads (Indiana University, Stable Isotope Laboratory). Values fitted well along a straight line with a linear regression R2 no slower than 0.999. Although pyrolysis may induce thermal cracking at high pyrolysis temperature (400ºC), significant changes in isotopic composition were not observed in light long chain molecules present in the standard solution nor the wood samples. Our results support the idea Py-CSIA can reveal major differences between compounds from different biogenic origin: polysaccharides, lignin-derived units and fatty acids. Despite that wood from different origins provide very similar Py-GC/MS pattern, Py-CSIA can make the difference giving additional valuable information.