Using GC-MS/Combustion/IRMS to determine the 13C/12C ratios of individual hydrocarbons produced from the combustion of biomass materials -: application to biomass burning

Simultaneous mass spectral detection and stable carbon isotope analysis was performed on individual indigenous n-alkanes isolated from single C-4 and C-3 plant species and on a series of aliphatic and polycyclic aromatic hydrocarbons (PAH) produced from the combustion of these same biomass materials. The analysis technique used a combined gas chromatograph-mass spectrometer/combustion/isotope ratio mass spectrometer (GC-MS/C/IRMS). Precision (2 sigma) for replicate measurements of individual compounds in standard solutions using this novel configuration ranged between 0.2 and 0.5 parts per thousand for n-alkanes and 0.3 and 0.8 parts per thousand for PAH. Accuracy of the n-alkane measurements ranged between 0.1 and 0.4 parts per thousand and that of the PAH measurements ranged between 0.2 and 0.9 parts per thousand. Replicate GC-MS/C/IRMS measurements on the combustion-derived n-alkene/alkane pairs were performed to within a precision of between 0.1 and 1.1 parts per thousand and the precision for the combustion PAH was similar to the standard PAH solution. No notable isotopic effects were observed when altering the temperature of the combustion process from 900 to 700 degrees C, or as a result of the individual n-alkenes/alkanes partitioning between the gaseous and condensate fractions. Combustion-derived n-alkenes/alkanes ranged from C-11 to C-31, and the C-4-derived n-alkenes/alkanes were approx. 8 parts per thousand more enriched in C-13 than the C-3-derived compounds. Both the C-4 and C-3-derived n-alkeneslalkanes (C-20-C-30) were isotopically similar to the indigenous n-alkanes and were 2-3 parts per thousand more depleted in C-13 than the lower mol. wt (C-11-C-19) n-alkenes/alkanes, suggesting an independent origin for the lower mol. wt compounds. Combustion-generated C-4 and C-3-derived 2-, 3-, and 4-ring PAH were also isotopically distinct (Delta delta = 10 parts per thousand). Unlike the n-alkenes/alkanes, no compound-to-compound variations were observed between the low and high mel. wt PAH. This study demonstrates that the isotopic composition of original plant biomass material is mainly preserved in the aliphatic hydrocarbons and PAH generated by its combustion. Consequently, analyses of these compounds in sediments impacted by fire occurrences may provide useful information about paleo-fire activity that may help elucidate the impact biomass burning may have had and could have on climate-biosphere interactions. (C) 1997 Elsevier Science Ltd. All rights reserved.