Stable carbon isotopic analyses of lignin-derived CuO oxidation products by isotope ratio monitoring-gas chromatography-mass spectrometry (irm-GC-MS)

In this paper, we evaluate compound-specific isotope analyses of lignin reaction products through the combined application of alkaline CuO oxidation and isotope ratio monitoring-gas chromatography-mass spectrometry (irm-GC-MS). Analyses of phenol standards as trimethylsilyl ethers/esters by irm-GC-MS yield accurate (±0.5‰) estimates of their isotopic signatures after correcting for added derivative carbon with precisions ≥1‰. CuO oxidation of purified lignin polymers yields reaction products that retain the isotopic composition of their polymeric precursors (within ±1‰). Analyses of a variety of vascular plant tissues reveal important differences in the isotopic compositions of lignin phenols from C3 (average of all COMPOUNDS = −30.4‰ ± 3.9‰) and C4 (−16.9 ± 2.7‰) plants. These differences are apparent in organic matter concentrates from several sediments and can be used to estimate the principal sources of lignin and vascular plant remains in a variety of settings. Finally, comparative analyses of bulk sediment and organic matter concentrate from the same sample show that it is possible to characterize directly the isotopic composition of sedimentary lignins by combined application of alkaline CuO oxidation and irm-GC-MS without the need for prior demineralization.