Stable carbon isotope analyses of nanogram quantities of particulate organic carbon (pollen) with laser ablation nano combustion gas chromatography/isotope ratio mass spectrometry

Abstract

Rationale: Analyses of stable carbon isotope ratios (δ13C values) of organic and inorganic matter remains have been instrumental for much of our understanding of present and past environmental and biological processes. Until recently, the analytical window of such analyses has been limited to samples containing at least several μg of carbon. Methods: Here we present a setup combining laser ablation, nano combustion gas chromatography and isotope ratio mass spectrometry (LA/nC/GC/IRMS). A deep UV (193 nm) laser is used for optimal fragmentation of organic matter with minimum fractionation effects and an exceptionally small ablation chamber and combustion oven are used to reduce the minimum sample mass requirement compared with previous studies. Results: Analyses of the international IAEA CH-7 polyethylene standard show optimal accuracy, and precision better than 0.5‰, when measuring at least 42 ng C. Application to untreated modern Eucalyptus globulus (C3 plant) and Zea mays (C4 plant) pollen grains shows a ~ 16‰ offset between these species. Within each single Z. mays pollen grain, replicate analyses show almost identical δ13C values. Conclusions: Isotopic offsets between individual pollen grains exceed analytical uncertainties, therefore probably reflecting interspecimen variability of ~0.5–0.9‰. These promising results set the stage for investigating both δ13C values and natural carbon isotopic variability between single specimens of a single population of all kinds of organic particles yielding tens of nanograms of carbon.