Spatial Microanalysis of Natural C/C Abundance in Environmental Samples Using Laser Ablation-Isotope Ratio Mass Spectrometry.

The stable C/C isotope composition usually varies among different organic materials due to isotope fractionation during biochemical synthesis and degradation processes. Here, we introduce a novel laser ablation-isotope ratio mass spectrometry (LA-IRMS) methodology that allows highly resolved spatial analysis of carbon isotope signatures in solid samples down to a spatial resolution of 10 μm. The presented instrumental setup includes in-house-designed exchangeable ablation cells (3.8 and 0.4 mL, respectively) and an improved sample gas transfer, which allow accurate δC measurements of an acryl plate standard down to 0.6 and 0.4 ng of ablated carbon, respectively (standard deviation 0.25‰). Initial testing on plant and soil samples confirmed that microheterogeneity of their natural C/C abundance can now be mapped at a spatial resolution down to 10 μm. The respective δC values in soils with C3/C4 crop sequence history varied by up to 14‰ across a distance of less than 100 μm in soil aggregates, while being partly sorted along rhizosphere gradients of <300 μm from Miscanthus plant roots into the surrounding soil. These very first demonstrations point to the appearance of very small metabolic hotspots originating from different natural isotope discrimination processes, now traceable via LA-IRMS.