NanoSIMS μm-scale in situ measurement of 13C/12C in early Precambrian organic matter, with permil precision

NanoSIMS is readily able to measure 13C−/12C− with 1σ internal precision <1‰ and reproducibility ∼1‰ on 5 μm × 5 μm areas of solid organic matter, using a rastered Cs+ primary ion beam and a pair of electron-multiplier secondary ion detectors. Similar results can be obtained on areas as small as 3 μm × 3 μm. Samples smaller than these frame sizes and samples with isotopic complexity on smaller scales can also be analysed, though with poorer precision corresponding to the reductions in analysed surface area. Data reduction requires consideration of the ratio drift resulting from ageing of the 12C− electron multiplier, variations in count rates within the analysed area, and the effects of quasi-simultaneous 12C− arrivals. Samples that do not fill the raster area also require image analysis to determine the effective sample areas; in complex cases the ratios may be better derived from images recorded with long acquisition times.