Determination of the nitrogen abundance in organic materials by NanoSIMS quantitative imaging

Abstract

We describe a procedure to determine precise and accurate elemental abundance by means of quantitative imaging using secondary ion mass spectrometry (on images covering ∼10 × 10 μm2) applied to natural Insoluble Organic Matter (IOM). Dynamic SIMS conditions are reached for a 16 keV Cs+ fluence of >2.0 × 1017 Cs+ cm−2 implanted at the surface of the IOM. Once the sample surface is saturated in cesium, steady-state implantation and sputtering yield are reached: constant secondary ion count rates are then observed. Two calibrations of the nitrogen abundance are presented. The nitrogen abundance is expressed either as [N] or N/C atomic ratio calibrated by means of 12C14N−/12C− or 12C14N−/12C2− ionic ratios respectively. The 12C14N−/12C− uncertainty is always larger than the uncertainty of 12C14N−/12C2−. At a smaller scale inside an image (∼1 × 1 μm2), a little larger than the primary beam size, processes induced by topographic irregularity of the IOM powder result in a variation of the 12C14N−,12C2− and 12C− count rates and increase the variability of the ionic ratios. The chemical contrast produced by a rough sample surface exposed to the Cs+ rastering is evaluated with a scratched epoxy resin for 12C14N−/12C− and 12C14N−/12C2− elemental ratios. Owing to similarities in emission parameters, the determination of the N/C ratio in rough organic surfaces is improved by using 12C 14N−/12C2− instead of 12C 14N−/12C−.