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⁻.