‘Box-Profile’ Ion Implants as Geochemical Reference Materials for Electron Probe Microanalysis and Secondary Ion Mass Spectrometry

EPMA (Electron Probe Microanalysis) and SIMS (Secondary Ion Mass Spectrometry) are widely used analytical techniques for geochemical and mineralogical applications. Nevertheless, metrologically rigorous quantification remains a major challenge for these methods. SIMS in particular is a matrix-sensitive method; for SIMS the use of matrix-matched reference materials (RMs) is essential in order to avoid significant analytical bias. A major problem is that the list of available RMs for SIMS is vanishingly short compared to the needs of the analyst. One approach for the production of matrix-specific RMs is the use of high-energy ion implantation that introduces a known amount of a selected isotope into a material. We chose the more elaborate way of implanting a so-called ‘box profile’ to generate a quasi-homogeneous concentration of the implanted isotope in three dimensions, which allows RMs not only to be used for ion beam analysis but also makes them suitable for EPMA. For proof of concept, we used the thoroughly studied mineralogically and chemically ‘simple’ SiO2 system, which addresses many interesting scientific challenges, such as the Ti-in-quartz geothermometer (Wark et al. 2006, Thomas et al. 2010). We implanted either 47Ti or 48Ti into synthetic, ultra-high purity silica glass. Several ‘box profiles’ with concentrations between 10 and 1000 µg g-1 Ti and maximum depths of homogeneous Ti distribution between 200 nm and 3 µm were produced at the Institute of Ion Beam Physics and Materials Research of Helmholtz-Zentrum Dresden-Rossendorf (HZDR). Multiple implantation steps using varying ion energies and ion doses were simulated with the SRIM (Stopping and Range of Ions in Matter) software (Ziegler et al. 2008), optimizing for the target concentrations, implantation-depths and technical limits of the implanter.
We characterized several implant test-samples having different concentrations and maximum implantation depths by means of SIMS and other analytical techniques. The results show that the implant samples are suitable for use as reference materials for SIMS measurements. The multi-energy ion implantation technique also looks to be very promising procedure for the production of EPMA-suitable reference materials.