Box-profile implants as geochemical reference materials for EMP and SIMS

EMP (Electron Microprobe) 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; 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 reference materials for SIMS is vanishingly short compared to the needs of the analyst. A current evaluation of the GeoReM database [1] shows a strong focus on using the well-known NIST SRM 610-617 glasses for trace element analysis along with several zircons for isotope analysis, with few other matrices being readily available.
One approach for the production of matrix specific RMs is the use of ion implantation that introduces a known amount of a selected isotope into a material. This strategy is widely-used for SIMS applications in materials science, but rarely used for geochemical applications. Bumett et al. (2015) [2] demonstrated the appropriateness of this method. We choose the more elaborate way of implanting a so-called box profile to generate a homogeneous concentration of the selected isotope in three dimensions.
For proof of concept we used the mineralogically and chemically “simple” SiO2 system which addresses many interesting scientific challenges, such as the Ti-in-quartz geothermometer [3]. We implanted either ⁴⁷Ti or ⁴⁸Ti into synthetic, ultra-high purity silica glass. Several box profiles with concentrations between 10 and 1000 µg/g Ti and a maximum depth of homogeneous Ti distribution between 200 and 3000 nm were produced at the Ion Beam Center in Dresden-Rossendorf. Multiple implantation steps using differing ion energies and ion doses were simulated with the SRIM (Stopping and Range of Ions in Matter) software [4], optimizing for the target concentrations, implantation-depths and technical limits of the implanter. A thorough TEM assessment showed that the Ti is dispersed throughout the glass structure. We characterized several implanted test-samples by means of SIMS, EMP and other analytical techniques. The homogeneity of the Ti-concentration is within ± 5% uncertainty in all 3 dimensions.
[1] http://georem.mpch-mainz.gwdg.de, [2] D. S. Bumett, et al., Geostandards and Geoanalytical Research 39(3), 2015, 265-276., [3] J. B. Thomas, et al., Contrib. Mineral. Petr. 160(5), 2010, 743–759., [4] J. F. Ziegler, Nucl. Instrum. Meth. B 219-220, 2004, 1027-1036.