In-house reference materials for the determination of low titanium concentration in SiO₂ by secondary ion mass spectrometry (SIMS)

SIMS (Secondary Ion Mass Spectrometry) is a routinely used analytical technique for geochemical and mineralogical applications. Nevertheless, quantification is still the major challenge of this method. Due to the fact, that each analysed matrix needs its own matrix-matched reference material (RM), the list of available reference materials is short compared to the needs of the analysts. 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 and several zircons for isotope analysis.

One approach for the production of suitable RMs is the use of ion implantation to introduce a known amount of an isotope into a matrix-matched material. This approach is widely-used for SIMS applications in materials science, but rarely used for geochemical problems. Bumett et al. (2014) [2] demonstrated the principal appropriateness of this method and ways to calibrate nominal implant fluence. We choose the more elaborate way of implanting a box profile to allow a homogeneous distribution of the respective isotope in all three dimensions.

The mineralogical and chemical “simple” SiO₂ system entails many interesting scientific challenges like the Ti-in-quartz geothermometer [3, 4]. We implanted ⁴⁷Ti respectively ⁴⁸Ti into synthetic ultra-high purity silica glass. Several box profiles with concentrations between 10 and 1000 ppm and a maximum depth of homogeneous ^47/48Ti distribution between 200 and 500 nm were produced at the Ion Beam Center in Dresden-Rossendorf. The single implantation steps with different ion-energies and -doses were simulated with the SRIM (Stopping and Range of Ions in Matter) software [5] and optimized to the necessary concentrations, implantation-depths and limits of the implanter. After the implantations, the surface-roughness of the implanted glasses was measured using atomic force microscopy (AFM). The roughness is still in the range necessary for SIMS measurements.

We characterized several implanted test-samples with different concentrations and maximum implantation-depths by means of SIMS and other analytical techniques. It has been shown, that Ti is dissolved in the glass structure. The homogeneity of the Ti-concentration is within ± 5% uncertainty in all 3 dimensions.

[1] http://georem.mpch-mainz.gwdg.de
[2] Bumett, D.S., et al. (2014). Ion Implants as Matrix-Appropriate Calibrators for Geochemical Ion Probe Analyses. Geostandards and Geoanalytical Research, 39(3), 265-276.
[3] Wark, D. A., Watson E. B. (2006). TitaniQ: a titanium-in-quartz geothermometer. Contributions to Mineralogy and Petrology, 152(6), 743-754.
[4] Thomas, J. B., Watson E. B., et al. (2010). TitaniQ under pressure: the effect of pressure and temperature on the solubility of Ti in quartz. Contributions to Mineralogy and Petrology, 160(5), 743–759.
[5] Ziegler J. F. (2004). SRIM-2003. Nuclear Instruments and Methods in Physics Research Section B, 219-220, 1027-1036.