Precise and Accurate Doping of Nanoporous Silica Gel for the Synthesis of Trace Element Microanalytical Reference Materials

摘要

This study presents an experimental procedure to fabricate high-purity silica glass containing a selected element at a specified mass fraction. The procedure was used to prepare glasses doped with trace-level mass fractions of Ti with the goal of improving analytical confidence when measuring trace elements in quartz. Systematic tests were performed to determine the ideal conditions and procedures for doping nanoporous silica gel with the highest efficiency of dopant recovery. Silica gel was cleaned in concentrated HCl, immersed in a non-polar doping medium at a controlled pH and doped with precise quantities of ICP-MS standard solution. Using liquids composed of longer chain molecules as the doping medium diminishes recovery, suggesting that large molecules could obstruct nanopores to inhibit capillary uptake of the dopant. A control experiment using crystalline quartz reinforced the effectiveness of nanoporous silica gel for doping with trace-level precision. Layered aggregates of silica gel doped with different Ti mass fractions were hot-pressed to create multi-layered reference materials that were analysed with multiple techniques at a variety of spatial scales. Analyses at the intra-grain scale (cathodoluminescence scanning electron microscopy, electron probe microanalysis), at the single grain scale (SIMS), at the sample layer scale (EPMA, laser ablation-ICP-MS) and at the bulk scale (ICP-OES) demonstrated acceptable homogeneity at sample volumes characteristic of most microanalysis techniques and show that nanoporous silica gel holds promise as a highly retentive doping substrate for preparing reference materials for laser-, electron-and ion-beam microanalysis.