Investigations of Pb ion formation in molten silicate glass using a microinterdigitated electrode array and TIMS

Abstract:

We have developed a microinterdigitated electrode array (IDA) for thermal ionization mass spectrometers to investigate the ionization mechanism for Pb metal atoms doped in molten silicate glasses and to assess methods of increasing ionization efficiencies of thermalized ion emitters. The array consists of a comb structure of 20 pairs of interleaved titanium electrode fingers (280 nm thickness) sputtered onto a sapphire wafer. Titanium electrode lengths and widths are 950µ and 25µ, respectively, and gap widths are 13µ. The sapphire wafer (330µ thickness) is used as a substrate for the electrode because of its high melting point >2000^oC, electrical insulation, and high thermal conductivity (42 W/m·K @20^oC). The wafer is held in place with a MACOR^® (machinable ceramic) holder with thin Ta plates used as electrical connections to external power supplies. The assembly is connected to a custom-designed potentiostat that allows the IDA to float at 10 kV while a differential voltage from -10V to +10V is applied across the IDA electrode. In the mass spectrometer, a Pb-doped silica “gel” is melted on the IDA surface by conductive heating from below by a metal Re ribbon resistive heater. Initial observations are that anodic currents within Pb-doped liquid glasses increase as increasing voltages are imposed across electrode pairs in the IDA, normally interpreted as the production of metal ions in electrolyte. When inserted in the mass spectrometer, the increasing anodic current is accompanied by increasing current of Pb⁺ ions through the mass spectrometer, confirming that the increasing current through the molten glass is due to ionization of Pb⁰ to Pb⁺ at anode in liquid glass. This result shows that it is possible to conduct electrochemistry on high temperature liquid glasses, in vacuo, and to determine the nature of electrochemical reactions by identifying metal ions that are emitted during evaporation of the electrolyte and then accelerated through the magnetic sector of the thermal ionization mass spectrometer. At present, we can increase ion formation by increasing electrode voltage, but future studies will address whether the main determinant on Pb ionization efficiency is the applied electrode voltage or the intrinsic work function of metal electrode surface.