Isotopic measurement of subnanogram quantities of rhenium and osmium by resonance ionization mass spectrometry

Resonance ionization mass spectrometry has been used to measure the isotopic compositions of microgram and picogram quantities of Re and Os. The high sensitivity required for these measurements was achieved through the optimization of sample atomization and efficient ionization from the resulting gas-phase reservoir. Re and Os are absorbed from chloride solutions onto anion exchange beads as a means of purifying and concentrating the sample and then loaded onto a miniaturized Ta filament. The molecular species of Re and Os are reduced to metal in the vacuum of the mass spectrometer by gradual heating in the presence of collodion and graphite. Both Re and Os atomize efficiently at 2173-2373 K. Os atomization is optimized further by pulsing the filament temperature to coincide with the pulse rate of the laser system. Pulsed ion signals of >2 ions/s are quantified by signal averaging using a transient digitizer. Ion fluxes <2 ions/s are quantified by ion counting. Measurements of microgram quantities of spike-standard mixtures are reported to precisions and accuracies of about 1%. Measurement precision and accuracy of picogram quantities of Os range from 1 to 5% and are primarily limited by counting statistics.