INSITU MEASUREMENT OF OSMIUM CONCENTRATIONS IN IRON-METEORITES BY RESONANCE IONIZATION OF SPUTTERED ATOMS

Resonance ionization of sputtered atoms followed by time-of-flight mass spectrometry was used for in situ quantitative measurement of Os with a spatial resolution of ∼70 μm. Osmium concentrations in synthetic metals and iron meteorites were measured to demonstrate the analytical capabilities of the technique. A linear correlation between Os+ signal intensity and the known Os concentration was observed over a range of nearly 104 in Os concentration with an accuracy of ∼ ±10%, a minimum detection limit of 7 parts per billion atomic, and a useful yield of 1%. Resonance ionization of sputtered atoms samples the dominant neutral-fraction of sputtered atoms and utilizes multiphoton resonance ionization to achieve high sensitivity and to eliminate atomic and molecular interferences. Matrix effects should be small compared to secondary ion mass spectrometry because ionization occurs in the gas phase and is largely independent of the physical properties of the matrix material. Resonance ionization of sputtered atoms can be applied to in situ chemical analysis of most high ionization-potential elements (including all of the Pt-group elements) in a wide range of natural and synthetic materials. The high useful yield and elemental selectivity of this method should eventually allow the in situ measurement of Os isotope ratios in some natural samples and in sample extracts enriched in Pt-group elements by fire assay fusion. © 1990.