Trace Element Microanalysis of Metals Using Laser Ablation ICPMS

A J Campbell and M Humayun (Both at: University of Chicago, Department of the Geophysical Sciences, Chicago, IL 60637; 773-834-1523; email: acampbel@midway.uchicago.edu, hum8@midway.uchicago.edu)

A laser ablation microanalysis system has been developed that can analyze trace elements with a sensitivity in the ppb range, using a CETAC LSX-200 laser ablation system with a Finnigan Element. This microprobe capability has been applied to a set of iron meteorites to demonstrate the laser microprobe's analytical capability for the determination of platinum group elements (PGEs) with a spatial resolution of ~20 µm, comparable to that of secondary ion mass spectrometry (SIMS). The laser is shown to provide an accurate means of solid sampling for magnetic sector inductively coupled plasma mass spectrometry (ICPMS), allowing the determination of bulk metal composition, chemical zoning within the sample at a spatial resolution of ~20 µm, and depth profiling at a spatial resolution of <1 µm. The ablation rate of metals is a strong function of the melting point, consistent with the laser ablation mechanism in these materials being that of melting and vaporization. Recovery of the chemical zoning in taenite lamellae in iron meteorites was achieved not only for the heavy PGEs but also for Ru, Rh and Pd, which was not previously possible using SIMS. Examination of ten meteorites from groups I, IIIAB, and pallasites revealed that all PGEs, Re and Au partition into the Ni-rich (taenite or plessite) phase relative to kamacite. The partitioning of light PGEs and Au is stronger than that of the heavy PGEs and Re. The methods presented here show that magnetic sector ICPMS can be successfully coupled to a laser ablation system, providing the advantages of higher sensitivity of the sector instrument, low background count rates (<0.1 cps), and flat-topped spectral peaks, while minimizing tradeoff against the speed of data acquisition required to handle the transient signals from the laser ablation system.