Mechanism of elemental fractionation in femtosecond laser ablation revealed by high-time-resolution MC-ICP-MS analysis of 208Pb/232Th ratios in single particles

Abstract

This study investigated the mechanism of elemental fractionation during laser ablation (LA) through elemental analysis of individual particles generated via femtosecond laser ablation (fsLA) using the minimised laser spot size of ca. 2 µm. The data acquisition was conducted with a short dwell time of 20 µs using a multiple collector-ICP-mass spectrometer equipped with high-time-resolution ion counting system (HTR-MC-ICP-MS), thereby avoiding overlap of signal events from multiple particles. Results obtained from monazite references showed that individual fsLA-generated particles fall into two distinct groups in terms of ²⁰⁸Pb/²³²Th: Pb-bearing (volatile-element-rich) vs. Pb-depleted (volatile-element-poor). The existence of these two populations is likely the major cause of elemental fractionation in LA. This decoupling is attributable to the different behaviours of volatile vs. refractory elements during particle generation. Particles that condense from the laser-induced vapor plume tend to retain volatile elements (e.g., Pb), whereas particles originating from the molten rim of the ablation pit (material that was molten but not vaporized) are depleted in volatile elements. The data obtained here demonstrates clearly that the presence of the elemental fractionation in the particle generation process is the main reason why using matrix-matched reference materials is essential for obtaining reliable elemental data in LA-ICP-MS.