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

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 a minimised laser spot size of approximately 2 μm. Data acquisition was conducted within a short dwell time of 20 μs using a multiple collector-ICP-mass spectrometer equipped with a high-time-resolution ion counting system (HTR-MC-ICP-MS), which avoids the 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 probably the major cause of elemental fractionation in LA. This decoupling is attributed to the different behaviours of volatile and refractory elements during particle generation. Particles that condense from 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 clearly demonstrated that elemental fractionation is involved in the particle generation process, and hence, using matrix-matched reference materials is essential for obtaining reliable elemental data in LA-ICP-MS.