Temporal changes of size distribution of mass and relative intensity for ablated particles during laser ablation inductively coupled plasma mass spectrometry

Abstract

To investigate elemental fractionation during laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) we measured mass fractions of ablated particles and chemical composition of ablated particles in this study. Temporal changes of fractionation indexes (FIs) were investigated under laser defocus conditions which caused a large variation of size distribution of abated particles. It was a useful technique for understanding the relationship between temporal changes of FIs and the size of ablated particles. Ablated particles were fractionated by aerodynamic diameters (<0.06, 0.06–0.22, 0.22–2.2, and >2.2 μm) with a low-pressure impactor and were digested with HNO₃ and HF; then As, Rb, Rh, La, Gd, Yb, W, Re, and Th were measured by ICPMS. Under 0.5 mm defocus and 1.0 mm defocus conditions, the mass fractions (ablated particle mass at 1–5 min divided by that at 0–1 min) of ablated particles larger than 0.22 μm were larger than the mass fractions of ablated particles smaller than 0.22 μm. Volatile elements such as As and Rb were enriched in particles smaller than 0.22 μm, owing to the large aspect ratio of the crater under defocus conditions. However, the magnitude of the enrichment for volatile elements did not change as ablation progressed. Therefore, we concluded that large particles could not be decomposed completely in the ICP and the FI peak observed at 2–3 min was caused by changes in elemental behavior due to changes in ablated particles larger than 0.22 μm.