Study on ablation behavior and mechanism for accurate determination of rare earth elements in CaF2 crystals by UV-LA-ICP-MS

Abstract

Catastrophic ablation of CaF₂ crystals is often observed in ultraviolet-laser ablation-inductively coupled plasma-mass spectrometry (UV-LA-ICP-MS) analysis, leading to variations in the volume of ablated material and resulting analytical errors in LA-ICP-MS microanalysis. Therefore, it is essential to understand the ablation behavior and the interaction mechanisms between the laser and CaF₂ crystals. In this study, we examined the effects of crystal surface roughness and laser fluence on ablation behavior. The CaF₂ crystal polished with 1500 mesh abrasive paper exhibited a stable signal profile, a regular ablation crater, and the formation of nanoparticles, which align with predictions from the thermo-mechanical coupling model. Both higher and lower roughness resulted in either unstable ablation or uncontrolled ejection of fragments; the latter occurred when the thermal stress from accumulated light energy significantly exceeded the compressive strength of CaF₂. Consequently, a roughness of 1500 mesh at the bottom of the ablation craters was deduced. We also identified a laser fluence of 14 J·cm⁻² as the threshold for effective LA-ICP-MS analysis of CaF₂ crystals, based on consistent signal profiles, elemental fractionation index (EFI), and crater morphology as revealed by LA-ICP-MS and scanning electron microscope (SEM) analyses, along with theoretical calculations from the thermo-mechanical coupling model. The optimal roughness (1500 mesh) and fluence (14 J·cm⁻²), together with the thermo-mechanical coupling model, can serve as guidelines for UV-LA-ICP-MS settings in further quantitative analyses of CaF₂ crystals.