Study of near infra red femtosecond laser induced particles using transmission electron microscopy and low pressure impaction: Implications for laser ablation–inductively coupled plasma-mass spectrometry analysis of natural monazite

The characteristics of infra red femtosecond laser-induced aerosols are studied for monazite (LREE, Th(PO₄)) ablation and correlations are established with inductively coupled plasma-mass spectrometry (ICP-MS) signals. Critical parameters are tested within wide ranges of values in order to cover the usual laser ablation -ICP-MS analysis conditions: pulse energy (0.15 < E₀ < 1 mJ/pulse), pulse width (60 < τ < 3000 fs), ablation time (t ≤ 10 min) and transport length (l ≤ 6.3 m). Transmission electron microscopy reveals that aerosols are made of agglomerates of ~ 10 nm particles and 20–300 nm phosphorus depleted condensed spherical particles. These structures are not affected by any laser ablation parameter. Particle counting is performed using electronic low pressure impaction. Small changes on particle size distribution are noticed. They may be induced either by a peak of ablation rate in the first 15 s at high fluence (larger particles) or the loss of small particles during transport. We found a positive correlation between I (ICP-MS mean signal intensity in cps) and N (particle density in cm^? 3) when varying E₀ and t, suggesting that N is controlled by the irradiance (P₀ in W·cm^? 2). Elemental ratio measurements show a steady state signal after the initial high ablation rate (mass load effect in the plasma torch) and before a late chemical fractionation, induced by poor extraction of bigger, early condensed spherical particles from the deepening crater. Such chemical fractionation effects remain within uncertainties, however. These effects can be limited by monitoring E₀ to shorten the initial transient state and delay the attainment of an unfavorable crater aspect ratio. Most adopted settings are for the first time deduced from aerosol characteristics, for infra red femtosecond laser ablation. A short transport (l < 4.0 m) limits the agglomeration of particles by collision process along the tube. Short τ is preferred because of higher P₀, yet no benefit is found on ICP-MS signal intensity under 200 fs. Under such pulse widths the increased particle production induces more agglomeration during transport, thereby resulting in higher mass load effects that reduce the ionization efficiency of the plasma torch. Thus, pulse energy must be set to get an optimal balance between the need for a high signal/background ratio and limitation of mass load effects in the plasma torch.