Contrasting particle size distributions and Fe isotope fractionations during nanosecond and femtosecond laser ablation of Fe minerals: Implications for LA-MC-ICP-MS analysis of stable isotopes

Laser ablation coupled to a multi-collector inductively coupled mass spectrometer (LA-MC-ICP-MS) is a promising tool for in situ analysis of metal and metalloid stable isotope ratios. Potential isotopic fractionation associated with laser ablation may, however, cause biased sampling of the substrate, posing a major challenge for precise and accurate isotope ratio measurements. To better characterize the nature of laser ablation induced isotopic fractionation, this study compared particle morphologies, sizes, and size-dependent Fe isotope fractionations produced by ablation of a suite of semi-conductive samples, including natural Fe oxide, sulfide, and carbonate minerals, under various conditions using a 193 nm ArF nanosecond (ns) laser and a 266 nm Ti:sapphire femtosecond (fs) laser. Ablation-produced particles were sorted based on aerodynamic size using a cascade impactor, and Fe isotope compositions of size-sorted particles were then measured offline using conventional solution nebulization ICP-MS to quantify isotopic fractionation produced by the laser ablation. Particle morphology and size distributions produced by ns-laser ablation are more substrate and fluence dependent as compared to fs-laser ablation, resulting from the thermal nature of ns-laser ablation. Often, a higher proportion of the ablated Fe mass resides in particles with large aerodynamic sizes during ns-laser ablation as compared to fs-laser ablation, posing a potential difficulty for LA-ICP-MS analysis due to the increased possibility of incomplete ionization of large particles. Significant size-dependent Fe isotope fractionations of up to several per mil can occur during both ns- and fs-laser ablation, highlighting the importance of quantitative transport of particles to the ICP-MS for accurate Fe isotope analysis. Size-dependent Fe isotope fractionation observed for fs-laser ablation of all Fe minerals can be explained by particle formation through a condensation model, but multiple processes need to be considered to explain the observed Fe isotope fractionation during ns-laser ablation. Mass-balance calculations suggest that ns-laser ablation does not sample magnetite stoichiometrically for Fe isotope compositions at low fluence (1 J/cm2), but does at higher fluences for all minerals. In contrast, fs-laser ablation always provides stoichiometric sampling for Fe isotopes regardless of fluence. Results of this study demonstrate that ns-laser ablation is substrate- and fluence-dependent, resulting in variable particle size distributions and Fe isotope fractionations, and possible non-stoichiometric sampling of semi-conductive samples for Fe isotope analysis. Instead, fs-laser ablation largely minimizes the substrate and fluence dependence, providing more consistent ablation.