Nanosecond Laser Ablation-Multicollector Inductively Coupled Plasma-Mass Spectrometry for in Situ Fe Isotopic Analysis of Micrometeorites: Application to Micrometer-Sized Glassy Cosmic Spherules

This work evaluates the use of nanosecond laser ablation-multicollector inductively coupled plasma-mass spectrometry (ns-LA-MC-ICP-MS) for Fe isotopic analysis of glassy cosmic spherules. Several protocols for data acquisition from the transient signals were compared, with the integration method, i.e., isotope ratios obtained by dividing the corresponding signal intensities integrated over the selected signal segment, providing the best precision. The bias caused by instrumental mass discrimination was corrected for by a combination of internal correction using Ni as an internal standard (coming from a conebulized standard solution) and external correction using a matrix-matched standard. Laser spot size and repetition rate were adapted to match the signal intensities for sample and standard within +/- 10%. For in situ isotopic analysis, the precision of the delta Fe-56 values ranged between 0.02 and 0.11 parts per thousand (1 SD, based on 4 measurement sessions, each based on ablation along 5 lines for 30 s each) and 0.03-0.17 parts per thousand (SD, based on 3 measurement sessions) for glass reference materials and micrometeorites, respectively. Despite this excellent reproducibility, the variation of the isotope ratios along a single ablation line indicated isotopic inhomogeneity exceeding 1 parts per thousand in some micrometeorites. Isotopic analysis via pneumatic nebulization MC-ICP-MS, after sample digestion and chromatographic Fe isolation, was performed to validate the results obtained by in situ isotopic analysis, and good agreement was achieved between the delta-values obtained via both approaches and with those reported in literature for MPI-DING and USGS glass reference materials. Also for the glassy cosmic spherules, overall, there was a good match between the ns-LA-MC-ICP-MS and solution MC-ICP-MS results.